LTC3858IGN-1TRPBF_技术文档

LTC3858-1

Low I , Dual

Q

2-Phase Synchronous

Step-Down Controller

FeaTures

DescripTion

TheꢀLTC^®3858-1ꢀisꢀaꢀhighꢀperformanceꢀdualꢀstep-downꢀ

switchingꢀregulatorꢀcontrollerꢀthatꢀdrivesꢀallꢀN-channelꢀ

synchronousꢀpowerꢀMOSFETꢀstages.ꢀAꢀconstantꢀfrequencyꢀ

currentꢀmodeꢀarchitectureꢀallowsꢀaꢀphase-lockableꢀfre-

quencyꢀofꢀupꢀtoꢀ850kHz.ꢀPowerꢀlossꢀandꢀnoiseꢀdueꢀtoꢀtheꢀ

inputꢀcapacitorꢀESRꢀareꢀminimizedꢀbyꢀoperatingꢀtheꢀtwoꢀ

controllerꢀoutputsꢀoutꢀofꢀphase.

n

ꢀ Low Operating I : 170µA (One Channel On)

Q

n

ꢀ Wide Output Voltage Range: 0.8V ≤ V

≤ 24V

OUT

ꢀ Wide V Range: 4V to 38V

IN

ꢀ R

or DCR Current Sensing

SENSE

ꢀ Out-of-PhaseꢀControllersꢀReduceꢀRequiredꢀInputꢀ

CapacitanceꢀandꢀPowerꢀSupplyꢀInducedꢀNoise

®

n

ꢀ OPTI-LOOP ꢀCompensationꢀMinimizesꢀC

OUT

ꢀ Phase-LockableꢀFrequencyꢀ(75kHz-850kHz)

ꢀ ProgrammableꢀFixedꢀFrequencyꢀ(50kHz-900kHz)

ꢀ SelectableꢀContinuous,ꢀPulse-Skippingꢀorꢀ

Theꢀ170μAꢀno-loadꢀquiescentꢀcurrentꢀextendsꢀoperatingꢀ

lifeꢀinꢀbatteryꢀpoweredꢀsystems.ꢀOPTI-LOOPꢀcompensa-

tionꢀallowsꢀtheꢀtransientꢀresponseꢀtoꢀbeꢀoptimizedꢀoverꢀ

aꢀwideꢀrangeꢀofꢀoutputꢀcapacitanceꢀandꢀESRꢀvalues.ꢀTheꢀ

LTC3858-1ꢀfeaturesꢀaꢀprecisionꢀ0.8Vꢀreferenceꢀandꢀaꢀpowerꢀ

goodꢀoutputꢀindicator.ꢀAꢀwideꢀ4Vꢀtoꢀ38Vꢀinputꢀsupplyꢀrangeꢀ

encompassesꢀaꢀwideꢀrangeꢀofꢀintermediateꢀbusꢀvoltagesꢀ

andꢀbatteryꢀchemistries.

ꢀ BurstꢀMode^®ꢀOperationꢀatꢀLightꢀLoads

n

ꢀ VeryꢀLowꢀDropoutꢀOperation:ꢀ99%ꢀDutyꢀCycle

n

ꢀ AdjustableꢀOutputꢀVoltageꢀSoft-Start

n

ꢀ PowerꢀGoodꢀOutputꢀVoltageꢀMonitor

n

ꢀ OutputꢀOvervoltageꢀProtection

n

ꢀ OutputꢀLatch-OffꢀProtectionꢀDuringꢀShortꢀCircuit

Independentꢀsoft-startꢀpinsꢀforꢀeachꢀcontrollerꢀrampꢀtheꢀ

outputꢀvoltagesꢀduringꢀstart-up.ꢀTheꢀoutputꢀlatch-offꢀfeatureꢀ

protectsꢀtheꢀcircuitꢀinꢀshort-circuitꢀconditions.

n

ꢀ LowꢀShutdownꢀI :ꢀ8µA

Q

n

ꢀ InternalꢀLDOꢀPowersꢀGateꢀDriveꢀfromꢀV ꢀorꢀEXTV

IN

CC

ꢀ NoꢀCurrentꢀFoldbackꢀDuringꢀStart-Up

ꢀ Tinyꢀ4mmꢀ×ꢀ5mmꢀQFNꢀandꢀNarrowꢀSSOPꢀPackages

Forꢀaꢀleadlessꢀ32-pinꢀQFNꢀpackageꢀwithꢀtheꢀadditionalꢀfea-

turesꢀofꢀadjustableꢀcurrentꢀlimit,ꢀclockꢀout,ꢀphaseꢀmodula-

tionꢀandꢀtwoꢀPGOODꢀoutputs,ꢀseeꢀtheꢀLTC3858ꢀdataꢀsheet.

L,ꢀLT,ꢀLTC,ꢀLTM,ꢀBurstꢀMode,ꢀOPTI-LOOP,ꢀµModule,ꢀLinearꢀTechnologyꢀandꢀtheꢀLinearꢀlogoꢀ

applicaTions

n

ꢀ AutomotiveꢀSystems

areꢀregisteredꢀtrademarksꢀandꢀNoꢀR ꢀandꢀUltraFastꢀareꢀtrademarksꢀofꢀLinearꢀTechnologyꢀ

SENSE

n

ꢀ BatteryꢀOperatedꢀDigitalꢀDevices

Corporation.ꢀAllꢀotherꢀtrademarksꢀareꢀtheꢀpropertyꢀofꢀtheirꢀrespectiveꢀowners.ꢀProtectedꢀbyꢀU.S.ꢀ

Patents,ꢀincludingꢀ5481178,ꢀ5705919,ꢀ5929620,ꢀ6100678,ꢀ6144194,ꢀ6177787,ꢀ6304066,ꢀ6580258.

n

ꢀ DistributedꢀDCꢀPowerꢀSystems

Typical applicaTion

High Efﬁciency Dual 8.5V/3.3V Step-Down Converter

V

Efﬁciency and Power Loss

IN

9V TO 38V

22µF

50V

vs Load Current

4.7µF

V

INTV

CC

100

90

10000

1000

100

10

IN

TG1

TG2

0.1µF

BOOST1

SW1

BOOST2

SW2

3.3µH

7.2µH

80

70

EFFICIENCY

BG1

BG2

60

50

LTC3858-1

PGND

POWER LOSS

= 12V

+

40

30

20

10

0

SENSE1

SENSE2

0.01Ω

193k

0.007Ω

–

1

–

V

8.5V

3.5A

SENSE2

OUT2

V

OUT1

3.3V

5A

IN

OUT

V

= 3.3V

FB1

FB2

I

TH2

62.5k

FIGURE 12 CIRCUIT

0.1 10

OUTPUT CURRENT (A)

I

TH1

SS1

0.1

150µF

680pF

15k

680pF

15k

150µF

SGND

SS2

0.0001 0.001

0.01

1

20k

0.1µF

38581 TA01b

38581 TA01

38581fb

ꢀ

PGOOD1ꢀVoltageꢀ......................................... –0.3Vꢀtoꢀ6V

ꢀ BOOST1,ꢀBOOST2ꢀ................................. –0.3Vꢀtoꢀ46V

(Noteꢀ2).................................................. –40°Cꢀtoꢀ125°C

RUN1,ꢀRUN2ꢀ

StorageꢀTemperatureꢀRange................... –65°Cꢀtoꢀ150°C

SENSE2 ꢀVoltages

...................................... –0.3Vꢀtoꢀ28V

EXTV ꢀ...................................................... –0.3Vꢀtoꢀ14V

LTC3858-1

absoluTe MaxiMuM raTings ^{(Note 1)}

InputꢀSupplyꢀVoltageꢀ(V )ꢀ......................... –0.3Vꢀtoꢀ40V

CC

IN

I

,ꢀI ,V ,ꢀV ꢀVoltagesꢀ...................... –0.3Vꢀtoꢀ6V

TH1 TH2 FB1 FB2

TopsideꢀDriverꢀVoltagesꢀ

SS1,ꢀSS2,ꢀINTV ꢀVoltagesꢀꢀ......................... –0.3Vꢀtoꢀ6V

SwitchꢀVoltageꢀ(SW1,ꢀSW2)ꢀꢀ........................ –5Vꢀtoꢀ40V

(BOOST1-SW1),ꢀ(BOOST2-SW2)ꢀ................ –0.3Vꢀtoꢀ6V

............................................... –0.3Vꢀtoꢀ8V

CC

OperatingꢀJunctionꢀTemperatureꢀRangeꢀ

MaximumꢀJunctionꢀTemperatureꢀ(Noteꢀ3)ꢀ............ 125°C

ꢀ MaximumꢀCurrentꢀSourcedꢀIntoꢀPinꢀ

ꢀ fromꢀSourceꢀ>8V...............................................100µA

+

–

+

–

LeadꢀTemperatureꢀ(Soldering,ꢀ10ꢀsec)

SENSE1 ,ꢀSENSE2 ,ꢀSENSE1

ꢀ SSOPꢀ................................................................ 300°C

PLLIN/MODE,ꢀFREQꢀVoltagesꢀꢀ.............. –0.3VꢀtoꢀINTV

CC

pin conFiguraTion

TOP VIEW

1

2

SS1

28

27

26

25

24

23

22

21

20

19

18

17

16

15

I

TH1

PGOOD1

TG1

V

FB1

+

3

SENSE1

28 27 26 25 24 23

+

–

SENSE1

1

2

3

4

5

6

7

8

22

21

20

19

18

17

16

15

BOOST1

BG1

4

SW1

–

5

BOOST1

BG1

FREQ

PLLIN/MODE

SGND

FREQ

PLLIN/MODE

SGND

V

IN

6

PGND

29

SGND

7

V

IN

EXTV

CC

8

PGND

RUN1

INTV

BG2

9

EXTV

CC

RUN2

–

10

11

12

13

14

INTV

CC

SENSE2

–

SENSE2

BOOST2

+

BG2

SENSE2

9

10 11 12 13 14

UFD PACKAGE

BOOST2

SW2

V

FB2

TH2

I

TG2

SS2

28-LEAD (4mm s 5mm) PLASTIC QFN

GN PACKAGE

28-LEAD PLASTIC SSOP

ꢀ

T

ꢀ=ꢀ125°C,ꢀθ ꢀ=ꢀ43°C/Wꢀ

JMAX

JA

ꢀ

EXPOSEDꢀPADꢀ(PINꢀ29)ꢀISꢀSGND,ꢀMUSTꢀBEꢀSOLDEREDꢀTOꢀPCB

T

ꢀ=ꢀ125°C,ꢀθ ꢀ=ꢀ90°C/W

JMAX JA

orDer inForMaTion

LEAD FREE FINISH

LTC3858EUFD-1#PBF

LTC3858IUFD-1#PBF

LTC3858EGN-1#PBF

LTC3858IGN-1#PBF

TAPE AND REEL

PART MARKING*

PACKAGE DESCRIPTION

TEMPERATURE RANGE

–40°Cꢀtoꢀ125°C

LTC3858EUFD-1#TRPBF 38581

28-Leadꢀ(4mmꢀ×ꢀ5mm)ꢀPlasticꢀQFN

28-LeadꢀPlasticꢀSSOP

LTC3858IUFD-1#TRPBF

LTC3858EGN-1#TRPBF

LTC3858IGN-1#TRPBF

38581

LTC3858GN-1

28-LeadꢀPlasticꢀSSOP

ConsultꢀLTCꢀMarketingꢀforꢀpartsꢀspeciﬁedꢀwithꢀwiderꢀoperatingꢀtemperatureꢀranges.ꢀꢀ*Theꢀtemperatureꢀgradeꢀisꢀidentiﬁedꢀbyꢀaꢀlabelꢀonꢀtheꢀshippingꢀcontainer.

ConsultꢀLTCꢀMarketingꢀforꢀinformationꢀonꢀnon-standardꢀleadꢀbasedꢀﬁnishꢀparts.

Forꢀmoreꢀinformationꢀonꢀleadꢀfreeꢀpartꢀmarking,ꢀgoꢀto:ꢀhttp://www.linear.com/leadfree/ꢀꢀ

Forꢀmoreꢀinformationꢀonꢀtapeꢀandꢀreelꢀspeciﬁcations,ꢀgoꢀto:ꢀhttp://www.linear.com/tapeandreel/

38581fb

ꢁ

LTC3858-1

elecTrical characTerisTics ^Thel denotes the speciﬁcations which apply over the full operating

junction temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= 12V, V_RUN1,2= 5V, EXTV_CC= 0V unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

V

InputꢀSupplyꢀOperatingꢀVoltageꢀRange

RegulatedꢀFeedbackꢀVoltage

4

38

V

IN

(Noteꢀ4)ꢀI

ꢀ=ꢀ1.2Vꢀ

TH1,2

ꢀ

FB1,2

l

ꢀꢀꢀ–40°Cꢀtoꢀ125°Cꢀ

ꢀꢀꢀ–40°Cꢀtoꢀ85°C

0.788ꢀ

0.792

0.800ꢀ

0.800

0.812ꢀ

0.808

Vꢀ

V

I

FeedbackꢀCurrent

(Noteꢀ4)

5

50

nA

FB1,2

V

ReferenceꢀVoltageꢀLineꢀRegulation

OutputꢀVoltageꢀLoadꢀRegulation

(Noteꢀ4)ꢀV ꢀ=ꢀ4.5Vꢀtoꢀ38V

0.002

0.02

%/V

REFLNREG

LOADREG

IN

(Note4)ꢀ

ꢀ

%

l

MeasuredꢀinꢀServoꢀLoop,ꢀꢀ

0.01

0.1

∆I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ0.7V

TH

(Note4)ꢀ

ꢀ

%

MeasuredꢀinꢀServoꢀLoop,ꢀꢀ

–0.01

–0.1

∆I ꢀVoltageꢀ=ꢀ1.2Vꢀtoꢀ2V

TH

g

ꢀ

TransconductanceꢀAmpliﬁerꢀg

InputꢀDCꢀSupplyꢀCurrent

(Noteꢀ4)ꢀI

ꢀ=ꢀ1.2V,ꢀSink/Sourceꢀ=ꢀ5µA

TH1,2

2

mmho

mA

m1,2

m

I

Q

(Noteꢀ5)

PulseꢀSkipꢀorꢀForcedꢀContinuousꢀModeꢀ

(OneꢀChannelꢀOn)

RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ

RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ

1.3

V

ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)

FB1

PulseꢀSkipꢀorꢀForcedꢀContinuousꢀModeꢀ

(BothꢀChannelsꢀOn)

RUN1,2ꢀ=ꢀ5V,ꢀV

ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)

2

mA

µA

FB1,2

SleepꢀModeꢀ(OneꢀChannelꢀOn)

RUN1ꢀ=ꢀ5VꢀandꢀRUN2ꢀ=ꢀ0Vꢀorꢀꢀ

RUN1ꢀ=ꢀ0VꢀandꢀRUN2ꢀ=ꢀ5V,ꢀꢀ

170

250

V

ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)

FB1

SleepꢀModeꢀ(BothꢀChannelsꢀOn)

Shutdown

RUN1,2ꢀ=ꢀ5V,ꢀV

RUN1,2ꢀ=ꢀ0V

ꢀ=ꢀ0.83Vꢀ(NoꢀLoad)

FB1,2

300

8

450

20

µA

l

UVLO

UndervoltageꢀLockout

INTV ꢀRampingꢀUpꢀ

ꢀ

4.0ꢀ

3.8

4.2ꢀ

4.0

Vꢀ

V

CC

INTV ꢀRampingꢀDown

3.6

CC

V

FeedbackꢀOvervoltageꢀProtection

MeasuredꢀatꢀV

EachꢀChannel

EachꢀChannelꢀ

,ꢀRelativeꢀtoꢀRegulatedꢀV

FB1,2

7

10

13

1

%

OVL

FB1,2

+

–

+

I

SENSE ꢀPinꢀCurrent

µA

SENSE

–

SENSE ꢀPinsꢀCurrent

ꢀ

µAꢀ

µA

V

ꢀ<ꢀINTV ꢀ–ꢀ0.5ꢀ

1ꢀ

OUT1,2

CC

ꢀ>ꢀINTVCCꢀ+ꢀ0.5

540

700

DF

MaximumꢀDutyꢀFactor

InꢀDropout,ꢀFREQꢀ=ꢀ0V

98

0.7

99.4

1.0

1.28

50

%

µA

V

MAX

I

Soft-StartꢀChargeꢀCurrent

RUNꢀPinꢀOnꢀThresholdꢀVoltage

V

ꢀ=ꢀ0V

SS1,2

1.4

SS1,2

l

V

ꢀOn

,ꢀV ꢀRising

RUN1 RUN2

1.23

1.33

RUN1,2

SS1,2

ꢀHyst RUNꢀPinꢀHysteresisꢀVoltage

mV

V

ꢀLA

SSꢀPinꢀLatch-OffꢀArmingꢀThresholdꢀ

Voltage

V

,ꢀV ꢀRisingꢀfromꢀ1V

1.9

2

2.1

SS1 SS2

V

ꢀLT

SSꢀPinꢀLatch-OffꢀThresholdꢀVoltage

SSꢀDischargeꢀCurrent

,ꢀV ꢀRisingꢀfromꢀ2V

SS1 SS2

1.3

7

1.5

10

1.7

13

V

SS1,2

I

ꢀLT

Short-CircuitꢀConditionꢀV

ꢀ=ꢀ0.5Vꢀ

FB1,2

µA

DSC1,2

V

ꢀ=ꢀ4.5V

SS1,2

l

V

MaximumꢀCurrentꢀSenseꢀThresholdꢀ

Voltage

V

ꢀ=ꢀ0.7V,ꢀV

FB1,2

–, –ꢀ=ꢀ3.3V

SENSE1 2

43

50

57

mV

SENSE(MAX)

Gate Driver

TG1,2

Pull-UpꢀOn-Resistanceꢀ

Pull-DownꢀOn-Resistance

2.5ꢀ

1.5

Ωꢀ

Ω

BG1,2

Pull-UpꢀOn-Resistanceꢀ

Pull-DownꢀOn-Resistance

2.4ꢀ

1.1

Ωꢀ

Ω

38581fb

ꢂ

LTC3858-1

elecTrical characTerisTics ^Thel denotes the speciﬁcations which apply over the full operating

junction temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= 12V, V_RUN1,2= 5V, EXTV_CC= 0V unless otherwise noted.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

ꢀ

TGꢀTransistionꢀTime:ꢀ

ꢀRiseꢀTimeꢀ

ꢀFallꢀTime

(Noteꢀ6)ꢀ

LOAD

ꢀ

nsꢀ

ns

TG1,2ꢀt ꢀ

C

ꢀ=ꢀ3300pFꢀ

ꢀ=ꢀ3300pF

25ꢀ

16

r

TG1,2ꢀt

f

ꢀ

BGꢀTransistionꢀTime:ꢀ

ꢀRiseꢀTimeꢀ

ꢀFallꢀTime

(Noteꢀ6)ꢀ

LOAD

ꢀ

nsꢀ

ns

BG1,2ꢀt ꢀ

C

ꢀ=ꢀ3300pFꢀ

ꢀ=ꢀ3300pF

28ꢀ

13

r

BG1,2ꢀt

f

TG/BGꢀt

TopꢀGateꢀOffꢀtoꢀBottomꢀGateꢀOnꢀDelayꢀ

SynchronousꢀSwitch-OnꢀDelayꢀTime

C

ꢀ=ꢀ3300pFꢀEachꢀDriver

30

95

ns

1D

LOAD

BG/TGꢀt

BottomꢀGateꢀOffꢀtoꢀTopꢀGateꢀOnꢀDelayꢀ

TopꢀSwitch-OnꢀDelayꢀTime

C

LOAD

ꢀ=ꢀ3300pFꢀEachꢀDriver

t

MinimumꢀOn-Time

(Noteꢀ7)

6Vꢀ<ꢀV ꢀ<ꢀ38V,ꢀV ꢀ=ꢀ0V

EXTVCC

ON(MIN)

INTV Linear Regulator

CC

V

InternalꢀV ꢀVoltage

4.85

4.5

5.1

0.7

5.1

0.6

4.7

250

5.35

1.1

V

%

V

INTVCCVIN

LDOVIN

CC

IN

INTV ꢀLoadꢀRegulation

I

CC

ꢀ=ꢀ0mAꢀtoꢀ50mA,ꢀV

ꢀ=ꢀ0V

CC

EXTVCC

InternalꢀV ꢀVoltage

6Vꢀ<ꢀV ꢀ<ꢀ13V

EXTVCC

5.35

1.1

INTVCCEXT

LDOEXT

CC

INTV ꢀLoadꢀRegulation

I

CC

ꢀ=ꢀ0mAꢀtoꢀ50mA,ꢀV

ꢀ=ꢀ8.5V

%

V

CC

EXTVCC

EXTV ꢀSwitchoverꢀVoltage

EXTV ꢀRampingꢀPositive

4.9

EXTVCC

CC

EXTV ꢀHysteresisꢀVoltage

mV

LDOHYS

CC

Oscillator and Phase-Locked Loop

f

ProgrammableꢀFrequency

LowꢀFixedꢀFrequency

R

ꢀ=ꢀ25k,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage

ꢀ=ꢀ65k,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage

ꢀ=ꢀ105k,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage

ꢀ=ꢀ0V,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage

105

440

835

350

535

kHz

25kΩ

65kΩ

105kΩ

LOW

FREQ

375

505

V

320

485

75

380

585

850

HighꢀFixedꢀFrequency

ꢀ=ꢀINTV ,ꢀPLLIN/MODEꢀ=ꢀDCꢀVoltage

CC

HIGH

SYNC

l

SynchronizableꢀFrequency

PLLIN/MODEꢀ=ꢀExternalꢀClock

PGOOD1 Output

V

PGOOD1ꢀVoltageꢀLow

PGOOD1ꢀLeakageꢀCurrent

PGOOD1ꢀTripꢀLevel

I

ꢀ=ꢀ2mA

0.2

0.4

1

V

PGL

PGOOD

I

V

ꢀ=ꢀ5V

PGOOD

µA

PGOOD

V

ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ

FB

ꢀ

PG

ꢀV ꢀRampingꢀNegativeꢀ

ꢀHysteresis

–13

–10ꢀ

2.5

–7

%ꢀ

%

FB

V

ꢀwithꢀRespectꢀtoꢀSetꢀRegulatedꢀVoltageꢀ

FB

ꢀ

7

ꢀ

FB

ꢀV ꢀRampingꢀPositiveꢀ

10ꢀ

2.5

13

%ꢀ

%

ꢀHysteresis

t

PG

DelayꢀforꢀReportingꢀaꢀFaultꢀ(PGOODꢀLow)

25

µs

Note 1:ꢀStressesꢀbeyondꢀthoseꢀlistedꢀunderꢀAbsoluteꢀMaximumꢀRatingsꢀ

mayꢀcauseꢀpermanentꢀdamageꢀtoꢀtheꢀdevice.ꢀExposureꢀtoꢀanyꢀAbsoluteꢀ

MaximumꢀRatingsꢀforꢀextendedꢀperiodsꢀmayꢀaffectꢀdeviceꢀreliabilityꢀandꢀ

lifetime.ꢀ

Note 4:ꢀTheꢀLTC3858-1ꢀisꢀtestedꢀinꢀaꢀfeedbackꢀloopꢀthatꢀservosꢀV

ꢀtoꢀ

ITH1,2

aꢀspeciﬁedꢀvoltageꢀandꢀmeasuresꢀtheꢀresultantꢀV .ꢀTheꢀspeciﬁcationꢀatꢀ

FB1,2

85°Cꢀisꢀnotꢀtestedꢀinꢀproduction.ꢀThisꢀspeciﬁcationꢀisꢀassuredꢀbyꢀdesign,ꢀ

characterizationꢀandꢀcorrelationꢀtoꢀproductionꢀtestingꢀatꢀ125°C.

Note 2:ꢀTheꢀLTC3858E-1ꢀisꢀguaranteedꢀtoꢀmeetꢀperformanceꢀspeciﬁcationsꢀ

fromꢀ0°Cꢀtoꢀ85°C.ꢀSpeciﬁcationsꢀoverꢀtheꢀ–40°Cꢀtoꢀ125°Cꢀoperatingꢀ

junctionꢀtemperatureꢀrangeꢀareꢀassuredꢀbyꢀdesign,ꢀcharacterizationꢀandꢀ

correlationꢀwithꢀstatisticalꢀprocessꢀcontrols.ꢀTheꢀLTC3858I-1ꢀisꢀguaranteedꢀ

overꢀtheꢀfullꢀ–40°Cꢀtoꢀ125°Cꢀoperatingꢀjunctionꢀtemperatureꢀrange.

Note 5:ꢀDynamicꢀsupplyꢀcurrentꢀisꢀhigherꢀdueꢀtoꢀtheꢀgateꢀchargeꢀbeingꢀ

deliveredꢀatꢀtheꢀswitchingꢀfrequency.ꢀSeeꢀApplicationsꢀinformation.

Note 6:ꢀRiseꢀandꢀfallꢀtimesꢀareꢀmeasuredꢀusingꢀ10%ꢀandꢀ90%ꢀlevels.ꢀDelayꢀ

timesꢀareꢀmeasuredꢀusingꢀ50%ꢀlevels

Note 7:ꢀTheꢀminimumꢀon-timeꢀconditionꢀisꢀspeciﬁedꢀforꢀanꢀinductorꢀpeak-

Note 3:ꢀT ꢀisꢀcalculatedꢀfromꢀtheꢀambientꢀtemperatureꢀT ꢀandꢀpowerꢀ

J

A

to-peakꢀrippleꢀcurrentꢀ≥ꢀofꢀI ꢀ(SeeꢀMinimumꢀOn-TimeꢀConsiderationsꢀinꢀ

MAX

dissipationꢀP ꢀaccordingꢀtoꢀtheꢀfollowingꢀformula:

D

theꢀApplicationsꢀInformationꢀsection).

ꢀ

T ꢀ=ꢀT ꢀ+ꢀ(P •ꢀθ )

J A Dꢀ JA

whereꢀθ ꢀ=ꢀ43°C/WꢀforꢀtheꢀQFNꢀpackageꢀandꢀθ ꢀ=ꢀ90°C/WꢀforꢀtheꢀSSOPꢀ

JA

package.

38581fb

ꢃ

LTC3858-1

Typical perForMance characTerisTics

Efﬁciency and Power Loss vs

Output Current

Efﬁciency vs Load Current

100

90

100

90

10000

1000

100

10

FIGURE 12 CIRCUIT

V

= 12V

IN

OUT

V

= 5V

IN

= 3.3V

80

70

V

= 12V

IN

60

50

60

50

Burst Mode

OPERATION

PULSE-

SKIPPING

MODE

FORCED

40

30

20

10

0

40

30

20

10

0

1

V

= 3.3V

CONTINUOUS

MODE

OUT

FIGURE 12 CIRCUIT

0.1

0.0001 0.001

0.01

0.1 10

1

0.0001 0.001

0.01

0.1

1

10

OUTPUT CURRENT (A)

3858 G02

3858 G01

Load Step

(Forced Continuous Mode)

Load Step (Burst Mode Operation)

Efﬁciency vs Input Voltage

98

96

94

92

90

88

86

84

82

80

FIGURE 12 CIRCUIT

V

I

= 3.3V

= 4A

OUT

V

OUT

100mV/DIV

AC-

100mV/DIV

AC-

COUPLED

I

L

I

L

2A/DIV

3858 G04

3858 G05

20 25

V

= 3.3V

20µs/DIV

V

= 3.3V

20µs/DIV

0

5

10 15

30 35 40

OUT

FIGURE 12 CIRCUIT

INPUT VOLTAGE (V)

3858 G03

Inductor Current at Light Load

Load Step (Pulse-Skipping Mode)

Soft-Start

V

OUT

FORCED

CONTINUOUS

MODE

V

OUT2

100mV/DIV

AC-

2V/DIV

COUPLED

Burst Mode

OPERATION

2A/DIV

V

OUT1

2V/DIV

I

L

2A/DIV

PULSE-

SKIPPING

MODE

3858 G06

3858 G07

3858 G08

V

= 3.3V

20µs/DIV

V

LOAD

FIGURE 12 CIRCUIT

= 3.3V

2µs/DIV

20ms/DIV

FIGURE 12 CIRCUIT

OUT

FIGURE 12 CIRCUIT

I

= 200µA

38581fb

ꢄ

LTC3858-1

Typical perForMance characTerisTics

Total Input Supply Current

vs Input Voltage

EXTV_CCSwitchover and INTV_CC

Voltages vs Temperature

INTV_CCLine Regulation

400

5.6

5.2

5.1

FIGURE 12 CIRCUIT

= 3.3V

V

OUT

350

300

5.4

5.2

ONE CHANNEL ON

INTV

CC

300µA LOAD

250

200

150

100

50

5.0

4.8

4.6

4.4

4.2

EXTV RISING

CC

NO LOAD

EXTV FALLING

CC

0

5.0

4.0

10

15

25

30

35

40

5

20

0

5

10 15 20 25 30 35 40

INPUT VOLTAGE (V)

–20

5

55

80 105 130

–45

30

INPUT VOLTAGE (V)

TEMPERATURE (°C)

3858 G10

3858 G12

3858 G11

Maximum Current Sense Voltage

vs I_THVoltage

Maximum Current Sense

Threshold vs Duty Cycle

SENSE^–Pin Input Bias Current

80

60

40

20

80

60

40

20

0

PULSE-SKIPPING MODE

FORCED CONTINUOUS MODE

Burst Mode OPERATION

(FALLING)

Burst Mode OPERATION

(RISING)

–50

–100

–150

–200

–250

–300

–350

–400

–450

–500

–550

–600

0

–20

–40

5% DUTY CYCLE

0.8

PIN VOLTAGE

1.2 1.4

0

10

15

20

25

10 20

50

60 70 80 90 100

0

0.2 0.4 0.6

1.0

5

0

30 40

V

COMMON MODE VOLTAGE (V)

I

DUTY CYCLE (%)

SENSE

TH

3858 G14

3858 G13

3858 G15

Shutdown Current vs Temperature

Foldback Current Limit

Quiescent Current vs Temperature

230

210

190

170

150

130

110

10

9

90

80

70

60

50

40

30

20

10

PLLIN/MODE = 0

V

= 12V

IN

OUT

= 3.3V

ONE CHANNEL ON

8

7

6

5

4

0

55

TEMPERATURE (°C)

105 130

–45 –20

5

30

80

–45 –20

5

30

55

80 105 130

0

0.1 0.2 0.3 0.4 0.5

0.9

0.6 0.7 0.8

TEMPERATURE (°C)

FEEDBACK VOLTAGE (V)

3858 G17

3858 G18

3858 G16

38581fb

ꢅ

LTC3858-1

Typical perForMance characTerisTics

Regulated Feedback Voltage

vs Temperature

Soft-Start Pull-Up Current

vs Temperature

Shutdown (RUN) Threshold

vs Temperature

1.40

1.35

1.30

1.25

1.20

1.15

1.10

1.05

1.00

0.95

0.90

1.20

808

1.15

1.10

806

804

1.05

1.00

0.95

0.90

0.85

802

800

798

796

794

0.80

792

–45

5

30

55

80 105 130

–20

5

55

80 105 130

–45

30

–20

5

55

80 105 130

–45

30

TEMPERATURE (°C)

3858 G20

3858 G19

22554 G21

SENSE– Pin Input Current

vs Temperature

Shutdown Input Current

vs Input Voltage

Oscillator Frequency

vs Temperature

50

0

–50

14

12

800

700

600

V

= 3.3V

OUT

–100

–150

–200

–250

–300

–350

–400

–450

–500

–550

–600

FREQ = INTV

CC

10

8

500

400

300

200

100

FREQ = GND

6

4

2

V

= 28V

55

OUT

0

25

INPUT VOLTAGE (V)

35

40

5

10

15

20

30

–45 –20

5

30

80 105 130

–45

–20

5

30

55

80 105 130

TEMPERATURE (°C)

3858 G22

3858 G23

3858 G24

Oscillator Frequency

vs Input Voltage

Undervoltage Lockout Threshold

vs Temperature

4.4

4.3

4.2

4.1

4.0

3.9

3.8

3.7

3.6

3.5

3.4

356

354

352

350

FREQ = GND

348

346

344

25

35

40

–45

5

30

55

80

130

5

10

15

20

30

–20

105

TEMPERATURE (°C)

INPUT VOLTAGE (V)

3858 G28

3858 G25

38581fb

ꢆ

LTC3858-1

Typical perForMance characTerisTics

Latch-Off Threshold Voltage

vs Temperature

INTV_CCvs Load Current

5.20

5.15

5.10

2.3

V

= 12V

IN

2.2

2.1

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.3

1.2

ARMING THRESHOLD

EXTV = 0V

CC

5.05

5.00

4.95

LATCH-OFF THRESHOLD

EXTV = 8V

CC

0

20 40 60 80 100 120 140 160 180 200

–45

5

30

55

80 105 130

–20

TEMPERATURE (°C)

LOAD CURRENT (mA)

3858 G26

3858 G27

pin FuncTions ^(QFN/SSOP)

–

LTC3858-1ꢀ operatesꢀ atꢀ lightꢀ loads.ꢀ Pullingꢀ thisꢀ pinꢀ toꢀ

groundꢀselectsꢀBurstꢀModeꢀoperation.ꢀAnꢀinternalꢀ100kꢀ

resistorꢀ toꢀ groundꢀ alsoꢀ invokesꢀ Burstꢀ Modeꢀ operationꢀ

SENSE1 , SENSE2 (Pin 2, Pin 4/Pin 8, Pin 10):ꢀTheꢀ(–)ꢀ

InputꢀtoꢀtheꢀDifferentialꢀCurrentꢀComparators.ꢀWhenꢀgreaterꢀ

–

thanꢀINTV ꢀ–ꢀ0.5V,ꢀtheꢀSENSE ꢀpinꢀsuppliesꢀcurrentꢀtoꢀ

CC

whenꢀtheꢀpinꢀisꢀﬂoated.ꢀTyingꢀthisꢀpinꢀtoꢀINTV ꢀforcesꢀ

theꢀcurrentꢀcomparator.

CC

continuousꢀinductorꢀcurrentꢀoperation.ꢀTyingꢀthisꢀpinꢀtoꢀ

FREQ (Pin 3/Pin 5):ꢀTheꢀFrequencyꢀControlꢀPinꢀforꢀtheꢀ

InternalꢀVoltage-ContolledꢀOscillatorꢀ(VCO).ꢀConnectingꢀ

thisꢀpinꢀtoꢀGNDꢀforcesꢀtheꢀVCOꢀtoꢀaꢀﬁxedꢀlowꢀfrequencyꢀ

aꢀvoltageꢀgreaterꢀthanꢀ1.2VꢀandꢀlessꢀthanꢀINTV ꢀ–ꢀ1.3Vꢀ

CC

selectsꢀpulse-skippingꢀoperation.ꢀ

SGND (Pin 5, Exposed Pad Pin 29/Pin 7):ꢀSmall-signalꢀ

groundꢀ commonꢀ toꢀ bothꢀ controllers,ꢀ mustꢀ beꢀ routedꢀ

separatelyꢀfromꢀhighꢀcurrentꢀgroundsꢀtoꢀtheꢀcommonꢀ(–)ꢀ

ofꢀ350kHz.ꢀConnectingꢀthisꢀpinꢀtoꢀINTV ꢀforcesꢀtheꢀVCOꢀ

CC

toꢀaꢀﬁxedꢀhighꢀfrequencyꢀofꢀ535kHz.ꢀOtherꢀfrequenciesꢀ

betweenꢀ50kHzꢀandꢀ900kHzꢀcanꢀbeꢀprogrammedꢀusingꢀaꢀ

resistorꢀbetweenꢀFREQꢀandꢀGND.ꢀAnꢀinternalꢀ20µAꢀpull-

upꢀcurrentꢀdevelopsꢀtheꢀvoltageꢀtoꢀbeꢀusedꢀbyꢀtheꢀVCOꢀtoꢀ

controlꢀtheꢀfrequencyꢀ

terminalsꢀofꢀtheꢀC ꢀcapacitors.ꢀTheꢀexposedꢀpadꢀ(QFNꢀ

IN

only)ꢀ mustꢀ beꢀ solderedꢀ toꢀ theꢀ PCBꢀ forꢀ ratedꢀ thermalꢀ

performance.

RUN1, RUN2 (Pin 6, Pin 8/Pin 7, Pin 9):ꢀDigitalꢀRunꢀ

ControlꢀInputsꢀforꢀEachꢀController.ꢀForcingꢀeitherꢀofꢀtheseꢀ

pinsꢀbelowꢀ1.2Vꢀshutsꢀdownꢀthatꢀcontroller.ꢀForcingꢀbothꢀofꢀ

theseꢀpinsꢀbelowꢀ0.7VꢀshutsꢀdownꢀtheꢀentireꢀLTC3858-1,ꢀ

reducingꢀquiescentꢀcurrentꢀtoꢀapproximatelyꢀ8µA.ꢀDoꢀNOTꢀ

ﬂoatꢀtheseꢀpins.

PLLIN/MODE (Pin 4/Pin 6):ꢀ Externalꢀ Synchronizationꢀ

InputꢀtoꢀPhaseꢀDetectorꢀandꢀForcedꢀContinuousꢀModeꢀ

Input.ꢀWhenꢀanꢀexternalꢀclockꢀisꢀappliedꢀtoꢀthisꢀpin,ꢀtheꢀ

phase-lockedꢀloopꢀwillꢀforceꢀtheꢀrisingꢀTG1ꢀsignalꢀtoꢀbeꢀ

synchronizedꢀwithꢀtheꢀrisingꢀedgeꢀofꢀtheꢀexternalꢀclock.ꢀ

Whenꢀnotꢀsynchronizingꢀtoꢀanꢀexternalꢀclock,ꢀthisꢀinput,ꢀ

whichꢀ actsꢀ onꢀ bothꢀ controllers,ꢀ determinesꢀ howꢀ theꢀ

38581fb

ꢇ

LTC3858-1

pin FuncTions ^(QFN/SSOP)

INTV (Pin 17/Pin 19):ꢀOutputꢀofꢀtheꢀInternalꢀLinearꢀLowꢀ

TG1, TG2 (Pin 24, Pin 26/Pin 13, Pin 15):ꢀHighꢀCurrentꢀ

GateꢀDrivesꢀforꢀTopꢀN-ChannelꢀMOSFETs.ꢀTheseꢀareꢀtheꢀ

outputsꢀofꢀﬂoatingꢀdriversꢀwithꢀaꢀvoltageꢀswingꢀequalꢀtoꢀ

CC

Dropoutꢀ Regulator.ꢀ Theꢀ driverꢀ andꢀ controlꢀ circuitsꢀ areꢀ

poweredꢀfromꢀthisꢀvoltageꢀsource.ꢀMustꢀbeꢀdecoupledꢀtoꢀ

powerꢀgroundꢀwithꢀaꢀminimumꢀofꢀ4.7µFꢀceramicꢀorꢀotherꢀ

INTV ꢀ–ꢀ0.5Vꢀsuperimposedꢀonꢀtheꢀswitchꢀnodeꢀvoltageꢀ

CC

lowꢀESRꢀcapacitor.ꢀDoꢀnotꢀuseꢀtheꢀINTV ꢀpinꢀforꢀanyꢀ

SW.

CC

otherꢀpurpose.

PGOOD1 (Pin 25/Pin 27):ꢀ Open-Drainꢀ Logicꢀ Output.ꢀ

EXTV (Pin 18/Pin 20):ꢀ Externalꢀ Powerꢀ Inputꢀ toꢀ anꢀ

PGOOD1ꢀisꢀpulledꢀtoꢀgroundꢀwhenꢀtheꢀvoltageꢀonꢀtheꢀV

pinꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀitsꢀsetꢀpoint.

ꢀ

CC

FB1

InternalꢀLDOꢀConnectedꢀtoꢀINTV .ꢀThisꢀLDOꢀsuppliesꢀ

CC

INTV ꢀpower,ꢀbypassingꢀtheꢀinternalꢀLDOꢀpoweredꢀfromꢀ

CC

SS1, SS2 (Pin 26, Pin 28/Pin 12, Pin 14):ꢀExternalꢀSoft-

StartꢀInput.ꢀTheꢀLTC3858-1ꢀregulatesꢀtheꢀV ꢀvoltageꢀ

V ꢀwheneverꢀEXTV ꢀisꢀhigherꢀthanꢀ4.7V.ꢀSeeꢀEXTV ꢀ

IN

CC

FB1,2

ConnectionꢀinꢀtheꢀApplicationsꢀInformationꢀsection.ꢀDoꢀ

toꢀtheꢀsmallerꢀofꢀ0.8VꢀorꢀtheꢀvoltageꢀonꢀtheꢀSS1,2ꢀpin.ꢀAnꢀ

internalꢀ1µAꢀpull-upꢀcurrentꢀsourceꢀisꢀconnectedꢀtoꢀthisꢀ

pin.ꢀAꢀcapacitorꢀtoꢀgroundꢀatꢀthisꢀpinꢀsetsꢀtheꢀrampꢀtimeꢀ

toꢀﬁnalꢀregulatedꢀoutputꢀvoltage.ꢀThisꢀpinꢀisꢀalsoꢀusedꢀasꢀ

theꢀshort-circuitꢀlatchoffꢀtimer.

notꢀexceedꢀ14Vꢀonꢀthisꢀpin.

PGND (Pin 19/Pin 21):ꢀDriverꢀPowerꢀGround.ꢀConnectsꢀtoꢀ

theꢀsourcesꢀofꢀbottomꢀ(synchronous)ꢀN-channelꢀMOSFETsꢀ

andꢀtheꢀ(–)ꢀterminal(s)ꢀofꢀC .

IN

V

(Pin 20/Pin 22):ꢀMainꢀInputꢀSupplyꢀPin.ꢀAꢀbypassꢀ

I

, I

(Pin 27, Pin 1/Pin 11, Pin 13):ꢀErrorꢀAmpliﬁerꢀ

IN

TH1 TH2

capacitorꢀshouldꢀbeꢀtiedꢀbetweenꢀthisꢀpinꢀandꢀtheꢀsignalꢀ

OutputsꢀandꢀSwitchingꢀRegulatorꢀCompensationꢀPoints.ꢀ

Eachꢀassociatedꢀchannel’sꢀcurrentꢀcomparatorꢀtripꢀpointꢀ

increasesꢀwithꢀthisꢀcontrolꢀvoltage.

groundꢀpin.

BG1, BG2 (Pin 21, Pin 23/Pin 16, Pin 18):ꢀHighꢀCur-

rentꢀGateꢀDrivesꢀforꢀBottomꢀ(Synchronous)ꢀN-Channelꢀ

MOSFETs.ꢀVoltageꢀswingꢀatꢀtheseꢀpinsꢀisꢀfromꢀgroundꢀ

V

, V

FB1 FB2

(Pin 28, Pin 2/Pin 10, Pin 12):ꢀReceivesꢀtheꢀ

remotelyꢀsensedꢀfeedbackꢀvoltageꢀforꢀeachꢀcontrollerꢀfromꢀ

anꢀexternalꢀresistiveꢀdividerꢀacrossꢀtheꢀoutput.

toꢀINTV .

CC

+

BOOST1, BOOST2 (Pin 22, Pin 24/Pin 15, Pin 17):ꢀBoot-

strappedꢀSuppliesꢀtoꢀtheꢀTopsideꢀFloatingꢀDrivers.ꢀCapaci-

torsꢀareꢀconnectedꢀbetweenꢀtheꢀBOOSTꢀandꢀSWꢀpinsꢀandꢀ

SENSE1 , SENSE2 (Pin 1, Pin 3/Pin 9, Pin 11):ꢀTheꢀ

(+)ꢀinputꢀtoꢀtheꢀdifferentialꢀcurrentꢀcomparatorsꢀthatꢀareꢀ

normallyꢀconnectedꢀtoꢀinductorꢀDCRꢀsensingꢀnetworksꢀorꢀ

SchottkyꢀdiodesꢀareꢀtiedꢀbetweenꢀtheꢀBOOSTꢀandꢀINTV ꢀ

currentꢀsensingꢀresistors.ꢀTheꢀI ꢀpinꢀvoltageꢀandꢀcontrolledꢀ

CC

TH

–

+

pins.ꢀVoltageꢀswingꢀatꢀtheꢀBOOSTꢀpinsꢀisꢀfromꢀINTV ꢀtoꢀ

offsetsꢀbetweenꢀtheꢀSENSE ꢀandꢀSENSE ꢀpinsꢀinꢀconjunc-

CC

(V ꢀ+ꢀINTV ).

tionꢀwithꢀR

ꢀsetꢀtheꢀcurrentꢀtripꢀthreshold.

IN

CC

SENSE

SW1, SW2 (Pin 23, Pin 25/Pin 14, Pin 16):ꢀSwitchꢀNodeꢀ

ConnectionsꢀtoꢀInductors.ꢀ

38581fb

ꢈ

LTC3858-1

FuncTional DiagraM

INTV

V

IN

CC

DUPLICATE FOR SECOND

CONTROLLER CHANNEL

BOOST

D

B

C

B

TG

DROP

OUT

DET

TOP

BOT

+

C

PGOOD1

0.88V

IN

D

BOT

–

SW

TOP ON

V

S

R

Q

FB1

+

INTV

CC

Q

–

SWITCH

LOGIC

0.72V

BG

SHDN

C

OUT

PGND

20µA

FREQ

V

OUT

VCO

CLK2

CLK1

+

–

R

SENSE

0.425V

SLEEP

L

ICMP

IR

+

–

+

–

PFD

+

–

+

3mV

SENSE

SYNC

DET

2(V

)

FB

0.45V

PLLIN/MODE

–

100k

SLOPE COMP

V

FB

R

B

+

V

IN

0.80V

TRACK/SS

EA

–

R

A

EXTV

CC

+

–

OV

C

0.88V

I

TH

5.1V

LDO

EN

5.1V

LDO

EN

0.5µA

10V

SHDN

RST

FB

C

C2

R

C

FOLDBACK

+

–

2(V

)

1µA

4.7V

SS

SGND

INTV

RUN

CC

SHORT CKT

LATCH-OFF

C

SHDN

10µA

SS

38581 FD

operaTion

Main Control Loop

theꢀV ꢀpinꢀ(whichꢀisꢀgeneratedꢀwithꢀanꢀexternalꢀresistorꢀ

FB

dividerꢀ connectedꢀ acrossꢀ theꢀ outputꢀ voltage,ꢀ V ,ꢀ toꢀ

OUTꢀ

TheꢀLTC3858-1ꢀusesꢀaꢀconstantꢀfrequency,ꢀcurrentꢀmodeꢀ

step-downꢀarchitectureꢀwithꢀtheꢀtwoꢀcontrollerꢀchannelsꢀ

operatingꢀ180ꢀdegreesꢀoutꢀofꢀphase.ꢀDuringꢀnormalꢀop-

eration,ꢀeachꢀexternalꢀtopꢀMOSFETꢀisꢀturnedꢀonꢀwhenꢀtheꢀ

clockꢀforꢀthatꢀchannelꢀsetsꢀtheꢀRSꢀlatch,ꢀandꢀisꢀturnedꢀoffꢀ

whenꢀtheꢀmainꢀcurrentꢀcomparator,ꢀICMP,ꢀresetsꢀtheꢀRSꢀ

latch.ꢀTheꢀpeakꢀinductorꢀcurrentꢀatꢀwhichꢀICMPꢀtripsꢀandꢀ

ground)ꢀtoꢀtheꢀinternalꢀ0.800Vꢀreferenceꢀvoltage.ꢀWhenꢀtheꢀ

loadꢀcurrentꢀincreases,ꢀitꢀcausesꢀaꢀslightꢀdecreaseꢀinꢀV ꢀ

FB

relativeꢀtoꢀtheꢀreference,ꢀwhichꢀcausesꢀtheꢀEAꢀtoꢀincreaseꢀ

theꢀI ꢀvoltageꢀuntilꢀtheꢀaverageꢀinductorꢀcurrentꢀmatchesꢀ

TH

theꢀnewꢀloadꢀcurrent.

AfterꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀeachꢀcycle,ꢀtheꢀbottomꢀ

MOSFETꢀisꢀturnedꢀonꢀuntilꢀeitherꢀtheꢀinductorꢀcurrentꢀstartsꢀ

toꢀreverse,ꢀasꢀindicatedꢀbyꢀtheꢀcurrentꢀcomparatorꢀIR,ꢀorꢀ

theꢀbeginningꢀofꢀtheꢀnextꢀclockꢀcycle.

resetsꢀtheꢀlatchꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀpin,ꢀ

TH

whichꢀisꢀtheꢀoutputꢀofꢀtheꢀerrorꢀampliﬁer,ꢀEA.ꢀTheꢀerrorꢀ

ampliﬁerꢀcomparesꢀtheꢀoutputꢀvoltageꢀfeedbackꢀsignalꢀatꢀ

38581fb

ꢀ0

LTC3858-1

operaTion ^{(Refer to the Functional Diagram)}

INTV /EXTV Power

internalꢀreference,ꢀtheꢀLTC3858-1ꢀregulatesꢀtheꢀV ꢀvolt-

CC

FB

ageꢀtoꢀtheꢀSSꢀpinꢀvoltageꢀinsteadꢀofꢀtheꢀ0.8Vꢀreference.ꢀ

ThisꢀallowsꢀtheꢀSSꢀpinꢀtoꢀbeꢀusedꢀtoꢀprogramꢀaꢀsoft-startꢀ

byꢀconnectingꢀanꢀexternalꢀcapacitorꢀfromꢀtheꢀSSꢀpinꢀtoꢀ

SGND.ꢀAnꢀinternalꢀ1µAꢀpull-upꢀcurrentꢀchargesꢀthisꢀca-

pacitorꢀcreatingꢀaꢀvoltageꢀrampꢀonꢀtheꢀSSꢀpin.ꢀAsꢀtheꢀSSꢀ

voltageꢀrisesꢀlinearlyꢀfromꢀ0Vꢀtoꢀ0.8Vꢀ(andꢀbeyondꢀupꢀtoꢀ

theꢀabsoluteꢀmaximumꢀratingꢀofꢀ6V),ꢀtheꢀoutputꢀvoltageꢀ

PowerꢀforꢀtheꢀtopꢀandꢀbottomꢀMOSFETꢀdriversꢀandꢀmostꢀ

otherꢀinternalꢀcircuitryꢀisꢀderivedꢀfromꢀtheꢀINTV ꢀpin.ꢀWhenꢀ

CC

theꢀEXTV ꢀpinꢀisꢀleftꢀopenꢀorꢀtiedꢀtoꢀaꢀvoltageꢀlessꢀthanꢀ

CC

4.7V,ꢀtheꢀV ꢀLDOꢀ(lowꢀdropoutꢀlinearꢀregulator)ꢀsuppliesꢀ

IN

5.1VꢀfromꢀV ꢀtoꢀINTV .ꢀIfꢀEXTV ꢀisꢀtakenꢀaboveꢀ4.7V,ꢀ

IN

CC

theꢀV ꢀLDOꢀisꢀturnedꢀoffꢀandꢀtheꢀEXTV ꢀLDOꢀisꢀturnedꢀon.ꢀ

IN

CC

Onceꢀenabled,ꢀtheꢀEXTV ꢀLDOꢀsuppliesꢀ5.1VꢀfromꢀEXTV ꢀ

CC

V

ꢀrisesꢀsmoothlyꢀfromꢀzeroꢀtoꢀitsꢀﬁnalꢀvalue.

OUT

toꢀINTV .ꢀUsingꢀtheꢀEXTV ꢀpinꢀallowsꢀtheꢀINTV ꢀpowerꢀ

CC

toꢀbeꢀderivedꢀfromꢀaꢀhighꢀefﬁciencyꢀexternalꢀsourceꢀsuchꢀ

Short-Circuit Latch-Off

asꢀoneꢀofꢀtheꢀLTC3858-1ꢀswitchingꢀregulatorꢀoutputs.

Afterꢀ theꢀ controllerꢀ hasꢀ beenꢀ startedꢀ andꢀ beenꢀ givenꢀ

adequateꢀ timeꢀ toꢀ rampꢀ upꢀ theꢀ outputꢀ voltage,ꢀ theꢀ SSꢀ

capacitorꢀisꢀusedꢀinꢀaꢀshort-circuitꢀtime-outꢀcircuit.ꢀSpe-

ciﬁcally,ꢀonceꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀrisesꢀaboveꢀ2Vꢀ

(theꢀarmingꢀthreshold),ꢀtheꢀshort-circuitꢀtimeoutꢀcircuitꢀisꢀ

enabledꢀ(seeꢀFigureꢀ1).ꢀIfꢀtheꢀoutputꢀvoltageꢀfallsꢀbelowꢀ

70%ꢀofꢀitsꢀnominalꢀregulatedꢀvoltage,ꢀtheꢀSSꢀcapacitorꢀ

beginsꢀdischargingꢀwithꢀaꢀnetꢀ9µAꢀpull-downꢀcurrentꢀonꢀ

theꢀassumptionꢀthatꢀtheꢀoutputꢀisꢀinꢀanꢀovercurrentꢀand/orꢀ

short-circuitꢀcondition.ꢀIfꢀtheꢀconditionꢀlastsꢀlongꢀenoughꢀ

toꢀallowꢀtheꢀSSꢀpinꢀvoltageꢀtoꢀfallꢀbelowꢀ1.5Vꢀ(theꢀlatchoffꢀ

threshold)ꢀ,ꢀtheꢀcontrollerꢀwillꢀshutꢀdownꢀ(latchꢀoff)ꢀuntilꢀ

EachꢀtopꢀMOSFETꢀdriverꢀisꢀbiasedꢀfromꢀtheꢀﬂoatingꢀboot-

strapꢀcapacitor,ꢀC ,ꢀwhichꢀnormallyꢀrechargesꢀduringꢀeachꢀ

B

switchingꢀcycleꢀthroughꢀanꢀexternalꢀdiodeꢀwhenꢀtheꢀtopꢀ

MOSFETꢀturnsꢀoff.ꢀIfꢀtheꢀinputꢀvoltageꢀV ꢀdecreasesꢀtoꢀ

IN

aꢀvoltageꢀcloseꢀtoꢀV ,ꢀtheꢀloopꢀmayꢀenterꢀdropoutꢀandꢀ

OUTꢀ

attemptꢀtoꢀturnꢀonꢀtheꢀtopꢀMOSFETꢀcontinuously.ꢀTheꢀ

dropoutꢀdetectorꢀdetectsꢀthisꢀandꢀforcesꢀtheꢀtopꢀMOSFETꢀ

offꢀforꢀaboutꢀone-twelfthꢀofꢀtheꢀclockꢀperiodꢀeveryꢀtenthꢀ

cycleꢀtoꢀallowꢀC ꢀtoꢀrecharge.

B

Shutdown and Start-Up (RUN1, RUN2 and

SS1, SS2 Pins)

theꢀRUNꢀpinꢀvoltageꢀorꢀtheꢀV ꢀvoltageꢀisꢀrecycled.

IN

TheꢀtwoꢀchannelsꢀofꢀtheꢀLTC3858-1ꢀcanꢀbeꢀindependentlyꢀ

shutꢀdownꢀusingꢀtheꢀRUN1ꢀandꢀRUN2ꢀpins.ꢀPullingꢀeitherꢀofꢀ

theseꢀpinsꢀbelowꢀ1.26Vꢀshutsꢀdownꢀtheꢀmainꢀcontrolꢀloopꢀ

forꢀthatꢀcontroller.ꢀPullingꢀbothꢀpinsꢀbelowꢀ0.7Vꢀdisablesꢀ

bothꢀcontrollersꢀandꢀmostꢀinternalꢀcircuits,ꢀincludingꢀtheꢀ

Theꢀdelayꢀtimeꢀfromꢀwhenꢀanꢀshort-circuitꢀoccursꢀuntilꢀ

theꢀcontrollerꢀlatchesꢀoffꢀcanꢀbeꢀcalculatedꢀusingꢀtheꢀfol-

lowingꢀequation:

V_SS– 1.5V

t_LATCH≈ C_SS

INTV ꢀLDOs.ꢀInꢀthisꢀstate,ꢀtheꢀLTC3858-1ꢀdrawsꢀonlyꢀ8µAꢀ

CC

9µA

ꢀ

ofꢀquiescentꢀcurrent.

whereꢀV ꢀisꢀtheꢀinitialꢀvoltageꢀ(mustꢀbeꢀgreaterꢀthanꢀ2V)ꢀ

SS

TheꢀRUNꢀpinꢀmayꢀbeꢀexternallyꢀpulledꢀupꢀorꢀdrivenꢀdirectlyꢀ

byꢀlogic.ꢀWhenꢀdrivingꢀtheꢀRUNꢀpinꢀwithꢀaꢀlowꢀimpedanceꢀ

source,ꢀdoꢀnotꢀexceedꢀtheꢀabsoluteꢀmaximumꢀratingꢀofꢀ

8Vꢀonꢀthisꢀpin.ꢀTheꢀRUNꢀpinꢀhasꢀanꢀinternalꢀ11Vꢀvoltageꢀ

clampꢀthatꢀallowsꢀtheꢀRUNꢀpinꢀtoꢀbeꢀconnectedꢀthroughꢀaꢀ

onꢀtheꢀSSꢀpinꢀatꢀtheꢀtimeꢀtheꢀshort-circuitꢀoccurs.ꢀNormallyꢀ

theꢀSSꢀpinꢀvoltageꢀwillꢀhaveꢀbeenꢀpulledꢀupꢀtoꢀtheꢀINTV ꢀ

CC

voltageꢀ(5.1V)ꢀbyꢀtheꢀinternalꢀ1µAꢀpull-upꢀcurrent.

NoteꢀthatꢀtheꢀtwoꢀcontrollersꢀonꢀtheꢀLTC3858-1ꢀhaveꢀsepa-

rate,ꢀindependentꢀshort-circuitꢀlatchoffꢀcircuits.ꢀLatchoffꢀ

canꢀbeꢀoverridden/defeatedꢀbyꢀconnectingꢀaꢀresistorꢀ150kꢀ

resistorꢀtoꢀaꢀhigherꢀvoltageꢀ(forꢀexample,ꢀV ),ꢀsoꢀlongꢀasꢀ

IN

theꢀmaximumꢀcurrentꢀintoꢀtheꢀRUNꢀpinꢀdoesꢀnotꢀexceedꢀ

100µA.

orꢀlessꢀfromꢀtheꢀSSꢀpinꢀtoꢀINTV .ꢀThisꢀresistorꢀprovidesꢀ

CC

enoughꢀpull-upꢀcurrentꢀtoꢀovercomeꢀtheꢀ9µAꢀpull-downꢀ

currentꢀpresentꢀduringꢀaꢀshort-circuit.ꢀNoteꢀthatꢀthisꢀresis-

torꢀalsoꢀshortensꢀtheꢀsoft-startꢀperiod.

Theꢀstart-upꢀofꢀeachꢀcontroller’sꢀoutputꢀvoltageꢀV ꢀisꢀ

OUT

controlledꢀbyꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀforꢀthatꢀchannel.ꢀ

WhenꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀisꢀlessꢀthanꢀtheꢀ0.8Vꢀ

38581fb

ꢀꢀ

LTC3858-1

operaTion ^{(Refer to the Functional Diagram)}

INTV

CC

WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀtheꢀ

minimumꢀpeakꢀcurrentꢀinꢀtheꢀinductorꢀisꢀsetꢀtoꢀapproxi-

matelyꢀ30%ꢀofꢀtheꢀmaximumꢀsenseꢀvoltageꢀevenꢀthoughꢀ

SS VOLTAGE

2V

1.5V

theꢀvoltageꢀonꢀtheꢀI ꢀpinꢀindicatesꢀaꢀlowerꢀvalue.ꢀIfꢀtheꢀ

0.8V

TH

averageꢀinductorꢀcurrentꢀisꢀhigherꢀthanꢀtheꢀloadꢀcurrent,ꢀ

LATCH-OFF

COMMAND

theꢀerrorꢀampliﬁer,ꢀEA,ꢀwillꢀdecreaseꢀtheꢀvoltageꢀonꢀtheꢀI ꢀ

TH

pin.ꢀWhenꢀtheꢀI ꢀvoltageꢀdropsꢀbelowꢀ0.425V,ꢀtheꢀinternalꢀ

TH

0V

SS PIN

CURRENT

sleepꢀsignalꢀgoesꢀhighꢀ(enablingꢀ“sleep”ꢀmode)ꢀandꢀbothꢀ

1µA

–9µA

externalꢀMOSFETsꢀareꢀturnedꢀoff.ꢀ

OUTPUT

VOLTAGE

Inꢀsleepꢀmode,ꢀmuchꢀofꢀtheꢀinternalꢀcircuitryꢀisꢀturnedꢀoff,ꢀ

reducingꢀtheꢀquiescentꢀcurrent.ꢀIfꢀoneꢀchannelꢀisꢀshutꢀdownꢀ

andꢀtheꢀotherꢀchannelꢀisꢀinꢀsleepꢀmode,ꢀtheꢀLTC3858-1ꢀ

drawsꢀonlyꢀ170µAꢀofꢀquiescentꢀcurrent.ꢀIfꢀbothꢀchannelsꢀ

areꢀinꢀsleepꢀmode,ꢀtheꢀLTC3858-1ꢀdrawsꢀonlyꢀ300µAꢀofꢀqui-

escentꢀcurrent.ꢀInꢀsleepꢀmode,ꢀtheꢀloadꢀcurrentꢀisꢀsuppliedꢀ

byꢀtheꢀoutputꢀcapacitor.ꢀAsꢀtheꢀoutputꢀvoltageꢀdecreases,ꢀ

theꢀEA’sꢀoutputꢀbeginsꢀtoꢀrise.ꢀWhenꢀtheꢀoutputꢀvoltageꢀ

38581 F01

LATCH-OFF

ENABLE

ARMING

SOFT-START INTERVAL

t

LATCH

Figure 1. Latch-Off Timing Diagram

Foldback Current

dropsꢀenough,ꢀtheꢀI ꢀpinꢀisꢀreconnectedꢀtoꢀtheꢀoutputꢀ

TH

ofꢀtheꢀEA,ꢀtheꢀsleepꢀsignalꢀgoesꢀlow,ꢀandꢀtheꢀcontrollerꢀ

resumesꢀnormalꢀoperationꢀbyꢀturningꢀonꢀtheꢀtopꢀexternalꢀ

MOSFETꢀonꢀtheꢀnextꢀcycleꢀofꢀtheꢀinternalꢀoscillator.

Onꢀtheꢀotherꢀhand,ꢀwhenꢀtheꢀoutputꢀvoltageꢀfallsꢀtoꢀlessꢀ

thanꢀ70%ꢀofꢀitsꢀnominalꢀlevel,ꢀfoldbackꢀcurrentꢀlimitingꢀ

isꢀalsoꢀactivated,ꢀprogressivelyꢀloweringꢀtheꢀpeakꢀcurrentꢀ

limitꢀinꢀproportionꢀtoꢀtheꢀseverityꢀofꢀtheꢀovercurrentꢀorꢀ

short-circuitꢀcondition.ꢀEvenꢀifꢀaꢀshort-circuitꢀisꢀpresentꢀ

andꢀtheꢀshort-circuitꢀlatch-offꢀisꢀnotꢀyetꢀenabledꢀ(whenꢀ

SSꢀvoltageꢀhasꢀnotꢀyetꢀreachedꢀ2V),ꢀaꢀsafe,ꢀlowꢀoutputꢀ

currentꢀisꢀprovidedꢀdueꢀtoꢀinternalꢀcurrentꢀfoldbackꢀandꢀ

actualꢀpowerꢀwastedꢀisꢀlowꢀdueꢀtoꢀtheꢀefﬁcientꢀnatureꢀofꢀ

theꢀcurrentꢀmodeꢀswitchingꢀregulator.ꢀFoldbackꢀcurrentꢀ

limitingꢀisꢀdisabledꢀduringꢀtheꢀsoft-startꢀintervalꢀ(asꢀlongꢀ

WhenꢀaꢀcontrollerꢀisꢀenabledꢀforꢀBurstꢀModeꢀoperation,ꢀ

theꢀinductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverse.ꢀTheꢀreverseꢀ

currentꢀ comparator,ꢀ IR,ꢀ turnsꢀ offꢀ theꢀ bottomꢀ externalꢀ

MOSFETꢀjustꢀbeforeꢀtheꢀinductorꢀcurrentꢀreachesꢀzero,ꢀ

preventingꢀitꢀfromꢀreversingꢀandꢀgoingꢀnegative.ꢀThus,ꢀ

theꢀcontrollerꢀisꢀinꢀdiscontinuousꢀoperation.

Inꢀforcedꢀcontinuousꢀoperationꢀorꢀwhenꢀclockedꢀbyꢀanꢀ

externalꢀclockꢀsourceꢀtoꢀuseꢀtheꢀphase-lockedꢀloopꢀ(seeꢀ

Frequencyꢀ Selectionꢀ andꢀ Phase-Lockedꢀ Loopꢀ section),ꢀ

theꢀinductorꢀcurrentꢀisꢀallowedꢀtoꢀreverseꢀatꢀlightꢀloadsꢀ

orꢀunderꢀlargeꢀtransientꢀconditions.ꢀTheꢀpeakꢀinductorꢀ

asꢀtheꢀV ꢀvoltageꢀisꢀkeepingꢀupꢀwithꢀtheꢀSSꢀvoltage).ꢀ

FB

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping or Forced Continuous Conduction)

(PLLIN/MODE Pin)

currentꢀisꢀdeterminedꢀbyꢀtheꢀvoltageꢀonꢀtheꢀI ꢀpin,ꢀjustꢀ

TH

asꢀinꢀnormalꢀoperation.ꢀInꢀthisꢀmode,ꢀtheꢀefﬁciencyꢀatꢀlightꢀ

loadsꢀisꢀlowerꢀthanꢀinꢀBurstꢀModeꢀoperation.ꢀHowever,ꢀ

continuousꢀoperationꢀhasꢀtheꢀadvantagesꢀofꢀlowerꢀoutputꢀ

voltageꢀrippleꢀandꢀlessꢀinterferenceꢀtoꢀaudioꢀcircuitry.ꢀInꢀ

forcedꢀcontinuousꢀmode,ꢀtheꢀoutputꢀrippleꢀisꢀindependentꢀ

ofꢀloadꢀcurrent.

TheꢀLTC3858-1ꢀcanꢀbeꢀenabledꢀtoꢀenterꢀhighꢀefﬁciencyꢀ

BurstꢀModeꢀoperation,ꢀconstantꢀfrequencyꢀpulse-skippingꢀ

mode,ꢀorꢀforcedꢀcontinuousꢀconductionꢀmodeꢀatꢀlowꢀloadꢀ

currents.ꢀToꢀselectꢀBurstꢀModeꢀoperation,ꢀtieꢀtheꢀPLLIN/ꢀ

MODEꢀpinꢀtoꢀground.ꢀToꢀselectꢀforcedꢀcontinuousꢀopera-

tion,ꢀtieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀINTV .ꢀToꢀselectꢀpulse-

CC

WhenꢀtheꢀPLLIN/MODEꢀpinꢀisꢀconnectedꢀforꢀpulse-skippingꢀ

mode,ꢀtheꢀLTC3858-1ꢀoperatesꢀinꢀPWMꢀpulse-skippingꢀ

modeꢀatꢀlightꢀloads.ꢀInꢀthisꢀmode,ꢀconstantꢀfrequencyꢀ

skippingꢀmode,ꢀtieꢀtheꢀPLLIN/MODEꢀpinꢀtoꢀaꢀDCꢀvoltageꢀ

greaterꢀthanꢀ1.2VꢀandꢀlessꢀthanꢀINTV ꢀ–ꢀ1.3V.

CC

38581fb

ꢀꢁ

LTC3858-1

operaTion ^{(Refer to the Functional Diagram)}

operationꢀisꢀmaintainedꢀdownꢀtoꢀapproximatelyꢀ1%ꢀofꢀ isꢀapplied.ꢀIfꢀprebiasedꢀnearꢀtheꢀexternalꢀclockꢀfrequency,ꢀ

designedꢀmaximumꢀoutputꢀcurrent.ꢀAtꢀveryꢀlightꢀloads,ꢀtheꢀ theꢀPLLꢀloopꢀonlyꢀneedsꢀtoꢀmakeꢀslightꢀchangesꢀtoꢀtheꢀ

currentꢀcomparator,ꢀICMP,ꢀmayꢀremainꢀtrippedꢀforꢀseveralꢀ VCOꢀinputꢀinꢀorderꢀtoꢀsynchronizeꢀtheꢀrisingꢀedgeꢀofꢀtheꢀ

cyclesꢀandꢀforceꢀtheꢀexternalꢀtopꢀMOSFETꢀtoꢀstayꢀoffꢀforꢀ externalꢀclock’sꢀtoꢀtheꢀrisingꢀedgeꢀofꢀTG1.ꢀTheꢀabilityꢀtoꢀ

theꢀsameꢀnumberꢀofꢀcyclesꢀ(i.e.,ꢀskippingꢀpulses).ꢀTheꢀ pre-biasꢀtheꢀloopꢀﬁlterꢀallowsꢀtheꢀPLLꢀtoꢀlock-inꢀrapidlyꢀ

inductorꢀcurrentꢀisꢀnotꢀallowedꢀtoꢀreverseꢀ(discontinuousꢀ withoutꢀdeviatingꢀfarꢀfromꢀtheꢀdesiredꢀfrequency.

operation).ꢀThisꢀmode,ꢀlikeꢀforcedꢀcontinuousꢀoperation,ꢀ

Theꢀ typicalꢀ captureꢀ rangeꢀ ofꢀ theꢀ phase-lockedꢀ loopꢀ isꢀ

exhibitsꢀlowꢀoutputꢀrippleꢀasꢀwellꢀasꢀlowꢀaudioꢀnoiseꢀandꢀ

fromꢀ approximatelyꢀ 55kHzꢀ toꢀ 1MHz,ꢀ withꢀ aꢀ guaranteeꢀ

reducedꢀRFꢀinterferenceꢀwhenꢀcomparedꢀtoꢀBurstꢀModeꢀ

overꢀallꢀmanufacturingꢀvariationsꢀtoꢀbeꢀbetweenꢀ75kHzꢀ

operation.ꢀ Itꢀ providesꢀ higherꢀ lightꢀ loadꢀ efﬁciencyꢀ thanꢀ

andꢀ850kHz.ꢀ

forcedꢀcontinuousꢀmode,ꢀbutꢀnotꢀnearlyꢀasꢀhighꢀasꢀBurstꢀ

TheꢀtypicalꢀinputꢀclockꢀthresholdsꢀonꢀtheꢀPLLIN/MODEꢀ

pinꢀareꢀ1.6Vꢀ(rising)ꢀandꢀ1.1Vꢀ(falling).

Modeꢀoperation.

Frequency Selection and Phase-Locked Loop

(FREQ and PLLIN/MODE Pins)

Output Overvoltage Protection

Anꢀovervoltageꢀcomparatorꢀguardsꢀagainstꢀtransientꢀover-

shootsꢀasꢀwellꢀasꢀotherꢀmoreꢀseriousꢀconditionsꢀthatꢀmayꢀ

Theꢀselectionꢀofꢀswitchingꢀfrequencyꢀisꢀaꢀtradeoffꢀbetweenꢀ

efﬁciencyꢀ andꢀ componentꢀ size.ꢀ Lowꢀ frequencyꢀ opera-

tionꢀincreasesꢀefﬁciencyꢀbyꢀreducingꢀMOSFETꢀswitchingꢀ

losses,ꢀbutꢀrequiresꢀlargerꢀinductanceꢀand/orꢀcapacitanceꢀ

toꢀmaintainꢀlowꢀoutputꢀrippleꢀvoltage.

overvoltageꢀtheꢀoutput.ꢀWhenꢀtheꢀV ꢀpinꢀrisesꢀbyꢀmoreꢀ

FB

thanꢀ10%ꢀaboveꢀitsꢀregulationꢀpointꢀofꢀ0.800V,ꢀtheꢀtopꢀ

MOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀMOSFETꢀisꢀturnedꢀ

onꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀcleared.

TheꢀswitchingꢀfrequencyꢀofꢀtheꢀLTC3858-1’sꢀcontrollersꢀ

canꢀbeꢀselectedꢀusingꢀtheꢀFREQꢀpin.

Power Good (PGOOD) Pin

IfꢀtheꢀPLLIN/MODEꢀpinꢀisꢀnotꢀbeingꢀdrivenꢀbyꢀanꢀexternalꢀ

clockꢀsource,ꢀtheꢀFREQꢀpinꢀcanꢀbeꢀtiedꢀtoꢀSGND,ꢀtiedꢀtoꢀ

TheꢀPGOOD1ꢀpinꢀisꢀconnectedꢀtoꢀanꢀopenꢀdrainꢀofꢀanꢀ

internalꢀN-channelꢀMOSFET.ꢀTheꢀMOSFETꢀturnsꢀonꢀandꢀ

INTV ꢀorꢀprogrammedꢀthroughꢀanꢀexternalꢀresistor.ꢀTyingꢀ

CC

pullsꢀtheꢀPGOOD1ꢀpinꢀlowꢀwhenꢀtheꢀcorrespondingꢀV ꢀpinꢀ

FB1

FREQꢀtoꢀSGNDꢀselectsꢀ350kHzꢀwhileꢀtyingꢀFREQꢀtoꢀINTV ꢀ

CC

voltageꢀisꢀnotꢀwithinꢀ 10%ꢀofꢀtheꢀ0.8Vꢀreferenceꢀvoltage.ꢀ

selectsꢀ 535kHz.ꢀ Placingꢀ aꢀ resistorꢀ betweenꢀ FREQꢀ andꢀ

SGNDꢀallowsꢀtheꢀfrequencyꢀtoꢀbeꢀprogrammedꢀbetweenꢀ

50kHzꢀandꢀ900kHz.

TheꢀPGOOD1ꢀpinꢀisꢀalsoꢀpulledꢀlowꢀwhenꢀtheꢀRUN1ꢀpinꢀ

isꢀlowꢀ(shutꢀdown).ꢀWhenꢀtheꢀV ꢀpinꢀvoltageꢀisꢀwithinꢀ

FB1

theꢀ 10%ꢀrequirement,ꢀtheꢀMOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀ

pinꢀisꢀallowedꢀtoꢀbeꢀpulledꢀupꢀbyꢀanꢀexternalꢀresistorꢀtoꢀaꢀ

sourceꢀnoꢀgreaterꢀthanꢀ6V.

Aꢀphase-lockedꢀloopꢀ(PLL)ꢀisꢀavailableꢀonꢀtheꢀLTC3858-1ꢀ

toꢀsynchronizeꢀtheꢀinternalꢀoscillatorꢀtoꢀanꢀexternalꢀclockꢀ

sourceꢀthatꢀisꢀconnectedꢀtoꢀtheꢀPLLIN/MODEꢀpin.ꢀTheꢀ

phaseꢀdetectorꢀadjustsꢀtheꢀvoltageꢀ(throughꢀanꢀinternalꢀ

lowpassꢀﬁlter)ꢀofꢀtheꢀVCOꢀinputꢀtoꢀalignꢀtheꢀturn-onꢀofꢀ

controllerꢀ1’sꢀexternalꢀtopꢀMOSFETꢀtoꢀtheꢀrisingꢀedgeꢀofꢀ

theꢀsynchronizingꢀsignal.ꢀThus,ꢀtheꢀturn-onꢀofꢀcontrollerꢀ

2’sꢀexternalꢀtopꢀMOSFETꢀisꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀtoꢀ

theꢀrisingꢀedgeꢀofꢀtheꢀexternalꢀclockꢀsource.

Theory and Beneﬁts of 2-Phase Operation

Whyꢀ theꢀ needꢀ forꢀ 2-phaseꢀ operation?ꢀ Upꢀ untilꢀ theꢀ

2-phaseꢀfamily,ꢀconstantꢀfrequencyꢀdualꢀswitchingꢀregula-

torsꢀoperatedꢀbothꢀchannelsꢀinꢀphaseꢀ(i.e.,ꢀsingleꢀphaseꢀ

operation).ꢀThisꢀmeansꢀthatꢀbothꢀswitchesꢀturnedꢀonꢀatꢀ

theꢀsameꢀtime,ꢀcausingꢀcurrentꢀpulsesꢀofꢀupꢀtoꢀtwiceꢀtheꢀ

amplitudeꢀofꢀthoseꢀforꢀoneꢀregulatorꢀtoꢀbeꢀdrawnꢀfromꢀtheꢀ

inputꢀcapacitorꢀandꢀbattery.ꢀTheseꢀlargeꢀamplitudeꢀcurrentꢀ

Theꢀ VCOꢀ inputꢀ voltageꢀ isꢀ pre-biasedꢀ toꢀ theꢀ operatingꢀ

frequencyꢀsetꢀbyꢀtheꢀFREQꢀpinꢀbeforeꢀtheꢀexternalꢀclockꢀ

38581fb

ꢀꢂ

LTC3858-1

operaTion ^{(Refer to the Functional Diagram)}

pulsesꢀincreasedꢀtheꢀtotalꢀRMSꢀcurrentꢀﬂowingꢀfromꢀtheꢀ Ofꢀcourse,ꢀtheꢀimprovementꢀaffordedꢀbyꢀ2-phaseꢀopera-

inputꢀcapacitor,ꢀrequiringꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀinputꢀ tionꢀisꢀaꢀfunctionꢀofꢀtheꢀdualꢀswitchingꢀregulator’sꢀrelativeꢀ

capacitorsꢀandꢀincreasingꢀbothꢀEMIꢀandꢀlossesꢀinꢀtheꢀinputꢀ dutyꢀcyclesꢀwhich,ꢀinꢀturn,ꢀareꢀdependentꢀuponꢀtheꢀinputꢀ

capacitorꢀandꢀbattery.

voltageꢀV ꢀ(DutyꢀCycleꢀ=ꢀV /V ).ꢀFigureꢀ3ꢀshowsꢀhowꢀ

IN OUT IN

theꢀRMSꢀinputꢀcurrentꢀvariesꢀforꢀsingle-phaseꢀandꢀ2-phaseꢀ

operationꢀforꢀ3.3Vꢀandꢀ5Vꢀregulatorsꢀoverꢀaꢀwideꢀinputꢀ

voltageꢀrange.

Withꢀ 2-phaseꢀ operation,ꢀ theꢀ twoꢀ channelsꢀ ofꢀ theꢀ dualꢀ

switchingꢀregulatorꢀareꢀoperatedꢀ180ꢀdegreesꢀoutꢀofꢀphase.ꢀ

Thisꢀeffectivelyꢀinterleavesꢀtheꢀcurrentꢀpulsesꢀdrawnꢀbyꢀtheꢀ

switches,ꢀgreatlyꢀreducingꢀtheꢀoverlapꢀtimeꢀwhereꢀtheyꢀaddꢀ Itꢀcanꢀreadilyꢀbeꢀseenꢀthatꢀtheꢀadvantagesꢀofꢀ2-phaseꢀop-

together.ꢀTheꢀresultꢀisꢀaꢀsigniﬁcantꢀreductionꢀinꢀtotalꢀRMSꢀ erationꢀareꢀnotꢀjustꢀlimitedꢀtoꢀaꢀnarrowꢀoperatingꢀrange,ꢀ

inputꢀcurrent,ꢀwhichꢀinꢀturnꢀallowsꢀlessꢀexpensiveꢀinputꢀ forꢀmostꢀapplicationsꢀisꢀthatꢀ2-phaseꢀoperationꢀwillꢀreduceꢀ

capacitorsꢀtoꢀbeꢀused,ꢀreducesꢀshieldingꢀrequirementsꢀforꢀ theꢀinputꢀcapacitorꢀrequirementꢀtoꢀthatꢀforꢀjustꢀoneꢀchannelꢀ

EMIꢀandꢀimprovesꢀrealꢀworldꢀoperatingꢀefﬁciency.

operatingꢀatꢀmaximumꢀcurrentꢀandꢀ50%ꢀdutyꢀcycle.

Figureꢀ2ꢀcomparesꢀtheꢀinputꢀwaveformsꢀforꢀaꢀrepresentativeꢀ

singleꢀphaseꢀdualꢀswitchingꢀregulatorꢀtoꢀtheꢀLTC3858-1ꢀ

2-phaseꢀ dualꢀ switchingꢀ regulator.ꢀ Anꢀ actualꢀ measure-

mentꢀofꢀtheꢀRMSꢀinputꢀcurrentꢀunderꢀtheseꢀconditionsꢀ

showsꢀthatꢀ2-phaseꢀoperationꢀdroppedꢀtheꢀinputꢀcurrentꢀ

3.0

SINGLE PHASE

DUAL CONTROLLER

2.5

2.0

1.5

1.0

0.5

0

fromꢀ2.53A

ꢀtoꢀ1.55A

.ꢀWhileꢀthisꢀisꢀanꢀimpressiveꢀ

RMS

2-PHASE

DUAL CONTROLLER

reductionꢀinꢀitself,ꢀrememberꢀthatꢀtheꢀpowerꢀlossesꢀareꢀ

2

proportionalꢀtoꢀI

,ꢀmeaningꢀthatꢀtheꢀactualꢀpowerꢀwastedꢀ

RMS

isꢀreducedꢀbyꢀaꢀfactorꢀofꢀ2.66.ꢀTheꢀreducedꢀinputꢀrippleꢀ

voltageꢀalsoꢀmeansꢀlessꢀpowerꢀisꢀlostꢀinꢀtheꢀinputꢀpowerꢀ

path,ꢀwhichꢀcouldꢀincludeꢀbatteries,ꢀswitches,ꢀtrace/con-

nectorꢀresistancesꢀandꢀprotectionꢀcircuitry.ꢀImprovementsꢀ

inꢀbothꢀconductedꢀandꢀradiatedꢀEMIꢀalsoꢀdirectlyꢀaccrueꢀasꢀ

aꢀresultꢀofꢀtheꢀreducedꢀRMSꢀinputꢀcurrentꢀandꢀvoltage.

V

O1

V

O2

= 5V/3A

= 3.3V/3A

0

10

20

30

40

INPUT VOLTAGE (V)

3858 F03

Figure 3. RMS Input Current Comparison

5V SWITCH

20V/DIV

3.3V SWITCH

20V/DIV

INPUT CURRENT

5A/DIV

INPUT VOLTAGE

500mV/DIV

38581 F01

I

= 2.53A

I

= 1.55A

IN(MEAS) RMS

IN(MEAS)

RMS

Figure 2. Input Waveforms Comparing Single-Phase (a) and 2-Phase (b) Operation for Dual Switching Regulators

Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple with the 2-Phase Regulator Allows

Less Expensive Input Capacitors, Reduces Shielding Requirements for EMI and Improves Efﬁciency

38581fb

ꢀꢃ

LTC3858-1

applicaTions inForMaTion

Theꢀ Typicalꢀ Applicationꢀ onꢀ theꢀ ﬁrstꢀ pageꢀ isꢀ aꢀ basicꢀ

LTC3858-1ꢀapplicationꢀcircuit.ꢀLTC3858-1ꢀcanꢀbeꢀconﬁguredꢀ

toꢀuseꢀeitherꢀDCRꢀ(inductorꢀresistance)ꢀsensingꢀorꢀlowꢀ

valueꢀresistorꢀsensing.ꢀTheꢀchoiceꢀbetweenꢀtheꢀtwoꢀcur-

rentꢀsensingꢀschemesꢀisꢀlargelyꢀaꢀdesignꢀtradeoffꢀbetweenꢀ

cost,ꢀpowerꢀconsumptionꢀandꢀaccuracy.ꢀDCRꢀsensingꢀisꢀ

becomingꢀ popularꢀ becauseꢀ itꢀ savesꢀ expensiveꢀ currentꢀ

sensingꢀresistorsꢀandꢀisꢀmoreꢀpowerꢀefﬁcient,ꢀespeciallyꢀ

inꢀ highꢀ currentꢀ applications.ꢀ However,ꢀ currentꢀ sensingꢀ

resistorsꢀprovideꢀtheꢀmostꢀaccurateꢀcurrentꢀlimitsꢀforꢀtheꢀ

controller.ꢀOtherꢀexternalꢀcomponentꢀselectionꢀisꢀdrivenꢀ

byꢀtheꢀloadꢀrequirement,ꢀandꢀbeginsꢀwithꢀtheꢀselectionꢀofꢀ

programmedꢀcurrentꢀlimitꢀunpredictable.ꢀIfꢀinductorꢀDCRꢀ

sensingꢀisꢀusedꢀ(Figureꢀ5b),ꢀresistorꢀR1ꢀshouldꢀbeꢀplacedꢀ

closeꢀtoꢀtheꢀswitchingꢀnode,ꢀtoꢀpreventꢀnoiseꢀfromꢀcouplingꢀ

intoꢀsensitiveꢀsmall-signalꢀnodes.

TO SENSE FILTER,

NEXT TO THE CONTROLLER

C

OUT

38581 F04

INDUCTOR OR R

SENSE

Figure 4. Sense Lines Placement with Inductor or Sense Resistor

R

ꢀ(ifꢀR

ꢀisꢀused)ꢀandꢀinductorꢀvalue.ꢀNext,ꢀtheꢀ

SENSE

V

IN

powerꢀMOSFETsꢀandꢀSchottkyꢀdiodesꢀareꢀselected.ꢀFinally,ꢀ

inputꢀandꢀoutputꢀcapacitorsꢀareꢀselected.

INTV

CC

BOOST

TG

+

–

SENSE and SENSE Pins

SW

V

OUT

+

–

LTC3858-1

TheꢀSENSE ꢀandꢀSENSE ꢀpinsꢀareꢀtheꢀinputsꢀtoꢀtheꢀcurrentꢀ

comparators.ꢀTheꢀcommonꢀmodeꢀvoltageꢀrangeꢀonꢀtheseꢀ

pinsꢀisꢀ0Vꢀtoꢀ28Vꢀ(AbsꢀMax),ꢀenablingꢀtheꢀLTC3858-1ꢀtoꢀ

regulateꢀoutputꢀvoltagesꢀupꢀtoꢀaꢀnominalꢀ24Vꢀ(allowingꢀ

marginꢀforꢀtolerancesꢀandꢀtransients).ꢀ

BG

+

SENSE

PLACE CAPACITOR NEAR

SENSE PINS

–

SENSE

SGND

+

38581 F05a

TheꢀSENSE ꢀpinꢀisꢀhighꢀimpedanceꢀoverꢀtheꢀfullꢀcommonꢀ

modeꢀrange,ꢀdrawingꢀatꢀmostꢀ 1µA.ꢀThisꢀhighꢀimpedanceꢀ

allowsꢀtheꢀcurrentꢀcomparatorsꢀtoꢀbeꢀusedꢀinꢀinductorꢀ

DCRꢀsensing.

(5a) Using a Resistor to Sense Current

V

INTV

V

IN

–

TheꢀimpedanceꢀofꢀtheꢀSENSE ꢀpinꢀchangesꢀdependingꢀonꢀ

CC

–

theꢀcommonꢀmodeꢀvoltage.ꢀWhenꢀSENSE ꢀisꢀlessꢀthanꢀ

INDUCTOR

DCR

BOOST

TG

INTV ꢀ–ꢀ0.5V,ꢀaꢀsmallꢀcurrentꢀofꢀlessꢀthanꢀ1µAꢀﬂowsꢀoutꢀ

CC

L

–

ofꢀtheꢀpin.ꢀWhenꢀSENSE ꢀisꢀaboveꢀINTV ꢀ+ꢀ0.5V,ꢀaꢀhigherꢀ

CC

SW

V

OUT

LTC3858-1

currentꢀ(~550µA)ꢀﬂowsꢀintoꢀtheꢀpin.ꢀBetweenꢀINTV ꢀ–ꢀ0.5Vꢀ

CC

BG

andꢀINTV ꢀ+ꢀ0.5V,ꢀtheꢀcurrentꢀtransitionsꢀfromꢀtheꢀsmallerꢀ

CC

R1

C1* R2

currentꢀtoꢀtheꢀhigherꢀcurrent.

+

SENSE

Filterꢀcomponentsꢀmutualꢀtoꢀtheꢀsenseꢀlinesꢀshouldꢀbeꢀ

placedꢀcloseꢀtoꢀtheꢀLTC3858-1,ꢀandꢀtheꢀsenseꢀlinesꢀshouldꢀ

runꢀcloseꢀtogetherꢀtoꢀaꢀKelvinꢀconnectionꢀunderneathꢀtheꢀ

currentꢀsenseꢀelementꢀ(shownꢀinꢀFigureꢀ4).ꢀSensingꢀcur-

rentꢀelsewhereꢀcanꢀeffectivelyꢀaddꢀparasiticꢀinductanceꢀ

andꢀcapacitanceꢀtoꢀtheꢀcurrentꢀsenseꢀelement,ꢀdegradingꢀ

theꢀinformationꢀatꢀtheꢀsenseꢀterminalsꢀandꢀmakingꢀtheꢀ

–

SENSE

SGND

38581 F05b

R2

R1 + R2

L

DCR

||

(R1 R2) • C1 =

*PLACE C1 NEAR

SENSE PINS

R

= DCR

SENSE(EQ)

(5b) Using the Inductor DCR to Sense Current

Figure 5. Current Sensing Methods

38581fb

ꢀꢄ

LTC3858-1

applicaTions inForMaTion

Low Value Resistors Current Sensing

usingꢀaꢀgoodꢀRLCꢀmeter,ꢀbutꢀtheꢀDCRꢀtoleranceꢀisꢀnotꢀ

alwaysꢀtheꢀsameꢀandꢀvariesꢀwithꢀtemperature;ꢀconsultꢀtheꢀ

manufacturers’ꢀdataꢀsheetsꢀforꢀdetailedꢀinformation.

Aꢀtypicalꢀsensingꢀcircuitꢀusingꢀaꢀdiscreteꢀresistorꢀisꢀshownꢀ

inꢀ Figureꢀ 5a.ꢀ R

outputꢀcurrent.

ꢀ isꢀ chosenꢀ basedꢀ onꢀ theꢀ requiredꢀ

SENSE

UsingꢀtheꢀinductorꢀrippleꢀcurrentꢀvalueꢀfromꢀtheꢀInductorꢀ

ValueꢀCalculationꢀsection,ꢀtheꢀtargetꢀsenseꢀresistorꢀvalueꢀ

is:

Theꢀ currentꢀ comparatorꢀ hasꢀ aꢀ maximumꢀ thresholdꢀ

ꢀofꢀ50mVꢀ(typ).ꢀTheꢀcurrentꢀcomparatorꢀthresh-

V

SENSE(MAX)

oldꢀvoltageꢀsetsꢀtheꢀpeakꢀofꢀtheꢀinductorꢀcurrent,ꢀyieldingꢀ

V_SENSE(MAX)

R_SENSE(EQUIV)

=

aꢀmaximumꢀaverageꢀoutputꢀcurrent,ꢀI

,ꢀequalꢀtoꢀtheꢀ

MAX

∆I_L

peakꢀvalueꢀlessꢀhalfꢀtheꢀpeak-to-peakꢀrippleꢀcurrent,ꢀ∆I .ꢀ

I_MAX+

L

2

ꢀ

Toꢀcalculateꢀtheꢀsenseꢀresistorꢀvalue,ꢀuseꢀtheꢀequation:

Toꢀensureꢀthatꢀtheꢀapplicationꢀwillꢀdeliverꢀfullꢀloadꢀcurrentꢀ

overꢀ theꢀ fullꢀ operatingꢀ temperatureꢀ range,ꢀ chooseꢀ theꢀ

minimumꢀvalueꢀforꢀtheꢀMaximumꢀCurrentꢀSenseꢀThresh-

V_SENSE(MAX)

R_SENSE

=

∆I_L

2

I_MAX

+

ꢀ

oldꢀVoltageꢀ(V

table.

)ꢀinꢀtheꢀElectricalꢀCharacteristicsꢀ

ꢀ

SENSE(MAX)

Whenꢀusingꢀtheꢀcontrollerꢀinꢀveryꢀlowꢀdropoutꢀconditions,ꢀ

theꢀmaximumꢀoutputꢀcurrentꢀlevelꢀwillꢀbeꢀreducedꢀdueꢀtoꢀtheꢀ

internalꢀcompensationꢀrequiredꢀtoꢀmeetꢀstabilityꢀcriterionꢀ

forꢀbuckꢀregulatorsꢀoperatingꢀatꢀgreaterꢀthanꢀ50%ꢀdutyꢀ

factor.ꢀAꢀcurveꢀisꢀprovidedꢀinꢀtheꢀTypicalꢀPerformanceꢀChar-

acteristicsꢀsectionꢀtoꢀestimateꢀthisꢀreductionꢀinꢀpeakꢀoutputꢀ

currentꢀdependingꢀuponꢀtheꢀoperatingꢀdutyꢀfactor.

Next,ꢀdetermineꢀtheꢀDCRꢀofꢀtheꢀinductor.ꢀWhenꢀprovided,ꢀ

useꢀtheꢀmanufacturer’sꢀmaximumꢀvalue,ꢀusuallyꢀgivenꢀatꢀ

20°C.ꢀIncreaseꢀthisꢀvalueꢀtoꢀaccountꢀforꢀtheꢀtemperatureꢀ

coefﬁcientꢀofꢀcopper,ꢀwhichꢀisꢀapproximatelyꢀ0.4%/°C.ꢀAꢀ

conservativeꢀvalueꢀforꢀT

ꢀisꢀ100°C.

L(MAX)

ToꢀscaleꢀtheꢀmaximumꢀinductorꢀDCRꢀtoꢀtheꢀdesiredꢀsenseꢀ

resistorꢀvalue,ꢀuseꢀtheꢀdividerꢀratio:

Inductor DCR Sensing

R_SENSE(EQUIV)

Forꢀapplicationsꢀrequiringꢀtheꢀhighestꢀpossibleꢀefﬁciencyꢀ

atꢀhighꢀloadꢀcurrents,ꢀtheꢀLTC3850ꢀisꢀcapableꢀofꢀsensingꢀ

theꢀvoltageꢀdropꢀacrossꢀtheꢀinductorꢀDCR,ꢀasꢀshownꢀinꢀ

Figureꢀ5b.ꢀTheꢀDCRꢀofꢀtheꢀinductorꢀrepresentsꢀtheꢀsmallꢀ

amountꢀofꢀDCꢀresistanceꢀofꢀtheꢀcopperꢀwire,ꢀwhichꢀcanꢀbeꢀ

lessꢀthanꢀ1mΩꢀforꢀtoday’sꢀlowꢀvalue,ꢀhighꢀcurrentꢀinductors.ꢀ

Inꢀaꢀhighꢀcurrentꢀapplicationꢀrequiringꢀsuchꢀanꢀinductor,ꢀ

powerꢀlossꢀthroughꢀaꢀsenseꢀresistorꢀwouldꢀcostꢀseveralꢀ

pointsꢀofꢀefﬁciencyꢀcomparedꢀtoꢀinductorꢀDCRꢀsensing.

R_D=

DCR_MAXatT

L(MAX)

ꢀ

C1ꢀisꢀusuallyꢀselectedꢀtoꢀbeꢀinꢀtheꢀrangeꢀofꢀ0.1µFꢀtoꢀ0.47µF.ꢀ

ThisꢀforcesꢀR1||R2ꢀtoꢀaroundꢀ2k,ꢀreducingꢀerrorꢀthatꢀmightꢀ

haveꢀbeenꢀcausedꢀbyꢀtheꢀSENSE ꢀpin’sꢀ 1µAꢀcurrent.

+

TheꢀequivalentꢀresistanceꢀR1||R2ꢀisꢀscaledꢀtoꢀtheꢀroomꢀ

temperatureꢀinductanceꢀandꢀmaximumꢀDCR:

L

R1||R2 =

IfꢀtheꢀexternalꢀR1||R2ꢀ•ꢀC1ꢀtimeꢀconstantꢀisꢀchosenꢀtoꢀbeꢀ

exactlyꢀequalꢀtoꢀtheꢀL/DCRꢀtimeꢀconstant,ꢀtheꢀvoltageꢀdropꢀ

acrossꢀtheꢀexternalꢀcapacitorꢀisꢀequalꢀtoꢀtheꢀdropꢀacrossꢀ

theꢀinductorꢀDCRꢀmultipliedꢀbyꢀR2/(R1ꢀ+ꢀR2).ꢀR2ꢀscalesꢀtheꢀ

voltageꢀacrossꢀtheꢀsenseꢀterminalsꢀforꢀapplicationsꢀwhereꢀ

theꢀDCRꢀisꢀgreaterꢀthanꢀtheꢀtargetꢀsenseꢀresistorꢀvalue.ꢀ

Toꢀproperlyꢀdimensionꢀtheꢀexternalꢀﬁlterꢀcomponents,ꢀtheꢀ

DCRꢀofꢀtheꢀinductorꢀmustꢀbeꢀknown.ꢀItꢀcanꢀbeꢀmeasuredꢀ

DCR at 20°C •C1

(

)

ꢀ

Theꢀsenseꢀresistorꢀvaluesꢀare:

R1•R_D

1–R_D

R1||R2

R_D

R1=

; R2 =

ꢀ

38581fb

ꢀꢅ

LTC3858-1

applicaTions inForMaTion

TheꢀmaximumꢀpowerꢀlossꢀinꢀR1ꢀisꢀrelatedꢀtoꢀdutyꢀcycle,ꢀ 30%ꢀofꢀtheꢀcurrentꢀlimitꢀdeterminedꢀbyꢀR

andꢀwillꢀoccurꢀinꢀcontinuousꢀmodeꢀatꢀtheꢀmaximumꢀinputꢀ inductorꢀvaluesꢀ(higherꢀ∆I )ꢀwillꢀcauseꢀthisꢀtoꢀoccurꢀatꢀ

voltage:

.ꢀLowerꢀ

SENSE

L

lowerꢀloadꢀcurrents,ꢀwhichꢀcanꢀcauseꢀaꢀdipꢀinꢀefﬁciencyꢀinꢀ

theꢀupperꢀrangeꢀofꢀlowꢀcurrentꢀoperation.ꢀInꢀBurstꢀModeꢀ

operation,ꢀlowerꢀinductanceꢀvaluesꢀwillꢀcauseꢀtheꢀburstꢀ

frequencyꢀtoꢀdecrease.

V

_IN(MAX)– V_OUT• V

(

)

OUT

P

R1=

LOSS

R1

ꢀ

EnsureꢀthatꢀR1ꢀhasꢀaꢀpowerꢀratingꢀhigherꢀthanꢀthisꢀvalue.ꢀ

Ifꢀhighꢀefﬁciencyꢀisꢀnecessaryꢀatꢀlightꢀloads,ꢀconsiderꢀthisꢀ

powerꢀlossꢀwhenꢀdecidingꢀtoꢀuseꢀinductorꢀDCRꢀsensingꢀ

orꢀsenseꢀresistors.ꢀLightꢀloadꢀpowerꢀlossꢀcanꢀbeꢀmodestlyꢀ

higherꢀwithꢀaꢀDCRꢀnetworkꢀthanꢀwithꢀaꢀsenseꢀresistor,ꢀdueꢀ

toꢀtheꢀextraꢀswitchingꢀlossesꢀincurredꢀthroughꢀR1.ꢀHowever,ꢀ

DCRꢀsensingꢀeliminatesꢀaꢀsenseꢀresistor,ꢀreducesꢀconduc-

tionꢀlossesꢀandꢀprovidesꢀhigherꢀefﬁciencyꢀatꢀheavyꢀloads.ꢀ

Peakꢀefﬁciencyꢀisꢀaboutꢀtheꢀsameꢀwithꢀeitherꢀmethod.

Inductor Core Selection

OnceꢀtheꢀvalueꢀforꢀLꢀisꢀknown,ꢀtheꢀtypeꢀofꢀinductorꢀmustꢀ

beꢀselected.ꢀHighꢀefﬁciencyꢀconvertersꢀgenerallyꢀcannotꢀ

affordꢀtheꢀcoreꢀlossꢀfoundꢀinꢀlowꢀcostꢀpowderedꢀironꢀcores,ꢀ

forcingꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀferriteꢀorꢀmolypermalloyꢀ

cores.ꢀActualꢀcoreꢀlossꢀisꢀindependentꢀofꢀcoreꢀsizeꢀforꢀaꢀ

ﬁxedꢀinductorꢀvalue,ꢀbutꢀitꢀisꢀveryꢀdependentꢀonꢀinductanceꢀ

valueꢀselected.ꢀAsꢀinductanceꢀincreases,ꢀcoreꢀlossesꢀgoꢀ

down.ꢀUnfortunately,ꢀincreasedꢀinductanceꢀrequiresꢀmoreꢀ

turnsꢀofꢀwireꢀandꢀthereforeꢀcopperꢀlossesꢀwillꢀincrease.

Inductor Value Calculation

Ferriteꢀdesignsꢀhaveꢀveryꢀlowꢀcoreꢀlossꢀandꢀareꢀpreferredꢀ

forꢀhighꢀswitchingꢀfrequencies,ꢀsoꢀdesignꢀgoalsꢀcanꢀcon-

centrateꢀonꢀcopperꢀlossꢀandꢀpreventingꢀsaturation.ꢀFerriteꢀ

coreꢀmaterialꢀsaturatesꢀ“hard,”ꢀwhichꢀmeansꢀthatꢀinduc-

tanceꢀcollapsesꢀabruptlyꢀwhenꢀtheꢀpeakꢀdesignꢀcurrentꢀisꢀ

exceeded.ꢀThisꢀresultsꢀinꢀanꢀabruptꢀincreaseꢀinꢀinductorꢀ

rippleꢀcurrentꢀandꢀconsequentꢀoutputꢀvoltageꢀripple.ꢀDoꢀ

notꢀallowꢀtheꢀcoreꢀtoꢀsaturate!

Theꢀoperatingꢀfrequencyꢀandꢀinductorꢀselectionꢀareꢀinter-

relatedꢀinꢀthatꢀhigherꢀoperatingꢀfrequenciesꢀallowꢀtheꢀuseꢀ

ofꢀsmallerꢀinductorꢀandꢀcapacitorꢀvalues.ꢀSoꢀwhyꢀwouldꢀ

anyoneꢀeverꢀchooseꢀtoꢀoperateꢀatꢀlowerꢀfrequenciesꢀwithꢀ

largerꢀcomponents?ꢀTheꢀanswerꢀisꢀefﬁciency.ꢀAꢀhigherꢀ

frequencyꢀgenerallyꢀresultsꢀinꢀlowerꢀefﬁciencyꢀbecauseꢀ

ofꢀMOSFETꢀgateꢀchargeꢀlosses.ꢀInꢀadditionꢀtoꢀthisꢀbasicꢀ

trade-off,ꢀtheꢀeffectꢀofꢀinductorꢀvalueꢀonꢀrippleꢀcurrentꢀandꢀ

lowꢀcurrentꢀoperationꢀmustꢀalsoꢀbeꢀconsidered.

Power MOSFET and Schottky Diode

(Optional) Selection

Theꢀinductorꢀvalueꢀhasꢀaꢀdirectꢀeffectꢀonꢀrippleꢀcurrent.ꢀ

Theꢀ inductorꢀ rippleꢀ currentꢀ ∆I ꢀ decreasesꢀ withꢀ higherꢀ

L

TwoꢀexternalꢀpowerꢀMOSFETsꢀmustꢀbeꢀselectedꢀforꢀeachꢀ

controllerꢀinꢀtheꢀLTC3858-1:ꢀoneꢀN-channelꢀMOSFETꢀforꢀ

theꢀtopꢀ(main)ꢀswitch,ꢀandꢀoneꢀN-channelꢀMOSFETꢀforꢀtheꢀ

bottomꢀ(synchronous)ꢀswitch.

inductanceꢀorꢀhigherꢀfrequencyꢀandꢀincreasesꢀwithꢀhigherꢀ

V :

IN





V

IN

1

^OUT

1–

_OUT

ΔI_L=

V

f L

( )( )





Theꢀpeak-to-peakꢀdriveꢀlevelsꢀareꢀsetꢀbyꢀtheꢀINTV ꢀvoltage.ꢀ

CC

ꢀ

Thisꢀvoltageꢀisꢀtypicallyꢀ5.1Vꢀduringꢀstart-upꢀ(seeꢀEXTV ꢀ

CC

Acceptingꢀ largerꢀ valuesꢀ ofꢀ ∆I ꢀ allowsꢀ theꢀ useꢀ ofꢀ lowꢀ

Pinꢀ Connection).ꢀ Consequently,ꢀ logic-levelꢀ thresholdꢀ

L

inductances,ꢀbutꢀresultsꢀinꢀhigherꢀoutputꢀvoltageꢀrippleꢀ

MOSFETsꢀmustꢀbeꢀusedꢀinꢀmostꢀapplications.ꢀTheꢀonlyꢀ

andꢀgreaterꢀcoreꢀlosses.ꢀAꢀreasonableꢀstartingꢀpointꢀforꢀ

exceptionꢀisꢀifꢀlowꢀinputꢀvoltageꢀisꢀexpectedꢀ(V ꢀ<ꢀ4V);ꢀ

IN

GS(TH)

settingꢀrippleꢀcurrentꢀisꢀ∆I ꢀ=ꢀ0.3(I

).ꢀTheꢀmaximumꢀ

MAX

then,ꢀsub-logicꢀlevelꢀthresholdꢀMOSFETsꢀ(V

ꢀ<ꢀ3V)ꢀ

L

∆I ꢀoccursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.

shouldꢀbeꢀused.ꢀPayꢀcloseꢀattentionꢀtoꢀtheꢀBV ꢀspeci-

L

DSS

ﬁcationꢀforꢀtheꢀMOSFETsꢀasꢀwell;ꢀmanyꢀofꢀtheꢀlogic-levelꢀ

Theꢀinductorꢀvalueꢀalsoꢀhasꢀsecondaryꢀeffects.ꢀTheꢀtran-

sitionꢀtoꢀBurstꢀModeꢀoperationꢀbeginsꢀwhenꢀtheꢀaverageꢀ

inductorꢀcurrentꢀrequiredꢀresultsꢀinꢀaꢀpeakꢀcurrentꢀbelowꢀ

MOSFETsꢀareꢀlimitedꢀtoꢀ30Vꢀorꢀless.

38581fb

ꢀꢆ

LTC3858-1

applicaTions inForMaTion

SelectionꢀcriteriaꢀforꢀtheꢀpowerꢀMOSFETsꢀincludeꢀtheꢀon-

synchronousꢀMOSFETꢀlossesꢀareꢀgreatestꢀatꢀhighꢀinputꢀ

resistance,ꢀ R ,ꢀ Millerꢀ capacitance,ꢀ C ,ꢀ inputꢀ voltageꢀwhenꢀtheꢀtopꢀswitchꢀdutyꢀfactorꢀisꢀlowꢀorꢀduringꢀ

DS(ON) MILLER

voltageꢀandꢀmaximumꢀoutputꢀcurrent.ꢀMillerꢀcapacitance,ꢀ aꢀshort-circuitꢀwhenꢀtheꢀsynchronousꢀswitchꢀisꢀonꢀcloseꢀ

,ꢀcanꢀbeꢀapproximatedꢀfromꢀtheꢀgateꢀchargeꢀcurveꢀ toꢀ100%ꢀofꢀtheꢀperiod.

C

MILLER

usuallyꢀ providedꢀ onꢀ theꢀ MOSFETꢀ manufacturers’ꢀ dataꢀ

sheet.ꢀC ꢀisꢀequalꢀtoꢀtheꢀincreaseꢀinꢀgateꢀchargeꢀ

Theꢀtermꢀ(1+ꢀδ)ꢀisꢀgenerallyꢀgivenꢀforꢀaꢀMOSFETꢀinꢀtheꢀ

MILLER

formꢀofꢀaꢀnormalizedꢀR

ꢀvsꢀTemperatureꢀcurve,ꢀbutꢀ

DS(ON)

alongꢀtheꢀhorizontalꢀaxisꢀwhileꢀtheꢀcurveꢀisꢀapproximatelyꢀ

δꢀ=ꢀ0.005/°Cꢀcanꢀbeꢀusedꢀasꢀanꢀapproximationꢀforꢀlowꢀ

voltageꢀMOSFETs.

ﬂatꢀdividedꢀbyꢀtheꢀspeciﬁedꢀchangeꢀinꢀV .ꢀThisꢀresultꢀisꢀ

DS

thenꢀmultipliedꢀbyꢀtheꢀratioꢀofꢀtheꢀapplicationꢀappliedꢀV ꢀ

DS

TheꢀoptionalꢀSchottkyꢀdiodesꢀD1ꢀandꢀD2ꢀshownꢀinꢀFigureꢀ10ꢀ

conductꢀduringꢀtheꢀdead-timeꢀbetweenꢀtheꢀconductionꢀofꢀ

theꢀtwoꢀpowerꢀMOSFETs.ꢀThisꢀpreventsꢀtheꢀbodyꢀdiodeꢀofꢀ

theꢀbottomꢀMOSFETꢀfromꢀturningꢀon,ꢀstoringꢀchargeꢀduringꢀ

theꢀdead-timeꢀandꢀrequiringꢀaꢀreverseꢀrecoveryꢀperiodꢀthatꢀ

toꢀtheꢀgateꢀchargeꢀcurveꢀspeciﬁedꢀV .ꢀWhenꢀtheꢀICꢀisꢀ

DS

operatingꢀinꢀcontinuousꢀmodeꢀtheꢀdutyꢀcyclesꢀforꢀtheꢀtopꢀ

andꢀbottomꢀMOSFETsꢀareꢀgivenꢀby:

V_OUT

Main Switch Duty Cycle =

V

couldꢀcostꢀasꢀmuchꢀasꢀ3%ꢀinꢀefﬁciencyꢀatꢀhighꢀV .ꢀAꢀ1Aꢀ

IN

toꢀ3AꢀSchottkyꢀisꢀgenerallyꢀaꢀgoodꢀcompromiseꢀforꢀbothꢀ

regionsꢀofꢀoperationꢀdueꢀtoꢀtheꢀrelativelyꢀsmallꢀaverageꢀ

current.ꢀLargerꢀdiodesꢀresultꢀinꢀadditionalꢀtransitionꢀlossesꢀ

dueꢀtoꢀtheirꢀlargerꢀjunctionꢀcapacitance.

V − V_OUT

IN

Synchronous Switch Duty Cycle =

V

IN

ꢀ

Theꢀ MOSFETꢀ powerꢀ dissipationsꢀ atꢀ maximumꢀ outputꢀ

currentꢀareꢀgivenꢀby:

C and C

Selection

IN

OUT

V_OUT

2

TheꢀselectionꢀofꢀC ꢀisꢀsimpliﬁedꢀbyꢀtheꢀ2-phaseꢀarchitec-

P_MAIN

=

I

1+ δ R

+

DS(ON)

IN

(

_MAX) (

)

V

IN

tureꢀandꢀitsꢀimpactꢀonꢀtheꢀworst-caseꢀRMSꢀcurrentꢀdrawnꢀ

throughꢀtheꢀinputꢀnetworkꢀ(battery/fuse/capacitor).ꢀItꢀcanꢀbeꢀ

shownꢀthatꢀtheꢀworst-caseꢀcapacitorꢀRMSꢀcurrentꢀoccursꢀ

whenꢀonlyꢀoneꢀcontrollerꢀisꢀoperating.ꢀTheꢀcontrollerꢀwithꢀ

₂











I_MAX

2

V

R

C

•

f

(

)

(

_DR)(

)

IN

MILLER









1

theꢀhighestꢀ(V )(I )ꢀproductꢀneedsꢀtoꢀbeꢀusedꢀinꢀtheꢀ

OUT OUT

+





( )

formulaꢀshownꢀinꢀEquationꢀ1ꢀtoꢀdetermineꢀtheꢀmaximumꢀ

RMSꢀcapacitorꢀcurrentꢀrequirement.ꢀIncreasingꢀtheꢀout-

putꢀcurrentꢀdrawnꢀfromꢀtheꢀotherꢀcontrollerꢀwillꢀactuallyꢀ

decreaseꢀtheꢀinputꢀRMSꢀrippleꢀcurrentꢀfromꢀitsꢀmaximumꢀ

value.ꢀTheꢀout-of-phaseꢀtechniqueꢀtypicallyꢀreducesꢀtheꢀ

inputꢀcapacitor’sꢀRMSꢀrippleꢀcurrentꢀbyꢀaꢀfactorꢀofꢀ30%ꢀ

toꢀ70%ꢀwhenꢀcomparedꢀtoꢀaꢀsingleꢀphaseꢀpowerꢀsupplyꢀ

solution.

V

_INTVCC– V_THMINV_THMIN

V – V_OUT

2

IN

P_SYNC

=

I

1+ δ R

(

_MAX) (

)

DS(ON)

V

IN

ꢀ

whereꢀδꢀisꢀtheꢀtemperatureꢀdependencyꢀofꢀR

ꢀandꢀ

DS(ON)

R ꢀ(approximatelyꢀ2Ω)ꢀisꢀtheꢀeffectiveꢀdriverꢀresistanceꢀ

DR

atꢀtheꢀMOSFET’sꢀMillerꢀthresholdꢀvoltage.ꢀV

ꢀisꢀtheꢀ

THMIN

typicalꢀMOSFETꢀminimumꢀthresholdꢀvoltage.

Inꢀcontinuousꢀmode,ꢀtheꢀsourceꢀcurrentꢀofꢀtheꢀtopꢀMOSFETꢀ

isꢀaꢀsquareꢀwaveꢀofꢀdutyꢀcycleꢀ(V )/(V ).ꢀToꢀpreventꢀ

2

BothꢀMOSFETsꢀhaveꢀI RꢀlossesꢀwhileꢀtheꢀtopsideꢀN-channelꢀ

equationꢀincludesꢀanꢀadditionalꢀtermꢀforꢀtransitionꢀlosses,ꢀ

OUT

IN

largeꢀvoltageꢀtransients,ꢀaꢀlowꢀESRꢀcapacitorꢀsizedꢀforꢀtheꢀ

maximumꢀRMSꢀcurrentꢀofꢀoneꢀchannelꢀmustꢀbeꢀused.ꢀTheꢀ

maximumꢀRMSꢀcapacitorꢀcurrentꢀisꢀgivenꢀby:

whichꢀareꢀhighestꢀatꢀhighꢀinputꢀvoltages.ꢀForꢀV ꢀ<ꢀ20Vꢀ

IN

theꢀhighꢀcurrentꢀefﬁciencyꢀgenerallyꢀimprovesꢀwithꢀlargerꢀ

MOSFETs,ꢀwhileꢀforꢀV ꢀ>ꢀ20Vꢀtheꢀtransitionꢀlossesꢀrapidlyꢀ

IN

I_MAX



^1/2



C_INRequired I_RMS

≈

V

V – V

IN OUT

increaseꢀtoꢀtheꢀpointꢀthatꢀtheꢀuseꢀofꢀaꢀhigherꢀR

ꢀdeviceꢀ

(1)

ꢀ

(

_OUT)(

)

DS(ON)



V

IN

withꢀlowerꢀC

ꢀactuallyꢀprovidesꢀhigherꢀefﬁciency.ꢀTheꢀ

MILLER

38581fb

ꢀꢇ

LTC3858-1

applicaTions inForMaTion

Equationꢀ1ꢀhasꢀaꢀmaximumꢀatꢀV ꢀ=ꢀ2V ,ꢀwhereꢀI ꢀ

whereꢀfꢀisꢀtheꢀoperatingꢀfrequency,ꢀC ꢀisꢀtheꢀoutputꢀ

IN

OUTꢀ

RMS

OUT

=ꢀI /2.ꢀThisꢀsimpleꢀworst-caseꢀconditionꢀisꢀcommonlyꢀ

capacitanceꢀandꢀ∆I ꢀisꢀtheꢀrippleꢀcurrentꢀinꢀtheꢀinductor.ꢀ

OUT

L

usedꢀforꢀdesignꢀbecauseꢀevenꢀsigniﬁcantꢀdeviationsꢀdoꢀnotꢀ

offerꢀmuchꢀrelief.ꢀNoteꢀthatꢀcapacitorꢀmanufacturers’ꢀrippleꢀ

currentꢀratingsꢀareꢀoftenꢀbasedꢀonꢀonlyꢀ2000ꢀhoursꢀofꢀlife.ꢀ

Thisꢀmakesꢀitꢀadvisableꢀtoꢀfurtherꢀderateꢀtheꢀcapacitor,ꢀorꢀ

toꢀchooseꢀaꢀcapacitorꢀratedꢀatꢀaꢀhigherꢀtemperatureꢀthanꢀ

required.ꢀSeveralꢀcapacitorsꢀmayꢀbeꢀparalleledꢀtoꢀmeetꢀ

sizeꢀorꢀheightꢀrequirementsꢀinꢀtheꢀdesign.ꢀDueꢀtoꢀtheꢀhighꢀ

operatingꢀfrequencyꢀofꢀtheꢀLTC3858-1,ꢀceramicꢀcapacitorsꢀ

Theꢀoutputꢀrippleꢀisꢀhighestꢀatꢀmaximumꢀinputꢀvoltageꢀ

sinceꢀ∆I ꢀincreasesꢀwithꢀinputꢀvoltage.

L

Setting Output Voltage

TheꢀLTC3858-1ꢀoutputꢀvoltagesꢀareꢀeachꢀsetꢀbyꢀanꢀexter-

nalꢀfeedbackꢀresistorꢀdividerꢀcarefullyꢀplacedꢀacrossꢀtheꢀ

output,ꢀasꢀshownꢀinꢀFigureꢀ6.ꢀTheꢀregulatedꢀoutputꢀvoltageꢀ

isꢀdeterminedꢀby:

canꢀalsoꢀbeꢀusedꢀforꢀC .ꢀAlwaysꢀconsultꢀtheꢀmanufacturerꢀ

IN





R

R_A

ifꢀthereꢀisꢀanyꢀquestion.

V_OUT= 0.8V 1+



^B





Theꢀbeneﬁtꢀofꢀtheꢀ2-phaseꢀoperationꢀcanꢀbeꢀcalculatedꢀ

byꢀusingꢀEquationꢀ1ꢀforꢀtheꢀhigherꢀpowerꢀcontrollerꢀandꢀ

thenꢀcalculatingꢀtheꢀlossꢀthatꢀwouldꢀhaveꢀresultedꢀifꢀbothꢀ

controllerꢀchannelsꢀswitchedꢀonꢀatꢀtheꢀsameꢀtime.ꢀTheꢀ

totalꢀRMSꢀpowerꢀlostꢀisꢀlowerꢀwhenꢀbothꢀcontrollersꢀareꢀ

operatingꢀdueꢀtoꢀtheꢀreducedꢀoverlapꢀofꢀcurrentꢀpulsesꢀ

requiredꢀthroughꢀtheꢀinputꢀcapacitor’sꢀESR.ꢀThisꢀisꢀwhyꢀ

theꢀinputꢀcapacitor’sꢀrequirementꢀcalculatedꢀaboveꢀforꢀtheꢀ

worst-caseꢀcontrollerꢀisꢀadequateꢀforꢀtheꢀdualꢀcontrollerꢀ

design.ꢀAlso,ꢀtheꢀinputꢀprotectionꢀfuseꢀresistance,ꢀbatteryꢀ

resistance,ꢀandꢀPCꢀboardꢀtraceꢀresistanceꢀlossesꢀareꢀalsoꢀ

reducedꢀdueꢀtoꢀtheꢀreducedꢀpeakꢀcurrentsꢀinꢀaꢀ2-phaseꢀ

system.ꢀTheꢀoverallꢀbeneﬁtꢀofꢀaꢀmultiphaseꢀdesignꢀwillꢀ

onlyꢀbeꢀfullyꢀrealizedꢀwhenꢀtheꢀsourceꢀimpedanceꢀofꢀtheꢀ

powerꢀsupply/batteryꢀisꢀincludedꢀinꢀtheꢀefﬁciencyꢀtesting.ꢀ

TheꢀsourcesꢀofꢀtheꢀtopꢀMOSFETsꢀshouldꢀbeꢀplacedꢀwithinꢀ

ꢀ

Toꢀimproveꢀtheꢀfrequencyꢀresponse,ꢀaꢀfeedforwardꢀca-

pacitor,ꢀC ,ꢀmayꢀbeꢀused.ꢀGreatꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀ

FFꢀ

routeꢀtheꢀV ꢀlineꢀawayꢀfromꢀnoiseꢀsources,ꢀsuchꢀasꢀtheꢀ

FB

inductorꢀorꢀtheꢀSWꢀline.

V

OUT

R

B

C

FF

1/2 LTC3858-1

V

FB

R

A

38581 F05

Figure 6. Setting Output Voltage

Soft-Start (SS Pins)

Theꢀstart-upꢀofꢀeachꢀV ꢀisꢀcontrolledꢀbyꢀtheꢀvoltageꢀonꢀ

1cmꢀofꢀeachꢀotherꢀandꢀshareꢀaꢀcommonꢀC (s).ꢀSeparatingꢀ

OUT

IN

theꢀrespectiveꢀSSꢀpin.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀ

theꢀsourcesꢀandꢀC ꢀmayꢀproduceꢀundesirableꢀvoltageꢀandꢀ

IN

isꢀlessꢀthanꢀtheꢀinternalꢀ0.8Vꢀreference,ꢀtheꢀLTC3858-1ꢀ

currentꢀresonancesꢀatꢀV .

IN

regulatesꢀtheꢀV ꢀpinꢀvoltageꢀtoꢀtheꢀvoltageꢀonꢀtheꢀSSꢀpinꢀ

FB

Aꢀsmallꢀ(0.1µFꢀtoꢀ1µF)ꢀbypassꢀcapacitorꢀbetweenꢀtheꢀchipꢀ

insteadꢀofꢀ0.8V.ꢀTheꢀSSꢀpinꢀcanꢀbeꢀusedꢀtoꢀprogramꢀanꢀ

V ꢀpinꢀandꢀground,ꢀplacedꢀcloseꢀtoꢀtheꢀLTC3858-1,ꢀisꢀ

IN

externalꢀsoft-startꢀfunction.

alsoꢀsuggested.ꢀAꢀ10ΩꢀresistorꢀplacedꢀbetweenꢀC ꢀ(C1)ꢀ

IN

Soft-startꢀisꢀenabledꢀbyꢀsimplyꢀconnectingꢀaꢀcapacitorꢀfromꢀ

theꢀSSꢀpinꢀtoꢀground,ꢀasꢀshownꢀinꢀFigureꢀ7.ꢀAnꢀinternalꢀ

1µAꢀ currentꢀ sourceꢀ chargesꢀ theꢀ capacitor,ꢀ providingꢀ aꢀ

andꢀtheꢀV ꢀpinꢀprovidesꢀfurtherꢀisolationꢀbetweenꢀtheꢀ

IN

twoꢀchannels.

TheꢀselectionꢀofꢀC ꢀisꢀdrivenꢀbyꢀtheꢀeffectiveꢀseriesꢀ

OUT

resistanceꢀ(ESR).ꢀTypically,ꢀonceꢀtheꢀESRꢀrequirementꢀ

1/2 LTC3858-1

SS

isꢀsatisﬁed,ꢀtheꢀcapacitanceꢀisꢀadequateꢀforꢀﬁltering.ꢀTheꢀ

C

SS

outputꢀrippleꢀ(∆V )ꢀisꢀapproximatedꢀby:

OUT

SGND









38581 F06

1

ΔV_OUT≈ ΔI ESR+

_L

8 • f • C_OUT



Figure 7. Using the SS Pin to Program Soft-Start

ꢀ

38581fb

ꢀꢈ

LTC3858-1

applicaTions inForMaTion

linearꢀrampingꢀvoltageꢀatꢀtheꢀSSꢀpin.ꢀTheꢀLTC3858-1ꢀwillꢀ

Toꢀpreventꢀtheꢀmaximumꢀjunctionꢀtemperatureꢀfromꢀbe-

ingꢀexceeded,ꢀtheꢀinputꢀsupplyꢀcurrentꢀmustꢀbeꢀcheckedꢀ

whileꢀoperatingꢀinꢀforcedꢀcontinuousꢀmodeꢀ(PLLIN/MODEꢀ

regulateꢀtheꢀV ꢀpinꢀ(andꢀhenceꢀV )ꢀaccordingꢀtoꢀtheꢀ

FB

OUT

voltageꢀonꢀtheꢀSSꢀpin,ꢀallowingꢀV ꢀtoꢀriseꢀsmoothlyꢀfromꢀ

OUT

0Vꢀtoꢀitsꢀﬁnalꢀregulatedꢀvalue.ꢀTheꢀtotalꢀsoft-startꢀtimeꢀwillꢀ

=ꢀINTV )ꢀatꢀmaximumꢀV .

CC

IN

beꢀapproximately:

WhenꢀtheꢀvoltageꢀappliedꢀtoꢀEXTV ꢀrisesꢀaboveꢀ4.7V,ꢀtheꢀ

CC

0.8V

1µA

V ꢀLDOꢀisꢀturnedꢀoffꢀandꢀtheꢀEXTV ꢀLDOꢀisꢀenabled.ꢀTheꢀ

IN

CC

t_SS= C_SS

•

EXTV ꢀLDOꢀremainsꢀonꢀasꢀlongꢀasꢀtheꢀvoltageꢀappliedꢀtoꢀ

CC

ꢀ

EXTV ꢀremainsꢀaboveꢀ4.5V.ꢀTheꢀEXTV ꢀLDOꢀattemptsꢀ

CC

INTV Regulators

toꢀregulateꢀtheꢀINTV ꢀvoltageꢀtoꢀ5.1V,ꢀsoꢀwhileꢀEXTV ꢀ

CC

isꢀlessꢀthanꢀ5.1V,ꢀtheꢀLDOꢀisꢀinꢀdropoutꢀandꢀtheꢀINTV ꢀ

CC

TheꢀLTC3858-1ꢀfeaturesꢀtwoꢀseparateꢀinternalꢀP-channelꢀ

lowꢀdropoutꢀlinearꢀregulatorsꢀ(LDO)ꢀthatꢀsupplyꢀpowerꢀatꢀ

voltageꢀisꢀapproximatelyꢀequalꢀtoꢀEXTV .ꢀWhenꢀEXTV ꢀ

CC

isꢀgreaterꢀthanꢀ5.1V,ꢀupꢀtoꢀanꢀabsoluteꢀmaximumꢀofꢀ14V,ꢀ

theꢀINTV ꢀpinꢀfromꢀeitherꢀtheꢀV ꢀsupplyꢀpinꢀorꢀtheꢀEXT-

CC

IN

INTV ꢀisꢀregulatedꢀtoꢀ5.1V.

CC

V ꢀpinꢀdependingꢀonꢀtheꢀconnectionꢀofꢀtheꢀEXTV ꢀpin.ꢀ

CC

INTV ꢀpowersꢀtheꢀgateꢀdriversꢀandꢀmuchꢀofꢀtheꢀinternalꢀ

UsingꢀtheꢀEXTV_CCꢀLDOꢀallowsꢀtheꢀMOSFETꢀdriverꢀandꢀ

controlꢀpowerꢀtoꢀbeꢀderivedꢀfromꢀoneꢀofꢀtheꢀswitchingꢀ

regulatorꢀoutputsꢀ(4.7Vꢀ≤ꢀV_OUTꢀ≤ꢀ14V)ꢀduringꢀnormalꢀ

operationꢀandꢀfromꢀtheꢀV_INꢀLDOꢀwhenꢀtheꢀoutputꢀisꢀoutꢀ

ofꢀregulationꢀ(e.g.,ꢀstart-up,ꢀshort-circuit).ꢀIfꢀmoreꢀcurrentꢀ

isꢀrequiredꢀthroughꢀtheꢀEXTV_CCꢀLDOꢀthanꢀisꢀspeciﬁed,ꢀanꢀ

externalꢀSchottkyꢀdiodeꢀcanꢀbeꢀaddedꢀbetweenꢀtheꢀEXTV_CCꢀ

andꢀINTV_CCꢀpins.ꢀInꢀthisꢀcase,ꢀdoꢀnotꢀapplyꢀmoreꢀthanꢀ6Vꢀ

toꢀtheꢀEXTV_CCꢀpinꢀandꢀmakeꢀsureꢀthatꢀEXTV_CCꢀ≤ꢀV_IN.

CC

circuitry.ꢀTheꢀV ꢀLDOꢀandꢀtheꢀEXTV ꢀLDOꢀregulateꢀIN-

IN

CC

TV ꢀtoꢀ5.1V.ꢀEachꢀofꢀtheseꢀcanꢀsupplyꢀaꢀpeakꢀcurrentꢀofꢀ

CC

50mAꢀandꢀmustꢀbeꢀbypassedꢀtoꢀgroundꢀwithꢀaꢀminimumꢀ

ofꢀ4.7µFꢀlowꢀESRꢀcapacitor.ꢀRegardlessꢀofꢀwhatꢀtypeꢀofꢀ

bulkꢀcapacitorꢀisꢀused,ꢀanꢀadditionalꢀ1µFꢀceramicꢀcapacitorꢀ

placedꢀdirectlyꢀadjacentꢀtoꢀtheꢀINTV ꢀandꢀPGNDꢀICꢀpinsꢀisꢀ

CC

highlyꢀrecommended.ꢀGoodꢀbypassingꢀisꢀneededꢀtoꢀsupplyꢀ

theꢀhighꢀtransientꢀcurrentsꢀrequiredꢀbyꢀtheꢀMOSFETꢀgateꢀ

driversꢀandꢀtoꢀpreventꢀinteractionꢀbetweenꢀtheꢀchannels.

Signiﬁcantꢀefﬁciencyꢀandꢀthermalꢀgainsꢀcanꢀbeꢀrealizedꢀ

HighꢀinputꢀvoltageꢀapplicationsꢀinꢀwhichꢀlargeꢀMOSFETsꢀ

areꢀbeingꢀdrivenꢀatꢀhighꢀfrequenciesꢀmayꢀcauseꢀtheꢀmaxi-

mumꢀjunctionꢀtemperatureꢀratingꢀforꢀtheꢀLTC3858-1ꢀtoꢀbeꢀ

byꢀpoweringꢀINTV ꢀfromꢀtheꢀoutput,ꢀsinceꢀtheꢀV ꢀcur-

CC

IN

rentꢀresultingꢀfromꢀtheꢀdriverꢀandꢀcontrolꢀcurrentsꢀwillꢀbeꢀ

scaledꢀbyꢀaꢀfactorꢀofꢀ(DutyꢀCycle)/(SwitcherꢀEfﬁciency).ꢀ

Forꢀ5Vꢀtoꢀ14Vꢀregulatorꢀoutputs,ꢀthisꢀmeansꢀconnectingꢀ

theꢀEXTV ꢀpinꢀdirectlyꢀtoꢀV .ꢀTyingꢀtheꢀEXTV ꢀpinꢀtoꢀ

exceeded.ꢀTheꢀINTV ꢀcurrent,ꢀwhichꢀisꢀdominatedꢀbyꢀtheꢀ

CC

gateꢀchargeꢀcurrent,ꢀmayꢀbeꢀsuppliedꢀbyꢀeitherꢀtheꢀV ꢀLDOꢀ

IN

CC

OUTꢀ

CC

orꢀtheꢀEXTV ꢀLDO.ꢀWhenꢀtheꢀvoltageꢀonꢀtheꢀEXTV ꢀpinꢀ

aꢀ8.5Vꢀsupplyꢀreducesꢀtheꢀjunctionꢀtemperatureꢀinꢀtheꢀ

previousꢀexampleꢀfromꢀ125°Cꢀto:

CC

isꢀlessꢀthanꢀ4.7V,ꢀtheꢀV ꢀLDOꢀisꢀenabled.ꢀPowerꢀdissipa-

IN

tionꢀforꢀtheꢀICꢀinꢀthisꢀcaseꢀisꢀhighestꢀandꢀisꢀequalꢀtoꢀV ꢀ•ꢀ

IN

ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(15mA)(8.5V)(90°C/W)ꢀ=ꢀ82°C

J

I

.ꢀTheꢀgateꢀchargeꢀcurrentꢀisꢀdependentꢀonꢀoperatingꢀ

INTVCC

However,ꢀforꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀoutputs,ꢀaddi-

frequencyꢀasꢀdiscussedꢀinꢀtheꢀEfﬁciencyꢀConsiderationsꢀ

section.ꢀTheꢀjunctionꢀtemperatureꢀcanꢀbeꢀestimatedꢀbyꢀ

usingꢀtheꢀequationsꢀgivenꢀinꢀNoteꢀ2ꢀofꢀtheꢀElectricalꢀChar-

tionalꢀcircuitryꢀisꢀrequiredꢀtoꢀderiveꢀINTV ꢀpowerꢀfromꢀ

CC

theꢀoutput.

acteristics.ꢀForꢀexample,ꢀtheꢀLTC3858-1ꢀINTV ꢀcurrentꢀ

CC

Theꢀfollowingꢀlistꢀsummarizesꢀtheꢀfourꢀpossibleꢀconnec-

isꢀlimitedꢀtoꢀlessꢀthanꢀ15mAꢀfromꢀaꢀ40Vꢀsupplyꢀwhenꢀnotꢀ

tionsꢀforꢀEXTV :

CC

usingꢀtheꢀEXTV ꢀsupplyꢀatꢀ70°Cꢀambientꢀtemperatureꢀinꢀ

CC

theꢀSSOPꢀpackage:

ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(15mA)(40V)(90°C/W)ꢀ=ꢀ125°C

J

38581fb

ꢁ0

2.ꢀ

3.ꢀ

4.ꢀ

EXTV ꢀConnectedꢀDirectlyꢀtoꢀV .ꢀThisꢀisꢀtheꢀnormalꢀ

CC OUTꢀ

connectionꢀforꢀaꢀ5Vꢀtoꢀ14Vꢀregulatorꢀandꢀprovidesꢀtheꢀ

highestꢀefﬁciency.

EXTV_CCꢀConnectedꢀtoꢀanꢀExternalꢀSupply.ꢀIfꢀanꢀexternalꢀ

supplyꢀisꢀavailableꢀinꢀtheꢀ5Vꢀtoꢀ14Vꢀrange,ꢀitꢀmayꢀbeꢀ

usedꢀtoꢀpowerꢀEXTV_CC.ꢀEnsureꢀthatꢀEXTV_CCꢀ<ꢀV_IN.

EXTV ꢀConnectedꢀtoꢀanꢀOutput-DerivedꢀBoostꢀNetwork.ꢀ

1.ꢀ

EXTV ꢀLeftꢀOpenꢀ(orꢀGrounded).ꢀThisꢀwillꢀcauseꢀINTV ꢀ

LTC3858-1

applicaTions inForMaTion

on,ꢀtheꢀboostꢀvoltageꢀisꢀaboveꢀtheꢀinputꢀsupply:ꢀV

ꢀ=ꢀ

CC CC

BOOST

toꢀbeꢀpoweredꢀfromꢀtheꢀinternalꢀ5.1Vꢀregulatorꢀresult-

ingꢀinꢀanꢀefﬁciencyꢀpenaltyꢀofꢀupꢀtoꢀ10%ꢀatꢀhighꢀinputꢀ

voltages.

V ꢀ+ꢀV

.ꢀTheꢀvalueꢀofꢀtheꢀboostꢀcapacitor,ꢀC ,ꢀneedsꢀ

IN

INTVCC

B

toꢀbeꢀ100ꢀtimesꢀthatꢀofꢀtheꢀtotalꢀinputꢀcapacitanceꢀofꢀtheꢀ

topsideꢀMOSFET(s).ꢀTheꢀreverseꢀbreakdownꢀofꢀtheꢀexternalꢀ

SchottkyꢀdiodeꢀmustꢀbeꢀgreaterꢀthanꢀV

.ꢀ

IN(MAX)

Whenꢀadjustingꢀtheꢀgateꢀdriveꢀlevel,ꢀtheꢀﬁnalꢀarbiterꢀisꢀtheꢀ

totalꢀinputꢀcurrentꢀforꢀtheꢀregulator.ꢀIfꢀaꢀchangeꢀisꢀmadeꢀ

andꢀtheꢀinputꢀcurrentꢀdecreases,ꢀthenꢀtheꢀefﬁciencyꢀhasꢀ

improved.ꢀIfꢀthereꢀisꢀnoꢀchangeꢀinꢀinputꢀcurrent,ꢀthenꢀthereꢀ

isꢀnoꢀchangeꢀinꢀefﬁciency.

Fault Conditions: Current Limit and Current Foldback

CC

Forꢀ3.3Vꢀandꢀotherꢀlowꢀvoltageꢀregulators,ꢀefﬁciencyꢀ

Whenꢀtheꢀoutputꢀcurrentꢀhitsꢀtheꢀcurrentꢀlimit,ꢀtheꢀoutputꢀ

voltageꢀbeginsꢀtoꢀdrop.ꢀIfꢀtheꢀoutputꢀfallsꢀbelowꢀ70%ꢀofꢀitsꢀ

nominalꢀoutputꢀlevel,ꢀthenꢀtheꢀmaximumꢀsenseꢀvoltageꢀisꢀ

progressivelyꢀloweredꢀtoꢀaboutꢀone-halfꢀofꢀitsꢀmaximumꢀ

selectedꢀvalue.ꢀUnderꢀshort-circuitꢀconditionsꢀwithꢀveryꢀ

lowꢀdutyꢀcycles,ꢀtheꢀLTC3858-1ꢀwillꢀbeginꢀcycleꢀskippingꢀ

inꢀorderꢀtoꢀlimitꢀtheꢀshort-circuitꢀcurrent.ꢀInꢀthisꢀsituationꢀ

theꢀbottomꢀMOSFETꢀwillꢀbeꢀdissipatingꢀmostꢀofꢀtheꢀpowerꢀ

butꢀlessꢀthanꢀinꢀnormalꢀoperation.ꢀTheꢀshort-circuitꢀrippleꢀ

gainsꢀcanꢀstillꢀbeꢀrealizedꢀbyꢀconnectingꢀEXTV ꢀtoꢀanꢀ

CC

output-derivedꢀvoltageꢀthatꢀhasꢀbeenꢀboostedꢀtoꢀgreaterꢀ

thanꢀ4.7V.ꢀThisꢀcanꢀbeꢀdoneꢀwithꢀtheꢀcapacitiveꢀchargeꢀ

pumpꢀshownꢀinꢀFigureꢀ8.ꢀEnsureꢀthatꢀEXTV ꢀ<ꢀV .

CC

IN

V

IN

C

IN

BAT85

V

IN

currentꢀisꢀdeterminedꢀbyꢀtheꢀminimumꢀon-time,ꢀt

,ꢀ

ON(MIN)

MTOP

MBOT

ofꢀtheꢀLTC3858-1ꢀ(≈90ns),ꢀtheꢀinputꢀvoltageꢀandꢀinductorꢀ

value:

VN2222LL

TG1

1/2 LTC3858-1

L

R

SENSE





V

EXTV

SW

OUT

CC

V

L

_ON(MIN) ^IN

ΔI_L(SC)= t





C

ꢀ

D

BG1

OUT

Theꢀresultingꢀaverageꢀshort-circuitꢀcurrentꢀis:

38581 F08

PGND

50% •I

1

2

I_SC=

ꢀ

^LIM(MAX)– ∆I_L(SC)

Figure 8. Capacitive Charge Pump for EXTV_CC

R_SENSE

ꢀ

Fault Conditions: Overvoltage Protection (Crowbar)

Topside MOSFET Driver Supply (C , D )

B

Theꢀovervoltageꢀcrowbarꢀisꢀdesignedꢀtoꢀblowꢀaꢀsystemꢀ

inputꢀfuseꢀwhenꢀtheꢀoutputꢀvoltageꢀofꢀtheꢀregulatorꢀrisesꢀ

muchꢀhigherꢀthanꢀnominalꢀlevels.ꢀTheꢀcrowbarꢀcausesꢀhugeꢀ

currentsꢀtoꢀﬂow,ꢀthatꢀblowꢀtheꢀfuseꢀtoꢀprotectꢀagainstꢀaꢀ

shortedꢀtopꢀMOSFETꢀifꢀtheꢀshortꢀoccursꢀwhileꢀtheꢀcontrol-

lerꢀisꢀoperating.

Externalꢀbootstrapꢀcapacitors,ꢀC ,ꢀconnectedꢀtoꢀtheꢀBOOSTꢀ

B

pinsꢀsupplyꢀtheꢀgateꢀdriveꢀvoltagesꢀforꢀtheꢀtopsideꢀMOSFETs.ꢀ

CapacitorꢀC ꢀinꢀtheꢀFunctionalꢀDiagramꢀisꢀchargedꢀthoughꢀ

B

externalꢀdiodeꢀD ꢀfromꢀINTV ꢀwhenꢀtheꢀSWꢀpinꢀisꢀlow.ꢀ

B

CC

WhenꢀoneꢀofꢀtheꢀtopsideꢀMOSFETsꢀisꢀtoꢀbeꢀturnedꢀon,ꢀtheꢀ

driverꢀplacesꢀtheꢀC ꢀvoltageꢀacrossꢀtheꢀgate-sourceꢀofꢀtheꢀ

B

desiredꢀMOSFET.ꢀThisꢀenhancesꢀtheꢀMOSFETꢀandꢀturnsꢀonꢀ

Aꢀcomparatorꢀmonitorsꢀtheꢀoutputꢀforꢀovervoltageꢀcondi-

tions.ꢀTheꢀcomparatorꢀdetectsꢀfaultsꢀgreaterꢀthanꢀ10%ꢀ

theꢀtopsideꢀswitch.ꢀTheꢀswitchꢀnodeꢀvoltage,ꢀSW,ꢀrisesꢀtoꢀ

V ꢀandꢀtheꢀBOOSTꢀpinꢀfollows.ꢀWithꢀtheꢀtopsideꢀMOSFETꢀ

aboveꢀtheꢀnominalꢀoutputꢀvoltage.ꢀWhenꢀthisꢀconditionꢀ

IN

38581fb

ꢁꢀ

LTC3858-1

applicaTions inForMaTion

isꢀsensed,ꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀoffꢀandꢀtheꢀbottomꢀ

MOSFETꢀisꢀturnedꢀonꢀuntilꢀtheꢀovervoltageꢀconditionꢀisꢀ

cleared.ꢀTheꢀbottomꢀMOSFETꢀremainsꢀonꢀcontinuouslyꢀ

forꢀasꢀlongꢀasꢀtheꢀovervoltageꢀconditionꢀpersists;ꢀifꢀV

returnsꢀtoꢀaꢀsafeꢀlevel,ꢀnormalꢀoperationꢀautomaticallyꢀ

resumes.ꢀ

1000

900

800

700

600

500

400

300

200

100

0

ꢀ

OUT

AꢀshortedꢀtopꢀMOSFETꢀwillꢀresultꢀinꢀaꢀhighꢀcurrentꢀconditionꢀ

whichꢀwillꢀopenꢀtheꢀsystemꢀfuse.ꢀTheꢀswitchingꢀregulatorꢀ

willꢀregulateꢀproperlyꢀwithꢀaꢀleakyꢀtopꢀMOSFETꢀbyꢀalteringꢀ

theꢀdutyꢀcycleꢀtoꢀaccommodateꢀtheꢀleakage.

15 25 35 45 55 65 75 85 95 105 115 125

FREQ PIN RESISTOR (kΩ)

38581 F09

Phase-Locked Loop and Frequency Synchronization

Figure 9. Relationship Between Oscillator Frequency

and Resistor Value at the FREQ Pin

TheꢀLTC3858-1ꢀhasꢀanꢀinternalꢀphase-lockedꢀloopꢀ(PLL)ꢀ

comprisedꢀofꢀaꢀphaseꢀfrequencyꢀdetector,ꢀaꢀlowpassꢀﬁlter,ꢀ

andꢀaꢀvoltage-controlledꢀoscillatorꢀ(VCO).ꢀThisꢀallowsꢀtheꢀ

turn-onꢀofꢀtheꢀtopꢀMOSFETꢀofꢀcontrollerꢀ1ꢀtoꢀbeꢀlockedꢀtoꢀ

theꢀrisingꢀedgeꢀofꢀanꢀexternalꢀclockꢀsignalꢀappliedꢀtoꢀtheꢀ

PLLIN/MODEꢀpin.ꢀTheꢀturn-onꢀofꢀcontrollerꢀ2’sꢀtopꢀMOSFETꢀ

isꢀthusꢀ180ꢀdegreesꢀoutꢀofꢀphaseꢀwithꢀtheꢀexternalꢀclock.ꢀ

Theꢀphaseꢀdetectorꢀisꢀanꢀedgeꢀsensitiveꢀdigitalꢀtypeꢀthatꢀ

providesꢀzeroꢀdegreesꢀphaseꢀshiftꢀbetweenꢀtheꢀexternalꢀ

andꢀinternalꢀoscillators.ꢀThisꢀtypeꢀofꢀphaseꢀdetectorꢀdoesꢀ

notꢀexhibitꢀfalseꢀlockꢀtoꢀharmonicsꢀofꢀtheꢀexternalꢀclock.

Typically,ꢀ theꢀ externalꢀ clockꢀ (onꢀ theꢀ PLLIN/MODEꢀ pin)ꢀ

inputꢀhighꢀthresholdꢀisꢀ1.6V,ꢀwhileꢀtheꢀinputꢀlowꢀthresholdꢀ

isꢀ1.1V.

RapidꢀphaseꢀlockingꢀcanꢀbeꢀachievedꢀbyꢀusingꢀtheꢀFREQꢀ

pinꢀ toꢀ setꢀ aꢀ free-runningꢀ frequencyꢀ nearꢀ theꢀ desiredꢀ

synchronizationꢀfrequency.ꢀTheꢀVCO’sꢀinputꢀvoltageꢀisꢀ

prebiasedꢀatꢀaꢀfrequencyꢀcorrespondingꢀtoꢀtheꢀfrequencyꢀ

setꢀbyꢀtheꢀFREQꢀpin.ꢀOnceꢀprebiased,ꢀtheꢀPLLꢀonlyꢀneedsꢀ

toꢀadjustꢀtheꢀfrequencyꢀslightlyꢀtoꢀachieveꢀphaseꢀlockꢀ

andꢀsynchronization.ꢀAlthoughꢀitꢀisꢀnotꢀrequiredꢀthatꢀtheꢀ

free-runningꢀfrequencyꢀbeꢀnearꢀexternalꢀclockꢀfrequency,ꢀ

doingꢀsoꢀwillꢀpreventꢀtheꢀoperatingꢀfrequencyꢀfromꢀpassingꢀ

throughꢀaꢀlargeꢀrangeꢀofꢀfrequenciesꢀasꢀtheꢀPLLꢀlocks.

Ifꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀgreaterꢀthanꢀtheꢀinternalꢀ

oscillator’sꢀfrequency,ꢀf ,ꢀthenꢀcurrentꢀisꢀsourcedꢀcontinu-

OSC

ouslyꢀfromꢀtheꢀphaseꢀdetectorꢀoutput,ꢀpullingꢀupꢀtheꢀVCOꢀ

input.ꢀWhenꢀtheꢀexternalꢀclockꢀfrequencyꢀisꢀlessꢀthanꢀf ,ꢀ

OSC

currentꢀisꢀsunkꢀcontinuously,ꢀpullingꢀdownꢀtheꢀVCOꢀinput.ꢀ

Ifꢀtheꢀexternalꢀandꢀinternalꢀfrequenciesꢀareꢀtheꢀsameꢀbutꢀ

exhibitꢀaꢀphaseꢀdifference,ꢀtheꢀcurrentꢀsourcesꢀturnꢀonꢀforꢀ

anꢀamountꢀofꢀtimeꢀcorrespondingꢀtoꢀtheꢀphaseꢀdifference.ꢀ

TheꢀvoltageꢀatꢀtheꢀVCOꢀinputꢀisꢀadjustedꢀuntilꢀtheꢀphaseꢀ

andꢀfrequencyꢀofꢀtheꢀinternalꢀandꢀexternalꢀoscillatorsꢀareꢀ

identical.ꢀAtꢀtheꢀstableꢀoperatingꢀpoint,ꢀtheꢀphaseꢀdetectorꢀ

outputꢀisꢀhighꢀimpedanceꢀandꢀtheꢀinternalꢀﬁlterꢀcapacitor,ꢀ

Tableꢀ2ꢀsummarizesꢀtheꢀdifferentꢀstatesꢀinꢀwhichꢀtheꢀFREQꢀ

pinꢀcanꢀbeꢀused.

Table 2

FREQ PIN

PLLIN/MODE PIN

DCꢀVoltage

FREQUENCY

350kHz

0V

INTV

DCꢀVoltage

535kHz

CC

Resistor

DCꢀVoltage

50kHz–900kHz

C ,ꢀholdsꢀtheꢀvoltageꢀatꢀtheꢀVCOꢀinput.

LPꢀ

AnyꢀofꢀtheꢀAbove

ExternalꢀClock

Phase–Lockedꢀtoꢀ

ExternalꢀClock

NoteꢀthatꢀtheꢀLTC3858-1ꢀcanꢀonlyꢀbeꢀsynchronizedꢀtoꢀanꢀ

externalꢀ clockꢀ whoseꢀ frequencyꢀ isꢀ withinꢀ rangeꢀ ofꢀ theꢀ

LTC3858-1’sꢀ internalꢀ VCO,ꢀ whichꢀ isꢀ nominallyꢀ 55kHzꢀ

toꢀ1MHz.ꢀThisꢀisꢀguaranteedꢀtoꢀbeꢀbetweenꢀ75kHzꢀandꢀ

850kHz.ꢀ

38581fb

ꢁꢁ

INTV ꢀcurrentꢀisꢀtheꢀsumꢀofꢀtheꢀMOSFETꢀdriverꢀandꢀ

3.ꢀ

I RꢀlossesꢀareꢀpredictedꢀfromꢀtheꢀDCꢀresistancesꢀofꢀtheꢀ

1.ꢀ

2.ꢀ

TheꢀV ꢀcurrentꢀisꢀtheꢀDCꢀinputꢀsupplyꢀcurrentꢀgivenꢀ

LTC3858-1

applicaTions inForMaTion

Minimum On-Time Considerations

IN

inꢀtheꢀElectricalꢀCharacteristicsꢀtable,ꢀwhichꢀexcludesꢀ

Minimumꢀon-time,ꢀt

,ꢀisꢀtheꢀsmallestꢀtimeꢀdura-

ON(MIN)

MOSFETꢀdriverꢀandꢀcontrolꢀcurrents.ꢀV ꢀcurrentꢀtypi-

IN

tionꢀthatꢀtheꢀLTC3858-1ꢀisꢀcapableꢀofꢀturningꢀonꢀtheꢀtopꢀ

MOSFET.ꢀItꢀisꢀdeterminedꢀbyꢀinternalꢀtimingꢀdelaysꢀandꢀtheꢀ

gateꢀchargeꢀrequiredꢀtoꢀturnꢀonꢀtheꢀtopꢀMOSFET.ꢀLowꢀdutyꢀ

cycleꢀapplicationsꢀmayꢀapproachꢀthisꢀminimumꢀon-timeꢀ

limitꢀandꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀensureꢀthat:

callyꢀresultsꢀinꢀaꢀsmallꢀ(<0.1%)ꢀloss.

CC

controlꢀcurrents.ꢀTheꢀMOSFETꢀdriverꢀcurrentꢀresultsꢀ

fromꢀ switchingꢀ theꢀ gateꢀ capacitanceꢀ ofꢀ theꢀ powerꢀ

MOSFETs.ꢀEachꢀtimeꢀaꢀMOSFETꢀgateꢀisꢀswitchedꢀfromꢀ

lowꢀtoꢀhighꢀtoꢀlowꢀagain,ꢀaꢀpacketꢀofꢀcharge,ꢀdQ,ꢀmovesꢀ

V_OUT

t_ON(MIN)

<

V

f

_IN

( )

fromꢀINTV ꢀtoꢀground.ꢀTheꢀresultingꢀdQ/dtꢀisꢀaꢀcurrentꢀ

CC

ꢀ

outꢀofꢀINTV ꢀthatꢀisꢀtypicallyꢀmuchꢀlargerꢀthanꢀtheꢀ

CC

Ifꢀtheꢀdutyꢀcycleꢀfallsꢀbelowꢀwhatꢀcanꢀbeꢀaccommodatedꢀ

byꢀtheꢀminimumꢀon-time,ꢀtheꢀcontrollerꢀwillꢀbeginꢀtoꢀskipꢀ

cycles.ꢀTheꢀoutputꢀvoltageꢀwillꢀcontinueꢀtoꢀbeꢀregulated,ꢀ

butꢀtheꢀrippleꢀvoltageꢀandꢀcurrentꢀwillꢀincrease.

controlꢀcircuitꢀcurrent.ꢀInꢀcontinuousꢀmode,ꢀI

ꢀ

GATECHG

=ꢀf(Q ꢀ+ꢀQ ),ꢀwhereꢀQ ꢀandꢀQ ꢀareꢀtheꢀgateꢀchargesꢀofꢀ

T

B

T

B

theꢀtopsideꢀandꢀbottomꢀsideꢀMOSFETs.

ꢀ SupplyingꢀINTV ꢀfromꢀanꢀoutput-derivedꢀpowerꢀsourceꢀ

CC

Theꢀminimumꢀon-timeꢀforꢀtheꢀLTC3858-1ꢀisꢀapproximatelyꢀ

95ns.ꢀHowever,ꢀasꢀtheꢀpeakꢀsenseꢀvoltageꢀdecreasesꢀtheꢀ

minimumꢀon-timeꢀgraduallyꢀincreasesꢀupꢀtoꢀaboutꢀ130ns.ꢀ

Thisꢀisꢀofꢀparticularꢀconcernꢀinꢀforcedꢀcontinuousꢀapplica-

tionsꢀwithꢀlowꢀrippleꢀcurrentꢀatꢀlightꢀloads.ꢀIfꢀtheꢀdutyꢀcycleꢀ

dropsꢀbelowꢀtheꢀminimumꢀon-timeꢀlimitꢀinꢀthisꢀsituation,ꢀ

aꢀsigniﬁcantꢀamountꢀofꢀcycleꢀskippingꢀcanꢀoccurꢀwithꢀcor-

respondinglyꢀlargerꢀcurrentꢀandꢀvoltageꢀripple.

throughꢀ EXTV ꢀ willꢀ scaleꢀ theꢀ V ꢀ currentꢀ requiredꢀ

CC

IN

forꢀtheꢀdriverꢀandꢀcontrolꢀcircuitsꢀbyꢀaꢀfactorꢀofꢀ(Dutyꢀ

Cycle)/(Efﬁciency).ꢀForꢀexample,ꢀinꢀaꢀ20Vꢀtoꢀ5Vꢀapplica-

tion,ꢀ10mAꢀofꢀINTV ꢀcurrentꢀresultsꢀinꢀapproximatelyꢀ

CC

2.5mAꢀofꢀV ꢀcurrent.ꢀThisꢀreducesꢀtheꢀmidcurrentꢀlossꢀ

IN

fromꢀ10%ꢀorꢀmoreꢀ(ifꢀtheꢀdriverꢀwasꢀpoweredꢀdirectlyꢀ

fromꢀV )ꢀtoꢀonlyꢀaꢀfewꢀpercent.

IN

2

fuseꢀ(ifꢀused),ꢀMOSFET,ꢀinductor,ꢀcurrentꢀsenseꢀresis-

tor,ꢀandꢀinputꢀandꢀoutputꢀcapacitorꢀESR.ꢀInꢀcontinuousꢀ

modeꢀtheꢀaverageꢀoutputꢀcurrentꢀﬂowsꢀthroughꢀLꢀandꢀ

Efﬁciency Considerations

Theꢀpercentꢀefﬁciencyꢀofꢀaꢀswitchingꢀregulatorꢀisꢀequalꢀtoꢀ

theꢀoutputꢀpowerꢀdividedꢀbyꢀtheꢀinputꢀpowerꢀtimesꢀ100%.ꢀ

Itꢀisꢀoftenꢀusefulꢀtoꢀanalyzeꢀindividualꢀlossesꢀtoꢀdetermineꢀ

whatꢀisꢀlimitingꢀtheꢀefﬁciencyꢀandꢀwhichꢀchangeꢀwouldꢀ

produceꢀtheꢀmostꢀimprovement.ꢀPercentꢀefﬁciencyꢀcanꢀ

beꢀexpressedꢀas:

R

,ꢀbutꢀisꢀ“chopped”ꢀbetweenꢀtheꢀtopsideꢀMOSFETꢀ

SENSE

andꢀtheꢀsynchronousꢀMOSFET.ꢀIfꢀtheꢀtwoꢀMOSFETsꢀhaveꢀ

approximatelyꢀtheꢀsameꢀR

,ꢀthenꢀtheꢀresistanceꢀ

DS(ON)

ofꢀoneꢀMOSFETꢀcanꢀsimplyꢀbeꢀsummedꢀwithꢀtheꢀresis-

2

tancesꢀofꢀL,ꢀR

ꢀandꢀESRꢀtoꢀobtainꢀI Rꢀlosses.ꢀForꢀ

DS(ON)

SENSE

example,ꢀifꢀeachꢀR

ꢀ=ꢀ30mΩ,ꢀR ꢀ=ꢀ50mΩ,ꢀR

ꢀ

ꢀ %Efﬁciencyꢀ=ꢀ100%ꢀ–ꢀ(L1ꢀ+ꢀL2ꢀ+ꢀL3ꢀ+ꢀ...)

L

SENSE

=ꢀ10mΩꢀandꢀR ꢀ=ꢀ40mΩꢀ(sumꢀofꢀbothꢀinputꢀandꢀ

ESR

whereꢀL1,ꢀL2,ꢀetc.ꢀareꢀtheꢀindividualꢀlossesꢀasꢀaꢀpercent-

ageꢀofꢀinputꢀpower.

outputꢀcapacitanceꢀlosses),ꢀthenꢀtheꢀtotalꢀresistanceꢀ

isꢀ130mΩ.ꢀThisꢀresultsꢀinꢀlossesꢀrangingꢀfromꢀ3%ꢀtoꢀ

13%ꢀasꢀtheꢀoutputꢀcurrentꢀincreasesꢀfromꢀ1Aꢀtoꢀ5Aꢀforꢀ

aꢀ5Vꢀoutput,ꢀorꢀaꢀ4%ꢀtoꢀ20%ꢀlossꢀforꢀaꢀ3.3Vꢀoutput.ꢀ

Althoughꢀallꢀdissipativeꢀelementsꢀinꢀtheꢀcircuitꢀproduceꢀ

losses,ꢀfourꢀmainꢀsourcesꢀusuallyꢀaccountꢀforꢀmostꢀofꢀ

theꢀlossesꢀinꢀLTC3858-1ꢀcircuits:ꢀ1)ꢀICꢀV ꢀcurrent,ꢀ2)ꢀIN-

EfﬁciencyꢀvariesꢀasꢀtheꢀinverseꢀsquareꢀofꢀV ꢀforꢀtheꢀ

IN

OUT

2

TV ꢀregulatorꢀcurrent,ꢀ3)ꢀI Rꢀlosses,ꢀ4)ꢀtopsideꢀMOSFETꢀ

sameꢀexternalꢀcomponentsꢀandꢀoutputꢀpowerꢀlevel.ꢀTheꢀ

combinedꢀeffectsꢀofꢀincreasinglyꢀlowerꢀoutputꢀvoltagesꢀ

andꢀhigherꢀcurrentsꢀrequiredꢀbyꢀhighꢀperformanceꢀdigitalꢀ

systemsꢀisꢀnotꢀdoublingꢀbutꢀquadruplingꢀtheꢀimportanceꢀ

ofꢀlossꢀtermsꢀinꢀtheꢀswitchingꢀregulatorꢀsystem!

CC

transitionꢀlosses.

38581fb

ꢁꢂ

LTC3858-1

applicaTions inForMaTion

4.ꢀTransitionꢀlossesꢀapplyꢀonlyꢀtoꢀtheꢀtopsideꢀMOSFET(s),ꢀ canꢀalsoꢀbeꢀestimatedꢀbyꢀexaminingꢀtheꢀriseꢀtimeꢀatꢀtheꢀ

andꢀbecomeꢀsigniﬁcantꢀonlyꢀwhenꢀoperatingꢀatꢀhighꢀ pin.ꢀTheꢀI_THꢀexternalꢀcomponentsꢀshownꢀinꢀFigureꢀ12ꢀ

inputꢀ voltagesꢀ (t

y

picallyꢀ 15Vꢀ orꢀ greater).ꢀ Transitionꢀ circuitꢀwillꢀprovideꢀanꢀadequateꢀstartingꢀpointꢀforꢀmostꢀ

applications.

lossesꢀcanꢀbeꢀestimatedꢀfrom:

ꢀ ꢀ TransitionꢀLossꢀ=ꢀ(1.7) •ꢀV •ꢀ2 •ꢀI

•ꢀC

•ꢀf

TheꢀI ꢀseriesꢀR -C ꢀﬁlterꢀsetsꢀtheꢀdominantꢀpole-zeroꢀ

TH C C

ꢀ

INꢀ

ꢀ

O(MAX)ꢀ RSSꢀ

loopꢀcompensation.ꢀTheꢀvaluesꢀcanꢀbeꢀmodiﬁedꢀslightlyꢀ

(fromꢀ0.5ꢀtoꢀ2ꢀtimesꢀtheirꢀsuggestedꢀvalues)ꢀtoꢀoptimizeꢀ

transientꢀresponseꢀonceꢀtheꢀﬁnalꢀPCꢀlayoutꢀisꢀdoneꢀandꢀ

theꢀparticularꢀoutputꢀcapacitorꢀtypeꢀandꢀvalueꢀhaveꢀbeenꢀ

determined.ꢀTheꢀoutputꢀcapacitorsꢀneedꢀtoꢀbeꢀselectedꢀ

becauseꢀtheꢀvariousꢀtypesꢀandꢀvaluesꢀdetermineꢀtheꢀloopꢀ

gainꢀandꢀphase.ꢀAnꢀoutputꢀcurrentꢀpulseꢀofꢀ20%ꢀtoꢀ80%ꢀ

ofꢀfull-loadꢀcurrentꢀhavingꢀaꢀriseꢀtimeꢀofꢀ1µsꢀtoꢀ10µsꢀwillꢀ

ꢀ Otherꢀ“hidden”ꢀlossesꢀsuchꢀasꢀcopperꢀtraceꢀandꢀinternalꢀ

batteryꢀresistancesꢀcanꢀaccountꢀforꢀanꢀadditionalꢀ5%ꢀtoꢀ

10%ꢀefﬁciencyꢀdegradationꢀinꢀportableꢀsystems.ꢀItꢀisꢀ

veryꢀimportantꢀtoꢀincludeꢀtheseꢀ“system”ꢀlevelꢀlossesꢀ

duringꢀtheꢀdesignꢀphase.ꢀTheꢀinternalꢀbatteryꢀandꢀfuseꢀ

resistanceꢀlossesꢀcanꢀbeꢀminimizedꢀbyꢀmakingꢀsureꢀthatꢀ

C ꢀhasꢀadequateꢀchargeꢀstorageꢀandꢀveryꢀlowꢀESRꢀatꢀ

IN

theꢀswitchingꢀfrequency.ꢀAꢀ25Wꢀsupplyꢀwillꢀtypicallyꢀ

requireꢀ aꢀ minimumꢀ ofꢀ 20µFꢀ toꢀ 40µFꢀ ofꢀ capacitanceꢀ

havingꢀaꢀmaximumꢀofꢀ20mΩꢀtoꢀ50mΩꢀofꢀESR.ꢀTheꢀ

LTC3858-1ꢀ2-phaseꢀarchitectureꢀtypicallyꢀhalvesꢀthisꢀ

inputꢀcapacitanceꢀrequirementꢀoverꢀcompetingꢀsolu-

produceꢀoutputꢀvoltageꢀandꢀI ꢀpinꢀwaveformsꢀthatꢀwillꢀ

TH

giveꢀaꢀsenseꢀofꢀtheꢀoverallꢀloopꢀstabilityꢀwithoutꢀbreakingꢀ

theꢀfeedbackꢀloop.ꢀ

Placingꢀ aꢀ resistiveꢀ loadꢀ andꢀ aꢀ powerꢀ MOSFETꢀ directlyꢀ

tions.ꢀOtherꢀlossesꢀincludingꢀSchottkyꢀconductionꢀlossesꢀ acrossꢀtheꢀoutputꢀcapacitorꢀandꢀdrivingꢀtheꢀgateꢀwithꢀanꢀ

duringꢀdead-timeꢀandꢀinductorꢀcoreꢀlossesꢀgenerallyꢀ appropriateꢀsignalꢀgeneratorꢀisꢀaꢀpracticalꢀwayꢀtoꢀproduceꢀ

accountꢀforꢀlessꢀthanꢀ2%ꢀtotalꢀadditionalꢀloss.

aꢀrealisticꢀloadꢀstepꢀcondition.ꢀTheꢀinitialꢀoutputꢀvoltageꢀ

stepꢀresultingꢀfromꢀtheꢀstepꢀchangeꢀinꢀoutputꢀcurrentꢀmayꢀ

notꢀbeꢀwithinꢀtheꢀbandwidthꢀofꢀtheꢀfeedbackꢀloop,ꢀsoꢀthisꢀ

signalꢀcannotꢀbeꢀusedꢀtoꢀdetermineꢀphaseꢀmargin.ꢀThisꢀ

Checking Transient Response

Theꢀregulatorꢀloopꢀresponseꢀcanꢀbeꢀcheckedꢀbyꢀlookingꢀatꢀ

theꢀloadꢀcurrentꢀtransientꢀresponse.ꢀSwitchingꢀregulatorsꢀ

takeꢀseveralꢀcyclesꢀtoꢀrespondꢀtoꢀaꢀstepꢀinꢀDCꢀ(resistive)ꢀ

loadꢀcurrent.ꢀWhenꢀaꢀloadꢀstepꢀoccurs,ꢀV_OUTꢀshiftsꢀbyꢀ

anꢀamountꢀequalꢀtoꢀ∆I_LOADꢀ(ESR),ꢀwhereꢀESRꢀisꢀtheꢀef-

fectiveꢀseriesꢀresistanceꢀofꢀC_OUTꢀ.ꢀ∆I_LOADꢀalsoꢀbeginsꢀtoꢀ

chargeꢀorꢀdischargeꢀC_OUTꢀgeneratingꢀtheꢀfeedbackꢀerrorꢀ

signalꢀthatꢀforcesꢀtheꢀregulatorꢀtoꢀadaptꢀtoꢀtheꢀcurrentꢀ

changeꢀandꢀreturnꢀV_OUTꢀtoꢀitsꢀsteady-stateꢀvalue.ꢀDuringꢀ

thisꢀrecoveryꢀtimeꢀV_OUTꢀcanꢀbeꢀmonitoredꢀforꢀexcessiveꢀ

overshootꢀorꢀ ringing,ꢀ whichꢀ wouldꢀindicateꢀ aꢀ stabilityꢀ

problem.ꢀOPTI-LOOPꢀcompensationꢀallowsꢀtheꢀtransientꢀ

responseꢀtoꢀbeꢀoptimizedꢀoverꢀaꢀwideꢀrangeꢀofꢀoutputꢀ

capacitanceꢀandꢀESRꢀvalues.ꢀThe availability of the I_THpin

not only allows optimization of control loop behavior, but

it also provides a DC coupled and AC ﬁltered closed-loop

response test point. The DC step, rise time and settling

at this test point truly reﬂects the closed-loop response.ꢀ

Assumingꢀaꢀpredominantlyꢀsecondꢀorderꢀsystem,ꢀphaseꢀ

marginꢀand/orꢀdampingꢀfactorꢀcanꢀbeꢀestimatedꢀusingꢀtheꢀ

percentageꢀofꢀovershootꢀseenꢀatꢀthisꢀpin.ꢀTheꢀbandwidthꢀ

isꢀwhyꢀitꢀisꢀbetterꢀtoꢀlookꢀatꢀtheꢀI ꢀpinꢀsignalꢀwhichꢀisꢀinꢀ

TH

theꢀfeedbackꢀloopꢀandꢀisꢀtheꢀﬁlteredꢀandꢀcompensatedꢀ

controlꢀloopꢀresponse.ꢀ

TheꢀgainꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀincreasingꢀR ꢀ

C

andꢀtheꢀbandwidthꢀofꢀtheꢀloopꢀwillꢀbeꢀincreasedꢀbyꢀde-

creasingꢀC .ꢀIfꢀR ꢀisꢀincreasedꢀbyꢀtheꢀsameꢀfactorꢀthatꢀC ꢀ

C

isꢀdecreased,ꢀtheꢀzeroꢀfrequencyꢀwillꢀbeꢀkeptꢀtheꢀsame,ꢀ

therebyꢀkeepingꢀtheꢀphaseꢀshiftꢀtheꢀsameꢀinꢀtheꢀmostꢀ

criticalꢀfrequencyꢀrangeꢀofꢀtheꢀfeedbackꢀloop.ꢀTheꢀoutputꢀ

voltageꢀsettlingꢀbehaviorꢀisꢀrelatedꢀtoꢀtheꢀstabilityꢀofꢀtheꢀ

closed-loopꢀsystemꢀandꢀwillꢀdemonstrateꢀtheꢀactualꢀoverallꢀ

supplyꢀperformance.

Aꢀsecond,ꢀmoreꢀsevereꢀtransientꢀisꢀcausedꢀbyꢀswitchingꢀ

inꢀloadsꢀwithꢀlargeꢀ(>1µF)ꢀsupplyꢀbypassꢀcapacitors.ꢀTheꢀ

dischargedꢀbypassꢀcapacitorsꢀareꢀeffectivelyꢀputꢀinꢀparallelꢀ

withꢀC ,ꢀcausingꢀaꢀrapidꢀdropꢀinꢀV .ꢀNoꢀregulatorꢀcanꢀ

OUTꢀ

alterꢀitsꢀdeliveryꢀofꢀcurrentꢀquicklyꢀenoughꢀtoꢀpreventꢀthisꢀ

suddenꢀstepꢀchangeꢀinꢀoutputꢀvoltageꢀifꢀtheꢀloadꢀswitchꢀ

resistanceꢀisꢀlowꢀandꢀitꢀisꢀdrivenꢀquickly.ꢀIfꢀtheꢀratioꢀofꢀ

38581fb

ꢁꢃ

LTC3858-1

applicaTions inForMaTion

C

ꢀtoꢀC ꢀisꢀgreaterꢀthanꢀ1:50,ꢀtheꢀswitchꢀriseꢀtimeꢀ TheꢀpowerꢀdissipationꢀonꢀtheꢀtopsideꢀMOSFETꢀcanꢀbeꢀeasilyꢀ

LOAD

OUT

shouldꢀbeꢀcontrolledꢀsoꢀthatꢀtheꢀloadꢀriseꢀtimeꢀisꢀlimitedꢀ estimated.ꢀChoosingꢀaꢀFairchildꢀFDS6982SꢀdualꢀMOSFETꢀ

toꢀapproximatelyꢀ25ꢀ•ꢀC .ꢀThusꢀaꢀ10µFꢀcapacitorꢀwouldꢀ resultsꢀin:ꢀR ꢀ=ꢀ0.035Ω/0.022Ω,ꢀC ꢀ=ꢀ215pF.ꢀAtꢀ

LOAD

DS(ON)

MILLER

requireꢀaꢀ250µsꢀriseꢀtime,ꢀlimitingꢀtheꢀchargingꢀcurrentꢀ maximumꢀinputꢀvoltageꢀwithꢀT(estimated)ꢀ=ꢀ50°C:

toꢀaboutꢀ200mA.

²



3.3V

22V

P_MAIN

=

6A 1+ 0.005 50°C – 25°C

(

)

(

)(

)





Design Example

²6A

Asꢀ aꢀ designꢀ exampleꢀ forꢀ oneꢀ channel,ꢀ assumeꢀ V ꢀ =ꢀ

0.035Ω + 22V

2.5Ω 215pF •

(

) (

)

1

(

)(

)

IN

2

12V(nominal),ꢀV ꢀ=ꢀ22Vꢀ(max),ꢀV ꢀ=ꢀ3.3V,ꢀI ꢀ=ꢀ6A,ꢀ

IN

OUT

MAX





1

V

ꢀ=ꢀ50mVꢀandꢀfꢀ=ꢀ350kHz.

SENSE(MAX)

+

350kHz = 433mW

(

)







5V – 2.3V 2.3V



Theꢀinductanceꢀvalueꢀisꢀchosenꢀﬁrstꢀbasedꢀonꢀaꢀ30%ꢀrippleꢀ

currentꢀassumption.ꢀTheꢀhighestꢀvalueꢀofꢀrippleꢀcurrentꢀ

occursꢀatꢀtheꢀmaximumꢀinputꢀvoltage.ꢀTieꢀtheꢀFREQꢀpinꢀ

toꢀ GND,ꢀ generatingꢀ 350kHzꢀ operation.ꢀ Theꢀ minimumꢀ

inductanceꢀforꢀ30%ꢀrippleꢀcurrentꢀis:

ꢀ

Aꢀshort-circuitꢀtoꢀgroundꢀwillꢀresultꢀinꢀaꢀfoldedꢀbackꢀcur-

rentꢀof:





95ns 22V

(

)

25mV

0.006Ω 2

1

I_SC=

ꢀ

–

= 3.9A













3.9µH

V_OUT

f L

( )( )

V

IN





^OUT

1–

ꢀ

ΔI_L=



ꢀ

withꢀaꢀtypicalꢀvalueꢀofꢀR

=ꢀ0.125.ꢀTheꢀresultingꢀpowerꢀdissipatedꢀinꢀtheꢀbottomꢀ

ꢀandꢀδꢀ=ꢀ(0.005/°C)(25°C)ꢀ

DS(ON)

Aꢀ3.9µHꢀinductorꢀwillꢀproduceꢀ29%ꢀrippleꢀcurrent.ꢀTheꢀ

peakꢀinductorꢀcurrentꢀwillꢀbeꢀtheꢀmaximumꢀDCꢀvalueꢀplusꢀ

oneꢀhalfꢀtheꢀrippleꢀcurrent,ꢀorꢀ6.88A.ꢀIncreasingꢀtheꢀrippleꢀ

currentꢀwillꢀalsoꢀhelpꢀensureꢀthatꢀtheꢀminimumꢀon-timeꢀ

ofꢀ95nsꢀisꢀnotꢀviolated.ꢀTheꢀminimumꢀon-timeꢀoccursꢀatꢀ

MOSFETꢀis:

2

) (

P

= 3.9A 1.125 0.022Ω = 376mW

(

whichꢀisꢀlessꢀthanꢀfull-loadꢀconditions.

)(

)

ꢀ

^SYNC

maximumꢀV :

IN

C ꢀisꢀchosenꢀforꢀanꢀRMSꢀcurrentꢀratingꢀofꢀatꢀleastꢀ3Aꢀatꢀ

temperatureꢀassumingꢀonlyꢀthisꢀchannelꢀisꢀon.ꢀC ꢀisꢀ

chosenꢀwithꢀanꢀESRꢀofꢀ0.02Ωꢀforꢀlowꢀoutputꢀrippleꢀvolt-

age.ꢀTheꢀoutputꢀrippleꢀinꢀcontinuousꢀmodeꢀwillꢀbeꢀhighestꢀ

atꢀtheꢀmaximumꢀinputꢀvoltage.ꢀTheꢀoutputꢀvoltageꢀrippleꢀ

dueꢀtoꢀESRꢀisꢀapproximately:

IN

V_OUT

V

_IN

( )

3.3V

OUT

t_ON(MIN)

=

= 429ns

f

22V 350kHz

(

)

ꢀ

TheꢀequivalentꢀR

ꢀresistorꢀvalueꢀcanꢀbeꢀcalculatedꢀbyꢀ

SENSE

usingꢀtheꢀminimumꢀvalueꢀforꢀtheꢀmaximumꢀcurrentꢀsenseꢀ

thresholdꢀ(43mV):

ꢀ V

ꢀ=ꢀR (∆I )ꢀ=ꢀ0.02Ω(1.75A)ꢀ=ꢀ35mV

ESR L P-P

ORIPPLE

43mV

6.88A

R_SENSE

ꢀ

≤

= 0.006Ω

ꢀ

Choosingꢀ0.5%ꢀresistors:ꢀR ꢀ=ꢀ24.9kꢀandꢀR ꢀ=ꢀ77.7kꢀyieldsꢀ

A

B

anꢀoutputꢀvoltageꢀofꢀ3.296V.

38581fb

ꢁꢄ

1.ꢀ

2.ꢀ

AreꢀtheꢀtopꢀN-channelꢀMOSFETsꢀMTOP1ꢀandꢀMTOP2ꢀ

locatedꢀwithinꢀ1cmꢀofꢀeachꢀotherꢀwithꢀaꢀcommonꢀdrainꢀ

Areꢀtheꢀsignalꢀandꢀpowerꢀgroundsꢀkeptꢀseparate?ꢀTheꢀ

PC Board Layout Checklist

6.ꢀ

Keepꢀtheꢀswitchingꢀnodesꢀ(SW1,ꢀSW2),ꢀtopꢀgateꢀnodesꢀ

(TG1,ꢀTG2),ꢀandꢀboostꢀnodesꢀ(BOOST1,ꢀBOOST2)ꢀawayꢀ

fromꢀsensitiveꢀsmall-signalꢀnodes,ꢀespeciallyꢀfromꢀtheꢀ

oppositesꢀchannel’sꢀvoltageꢀandꢀcurrentꢀsensingꢀfeed-

backꢀpins.ꢀAllꢀofꢀtheseꢀnodesꢀhaveꢀveryꢀlargeꢀandꢀfastꢀ

movingꢀsignalsꢀandꢀthereforeꢀshouldꢀbeꢀkeptꢀonꢀtheꢀ

“outputꢀside”ꢀofꢀtheꢀLTC3858-1ꢀandꢀoccupyꢀminimumꢀ

PCꢀtraceꢀarea.

LTC3858-1

applicaTions inForMaTion

Whenꢀlayingꢀoutꢀtheꢀprintedꢀcircuitꢀboard,ꢀtheꢀfollowingꢀ

checklistꢀshouldꢀbeꢀusedꢀtoꢀensureꢀproperꢀoperationꢀofꢀ

theꢀIC.ꢀTheseꢀitemsꢀareꢀalsoꢀillustratedꢀgraphicallyꢀinꢀtheꢀ

layoutꢀdiagramꢀofꢀFigureꢀ10.ꢀFigureꢀ11ꢀillustratesꢀtheꢀcurrentꢀ

waveformsꢀpresentꢀinꢀtheꢀvariousꢀbranchesꢀofꢀtheꢀ2-phaseꢀ

synchronousꢀregulatorsꢀoperatingꢀinꢀtheꢀcontinuousꢀmode.ꢀ

Checkꢀtheꢀfollowingꢀinꢀyourꢀlayout:

7.ꢀUseꢀaꢀmodiﬁedꢀ“starꢀground”ꢀtechnique:ꢀaꢀlowꢀimped-

ance,ꢀ largeꢀ copperꢀ areaꢀ centralꢀ groundingꢀ pointꢀ onꢀ

theꢀsameꢀsideꢀofꢀtheꢀPCꢀboardꢀasꢀtheꢀinputꢀandꢀoutputꢀ

connectionꢀatꢀC ?ꢀDoꢀnotꢀattemptꢀtoꢀsplitꢀtheꢀinputꢀ

IN

capacitorsꢀwithꢀtie-insꢀforꢀtheꢀbottomꢀofꢀtheꢀINTV ꢀ

CC

decouplingꢀforꢀtheꢀtwoꢀchannelsꢀasꢀitꢀcanꢀcauseꢀaꢀlargeꢀ

resonantꢀloop.

decouplingꢀcapacitor,ꢀtheꢀbottomꢀofꢀtheꢀvoltageꢀfeedbackꢀ

resistiveꢀdividerꢀandꢀtheꢀSGNDꢀpinꢀofꢀtheꢀIC.

combinedꢀICꢀsignalꢀgroundꢀpinꢀandꢀtheꢀgroundꢀreturnꢀ PC Board Layout Debugging

ofꢀC

ꢀmustꢀreturnꢀtoꢀtheꢀcombinedꢀC ꢀ(–)ꢀter-

INTVCC

OUT

Startꢀwithꢀoneꢀcontrollerꢀonꢀatꢀaꢀtime.ꢀItꢀisꢀhelpfulꢀtoꢀuseꢀ

aꢀDC-50MHzꢀcurrentꢀprobeꢀtoꢀmonitorꢀtheꢀcurrentꢀinꢀtheꢀ

inductorꢀ whileꢀ testingꢀ theꢀ circuit.ꢀ Monitorꢀ theꢀ outputꢀ

switchingꢀnodeꢀ(SWꢀpin)ꢀtoꢀsynchronizeꢀtheꢀoscilloscopeꢀ

toꢀtheꢀinternalꢀoscillatorꢀandꢀprobeꢀtheꢀactualꢀoutputꢀvoltageꢀ

asꢀwell.ꢀCheckꢀforꢀproperꢀperformanceꢀoverꢀtheꢀoperatingꢀ

voltageꢀandꢀcurrentꢀrangeꢀexpectedꢀinꢀtheꢀapplication.ꢀTheꢀ

frequencyꢀofꢀoperationꢀshouldꢀbeꢀmaintainedꢀoverꢀtheꢀinputꢀ

minals.ꢀTheꢀpathꢀformedꢀbyꢀtheꢀtopꢀN-channelꢀMOSFET,ꢀ

SchottkyꢀdiodeꢀandꢀtheꢀC ꢀcapacitorꢀshouldꢀhaveꢀshortꢀ

IN

leadsꢀandꢀPCꢀtraceꢀlengths.ꢀTheꢀoutputꢀcapacitorꢀ(–)ꢀ

terminalsꢀshouldꢀbeꢀconnectedꢀasꢀcloseꢀasꢀpossibleꢀ

toꢀtheꢀ(–)ꢀterminalsꢀofꢀtheꢀinputꢀcapacitorꢀbyꢀplacingꢀ

theꢀcapacitorsꢀnextꢀtoꢀeachꢀotherꢀandꢀawayꢀfromꢀtheꢀ

Schottkyꢀloopꢀdescribedꢀabove.

3.ꢀDoꢀtheꢀLTC3858-1ꢀV ꢀpins’ꢀresistiveꢀdividersꢀconnectꢀ voltageꢀrangeꢀdownꢀtoꢀdropoutꢀandꢀuntilꢀtheꢀoutputꢀloadꢀ

FB

toꢀ theꢀ (+)ꢀ terminalsꢀ ofꢀ C ?ꢀ Theꢀ resistiveꢀ dividerꢀ dropsꢀbelowꢀtheꢀlowꢀcurrentꢀoperationꢀthreshold—typi-

OUT

mustꢀbeꢀconnectedꢀbetweenꢀtheꢀ(+)ꢀterminalꢀofꢀC

ꢀ

callyꢀ10%ꢀofꢀtheꢀmaximumꢀdesignedꢀcurrentꢀlevelꢀinꢀBurstꢀ

OUT

andꢀsignalꢀground.ꢀTheꢀfeedbackꢀresistorꢀconnectionsꢀ Modeꢀoperation.

shouldꢀnotꢀbeꢀalongꢀtheꢀhighꢀcurrentꢀinputꢀfeedsꢀfromꢀ

Theꢀdutyꢀcycleꢀpercentageꢀshouldꢀbeꢀmaintainedꢀfromꢀcycleꢀ

theꢀinputꢀcapacitor(s).

toꢀcycleꢀinꢀaꢀwell-designed,ꢀlowꢀnoiseꢀPCBꢀimplementation.ꢀ

–

+

4.ꢀAreꢀtheꢀSENSE ꢀandꢀSENSE ꢀleadsꢀroutedꢀtogetherꢀwithꢀ Variationꢀinꢀtheꢀdutyꢀcycleꢀatꢀaꢀsubharmonicꢀrateꢀcanꢀsug-

minimumꢀPCꢀtraceꢀspacing?ꢀTheꢀﬁlterꢀcapacitorꢀbetweenꢀ gestꢀnoiseꢀpickupꢀatꢀtheꢀcurrentꢀorꢀvoltageꢀsensingꢀinputsꢀ

+

–

SENSE ꢀandꢀSENSE ꢀshouldꢀbeꢀasꢀcloseꢀasꢀpossibleꢀ orꢀinadequateꢀloopꢀcompensation.ꢀOvercompensationꢀofꢀ

toꢀtheꢀIC.ꢀEnsureꢀaccurateꢀcurrentꢀsensingꢀwithꢀKelvinꢀ theꢀloopꢀcanꢀbeꢀusedꢀtoꢀtameꢀaꢀpoorꢀPCꢀlayoutꢀifꢀregula-

connectionsꢀatꢀtheꢀSENSEꢀresistor.

torꢀ bandwidthꢀ optimizationꢀ isꢀ notꢀ required.ꢀ Onlyꢀ afterꢀ

eachꢀcontrollerꢀisꢀcheckedꢀforꢀitsꢀindividualꢀperformanceꢀ

shouldꢀbothꢀcontrollersꢀbeꢀturnedꢀonꢀatꢀtheꢀsameꢀtime.ꢀ

Aꢀparticularlyꢀdifﬁcultꢀregionꢀofꢀoperationꢀisꢀwhenꢀoneꢀ

controllerꢀchannelꢀisꢀnearingꢀitsꢀcurrentꢀcomparatorꢀtripꢀ

pointꢀwhenꢀtheꢀotherꢀchannelꢀisꢀturningꢀonꢀitsꢀtopꢀMOSFET.ꢀ

Thisꢀoccursꢀaroundꢀ50%ꢀdutyꢀcycleꢀonꢀeitherꢀchannelꢀdueꢀ

5.ꢀIsꢀtheꢀINTV ꢀdecouplingꢀcapacitorꢀconnectedꢀcloseꢀ

CC

toꢀtheꢀIC,ꢀbetweenꢀtheꢀINTV ꢀandꢀtheꢀpowerꢀgroundꢀ

CC

pins?ꢀThisꢀcapacitorꢀcarriesꢀtheꢀMOSFETꢀdrivers’ꢀcur-

rentꢀpeaks.ꢀAnꢀadditionalꢀ1µFꢀceramicꢀcapacitorꢀplacedꢀ

immediatelyꢀnextꢀtoꢀtheꢀINTV ꢀandꢀPGNDꢀpinsꢀcanꢀhelpꢀ

CC

improveꢀnoiseꢀperformanceꢀsubstantially.

38581fb

ꢁꢅ

LTC3858-1

applicaTions inForMaTion

SS1

LTC3858-1

I

R

TH1

PU1

V

PULL-UP

(<6V)

V

PGOOD1

TG1

PGOOD1

FB1

L1

R

SENSE

+

–

V

SENSE1

FREQ

OUT1

SW1

C

B1

M1

M2

D1

BOOST1

BG1

C

OUT1

V

f

IN

1µF

IN

PLLIN/MODE

RUN1

R

C

IN

VIN

CERAMIC

PGND

GND

RUN2

EXTV

CC

V

OUT1

C

IN

C

SGND

INTVCC

V

IN

–

INTV

CC

SENSE2

OUT2

D2

1µF

CERAMIC

+

BG2

SENSE2

M4

M3

BOOST2

V

FB2

TH2

C

B2

SW2

TG2

I

R

SENSE

V

OUT2

SS2

L2

38581 F10

Figure 10. Recommended Printed Circuit Layout Diagram

toꢀtheꢀphasingꢀofꢀtheꢀinternalꢀclocksꢀandꢀmayꢀcauseꢀminorꢀ coincideꢀwithꢀhighꢀinputꢀvoltagesꢀandꢀlowꢀoutputꢀcurrents,ꢀ

dutyꢀcycleꢀjitter.

lookꢀforꢀcapacitiveꢀcouplingꢀbetweenꢀtheꢀBOOST,ꢀSW,ꢀTG,ꢀ

andꢀpossiblyꢀBGꢀconnectionsꢀandꢀtheꢀsensitiveꢀvoltageꢀ

andꢀcurrentꢀpins.ꢀTheꢀcapacitorꢀplacedꢀacrossꢀtheꢀcurrentꢀ

sensingꢀpinsꢀneedsꢀtoꢀbeꢀplacedꢀimmediatelyꢀadjacentꢀtoꢀ

theꢀpinsꢀofꢀtheꢀIC.ꢀThisꢀcapacitorꢀhelpsꢀtoꢀminimizeꢀtheꢀ

effectsꢀofꢀdifferentialꢀnoiseꢀinjectionꢀdueꢀtoꢀhighꢀfrequencyꢀ

capacitiveꢀ coupling.ꢀ Ifꢀ problemsꢀ areꢀ encounteredꢀ withꢀ

highꢀcurrentꢀoutputꢀloadingꢀatꢀlowerꢀinputꢀvoltages,ꢀlookꢀ

Reduceꢀ V ꢀ fromꢀ itsꢀ nominalꢀ levelꢀ toꢀ verifyꢀ operationꢀ

IN

ofꢀtheꢀregulatorꢀinꢀdropout.ꢀCheckꢀtheꢀoperationꢀofꢀtheꢀ

undervoltageꢀlockoutꢀcircuitꢀbyꢀfurtherꢀloweringꢀV ꢀwhileꢀ

IN

monitoringꢀtheꢀoutputsꢀtoꢀverifyꢀoperation.

Investigateꢀwhetherꢀanyꢀproblemsꢀexistꢀonlyꢀatꢀhigherꢀout-

putꢀcurrentsꢀorꢀonlyꢀatꢀhigherꢀinputꢀvoltages.ꢀIfꢀproblemsꢀ

38581fb

ꢁꢆ

LTC3858-1

applicaTions inForMaTion

SW1

L1

R

SENSE1

V

OUT1

D1

C

R

L1

OUT1

V

IN

R

IN

C

IN

SW2

L2

R

SENSE2

V

OUT2

D2

C

R

L2

OUT2

BOLD LINES INDICATE

HIGH SWITCHING

CURRENT. KEEP LINES

TO A MINIMUM LENGTH.

38581 F11

Figure 11. Branch Current Waveforms

forꢀinductiveꢀcouplingꢀbetweenꢀC ,ꢀSchottkyꢀandꢀtheꢀtopꢀ Theꢀoutputꢀvoltageꢀunderꢀthisꢀimproperꢀhookupꢀwillꢀstillꢀ

IN

MOSFETꢀcomponentsꢀtoꢀtheꢀsensitiveꢀcurrentꢀandꢀvoltageꢀ beꢀmaintainedꢀbutꢀtheꢀadvantagesꢀofꢀcurrentꢀmodeꢀcontrolꢀ

sensingꢀtraces.ꢀInꢀaddition,ꢀinvestigateꢀcommonꢀgroundꢀ willꢀnotꢀbeꢀrealized.ꢀCompensationꢀofꢀtheꢀvoltageꢀloopꢀwillꢀ

pathꢀvoltageꢀpickupꢀbetweenꢀtheseꢀcomponentsꢀandꢀtheꢀ beꢀ muchꢀ moreꢀ sensitiveꢀ toꢀ componentꢀ selection.ꢀ Thisꢀ

SGNDꢀpinꢀofꢀtheꢀIC.

behaviorꢀcanꢀbeꢀinvestigatedꢀbyꢀtemporarilyꢀshortingꢀoutꢀ

theꢀcurrentꢀsensingꢀresistor—don’tꢀworry,ꢀtheꢀregulatorꢀ

willꢀstillꢀmaintainꢀcontrolꢀofꢀtheꢀoutputꢀvoltage.

Anꢀembarrassingꢀproblem,ꢀwhichꢀcanꢀbeꢀmissedꢀinꢀanꢀ

otherwiseꢀproperlyꢀworkingꢀswitchingꢀregulator,ꢀresultsꢀ

whenꢀtheꢀcurrentꢀsensingꢀleadsꢀareꢀhookedꢀupꢀbackwards.ꢀ

38581fb

ꢁꢇ

LTC3858-1

Typical applicaTions

R

B1

215k

LTC3858-1

+

C

SENSE1

F1

INTV

CC

C1

1nF

15pF

100k

–

R

A1

68.1k

SENSE1

PGOOD1

BG1

L1

MBOT1

MTOP1

V

FB1

3.3µH

V

3.3V

5A

OUT1

C

150pF

ITH1A

SW1

R

C

SENSE1

7mΩ

OUT1

B1

0.47µF

BOOST1

TG1

150µF

R

15k

SS1

ITH1

I

TH1

D1

D2

C

820pF

ITH1

C

0.1µF

V

IN

V

IN

9V TO 38V

C

IN

22µF

SS1

INTV

CC

C

INT

4.7µF

PGND

PLLIN/MODE

SGND

MTOP2

MBOT2

EXTV

TG2

CC

RUN1

RUN2

FREQ

C

B2

0.47µF

BOOST2

L2

7.2µH

R

SENSE2

10mΩ

C

0.1µF

SS2

V

8.5V

3A

OUT2

SW2

BG2

SS2

C

680pF

OUT2

ITH2

R

27k

150µF

ITH2

I

TH2

C

100pF

C2

ITH2A

V

FB2

R

A2

44.2k

–

+

SENSE2

C

1nF

F2

39pF

SENSE2

R

B2

442k

38581 F12

C

, C : SANYO 10TPD150M

OUT1 OUT2

L1: SUMIDA CDEP105-3R2M

L2: SUMIDA CDEP105-7R2M

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

Start-Up

SW Node Waveforms

Efﬁciency vs Output Current

100

90

V

80

OUT2

V

= 8.5V

V

= 3.3V

OUT

2V/DIV

OUT

70

SW1

5V/DIV

60

50

V

OUT1

2V/DIV

40

30

20

10

0

SW2

5V/DIV

V

= 12V

IN

Burst Mode OPERATION

0.1 10

OUTPUT CURRENT (A)

3858 F12c

3858 F12d

20ms/DIV

1µs/DIV

0.000010.0001 0.001 0.01

1

38581 F12b

Figure 12. High Efﬁciency Dual 8.5V/3.3V Step-Down Converter

38581fb

ꢁꢈ

LTC3858-1

Typical applicaTions

High Efﬁciency Dual 2.5V/3.3V Step-Down Converter

R

B1

143k

LTC3858-1

+

C

SENSE1

INTV

CC

F1

C1

1nF

22pF

100k

–

R

A1

68.1k

SENSE1

PGOOD1

BG1

L1

2.4µH

MBOT1

MTOP1

V

FB1

V

2.5V

5A

OUT1

C

100pF

ITH1A

SW1

R

C

SENSE1

7mΩ

OUT1

B1

0.47µF

BOOST1

TG1

150µF

R

ITH1

22k

I

TH1

D1

D2

C

820pF

ITH1

C

SS1

0.01µF

V

IN

V

IN

4V TO 38V

C

IN

22µF

SS1

INTV

CC

C

INT

4.7µF

PGND

PLLIN/MODE

SGND

MTOP2

MBOT2

EXTV

TG2

CC

RUN1

RUN2

FREQ

C

B2

0.47µF

BOOST2

L2

3.2µH

R

SENSE2

7mΩ

C

SS2

0.01µF

V

3.3V

5A

OUT2

SW2

BG2

SS2

C

820pF

OUT2

ITH2

R

15k

150µF

ITH2

I

TH2

C

150pF

C2

ITH2A

V

FB2

R

A2

68.1k

–

+

SENSE2

C

1nF

F2

15pF

SENSE2

R

B2

215k

38581 F13

C

, C : SANYO 10TPD150M

OUT1 OUT2

L1: SUMIDA CDEP105-2R5

L2: SUMIDA CDEP105-3R2M

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

38581fb

ꢂ0

LTC3858-1

Typical applicaTions

High Efﬁciency Dual 12V/5V Step-Down Converter

R

B1

422k

+

C

SENSE1

INTV

F1

CC

C1

1nF

33pF

100k

–

R

A1

SENSE1

PGOOD1

BG1

30.1k

L1

8.8µH

MBOT1

MTOP1

V

FB1

V

12V

3A

OUT1

C

100pF

ITH1A

SW1

R

C

SENSE1

OUT1

B1

BOOST1

TG1

10mΩ

47µF

0.47µF

R

ITH1

33k

I

TH1

D1

D2

C

SS1

0.01µF

LTC3858-1

C

680pF

ITH1

V

IN

V

SS1

IN

12.5V TO 38V

C

IN

INTV

CC

C

22µF

INT

4.7µF

PGND

PLLIN/MODE

SGND

MTOP2

MBOT2

EXTV

TG2

CC

RUN1

RUN2

FREQ

R

C

FREQ

B2

BOOST2

60k

0.47µF

L2

4.3µH

R

SENSE2

7mΩ

C

0.01µF

SS2

V

OUT2

5V

SW2

BG2

SS2

5.5A

C

680pF

OUT2

ITH2

R

17k

150µF

ITH2

I

TH2

C

100pF

C2

ITH2A

V

FB2

C

: KEMET T525D476M016E035

: SANYO 10TPD150M

R

OUT1

OUT2

A2

–

+

SENSE2

C

75k

L1: SUMIDA CDEP105-8R8M

L2: SUMIDA CDEP105-4R3M

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

C

1nF

F2

15pF

SENSE2

R

B2

393k

38581 TA02a

38581fb

ꢂꢀ

LTC3858-1

Typical applicaTions

High Efﬁciency Dual 24V/5V Step-Down Converter

R

B1

487k

+

C

SENSE1

INTV

F1

CC

C1

1nF

18pF

100k

–

R

A1

SENSE1

PGOOD1

BG1

16.9k

L1

22µH

MBOT1

MTOP1

V

FB1

V

24V

1A

OUT1

C

100pF

ITH1A

SW1

R

C

SENSE1

OUT1

B1

BOOST1

TG1

25mΩ

22µF

25V

×2

0.47µF

R

46k

ITH1

I

TH1

D1

D2

C

0.01µF

SS1

CERAMIC

LTC3858-1

C

680pF

ITH1

V

IN

V

SS1

IN

24.5V TO 38V

C

IN

INTV

CC

C

4.7µF

22µF

INT

PGND

PLLIN/MODE

SGND

MTOP2

MBOT2

EXTV

TG2

CC

RUN1

RUN2

FREQ

R

C

FREQ

B2

BOOST2

60k

0.47µF

L2

4.3µH

R

SENSE2

7mΩ

C

0.01µF

SS2

V

5V

5A

OUT2

SW2

BG2

SS2

C

680pF

OUT2

ITH2

R

17k

150µF

ITH2

I

TH2

C

100pF

C2

ITH2A

V

FB2

R

A2

–

+

SENSE2

75k

C

: SANYO 10TPD150M

OUT2

L1: SUMIDA CDRH105R-220M

L2: SUMIDA CDEP105-4R3M

C

1nF

F2

15pF

SENSE2

MTOP1, MTOP2, MBOT1, MBOT2: VISHAY Si7848DP

R

B2

392k

38581 TA04

38581fb

ꢂꢁ

LTC3858-1

Typical applicaTions

High Efﬁciency Dual 1V/1.2V Step-Down Converter

R

B1

28.7k

+

C

SENSE1

F1

INTV

CC

C1

1nF

56pF

100k

–

R

A1

SENSE1

PGOOD1

BG1

115k

L1

0.47µH

MBOT1

MTOP1

V

FB1

V

OUT1

C

220pF

ITH1A

1V

SW1

C

R

OUT1 8A

C

SENSE1

4mΩ

B1

BOOST1

TG1

220µF

0.47µF

R

ITH1

3.93k

s2

I

TH1

D1

D2

LTC3858-1

C

1000pF

ITH1

C

SS1

0.01µF

V

IN

V

IN

12V

C

IN

SS1

22µF

INTV

CC

C

INT

4.7µF

PGND

PLLIN/MODE

SGND

MTOP2

MBOT2

EXTV

TG2

CC

RUN1

RUN2

FREQ

R

C

FREQ

B2

BOOST2

60k

0.47µF

L2

0.47µH

R

SENSE2

4mΩ

C

0.01µF

SS2

V

OUT2

1.2V

SW2

BG2

SS2

C

OUT2 8A

C

1000pF

ITH2

220µF

R

3.43k

ITH2

s2

I

TH2

C

220pF

C2

ITH2A

V

FB2

R

C

, C

: SANYO 2RSTPE220M

A2

OUT1 OUT2

–

+

SENSE2

115k

L1: SUMIDA CDEP105-3R2M

L2: SUMIDA CDEP105-7R2M

MTOP1, MTOP2: RENESAS RJK0305

MBOT1, MBOT2: RENESAS RJK0328

C

1nF

F2

56pF

SENSE2

R

B2

38581 TA03a

57.6k

38581fb

ꢂꢂ

LTC3858-1

Typical applicaTions

High Efﬁciency Dual 1V/1.2V Step-Down Converter with Inductor DCR Current Sensing

R

S1

1.18k

B1

28.7k

+

C

SENSE1

F1

INTV

CC

C1

0.1µF

56pF

100k

–

R

A1

PGOOD1

115k

L1

0.47µH

MBOT1

MTOP1

V

BG1

SW1

FB1

V

OUT1

C

200pF

ITH1A

1V

C

OUT1 8A

C

B1

BOOST1

TG1

220µF

0.47µF

R

ITH1

3.93k

s2

I

TH1

D1

D2

LTC3858-1

C

1000pF

ITH1

C

SS1

0.01µF

V

IN

V

IN

12V

C

IN

SS1

22µF

INTV

CC

C

INT

4.7µF

PGND

PLLIN/MODE

SGND

MTOP2

MBOT2

EXTV

TG2

CC

RUN1

RUN2

FREQ

R

C

FREQ

B2

BOOST2

65k

0.47µF

L2

0.47µH

C

0.01µF

SS2

V

OUT2

1.2V

SW2

BG2

SS2

C

OUT2 8A

C

1000pF

ITH2

220µF

R

3.93k

ITH2

s2

I

TH2

C

220pF

C2

ITH2A

V

FB2

R

A2

–

+

SENSE2

115k

C

, C

: SANYO 2R5TPE220M

OUT1 OUT2

L1, L2: SUMIDA IHL P2525CZERR47M06

MTOP1, MTOP2: RENESAS RJK0305

MBOT1, MBOT2: RENESAS RJK0328

C

0.1µF

F2

56pF

SENSE2

R

S2

1.18k

R

B2

57.6k

38581 TA05

38581fb

ꢂꢃ

LTC3858-1

package DescripTion

UFD Package

28-Lead Plastic QFN (4mm × 5mm)

(ReferenceꢀLTCꢀDWGꢀ#ꢀ05-08-1712ꢀRevꢀB)

0.70 p0.05

4.50 p 0.05

3.10 p 0.05

2.50 REF

2.65 p 0.05

3.65 p 0.05

PACKAGE

OUTLINE

0.25 p0.05

0.50 BSC

3.50 REF

4.10 p 0.05

5.50 p 0.05

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

PIN 1 NOTCH

R = 0.20 OR 0.35

s 45o CHAMFER

2.50 REF

R = 0.115

TYP

R = 0.05

TYP

0.75 p 0.05

4.00 p 0.10

(2 SIDES)

27

28

0.40 p 0.10

PIN 1

TOP MARK

(NOTE 6)

1

2

5.00 p 0.10

(2 SIDES)

3.50 REF

3.65 p 0.10

2.65 p 0.10

(UFD28) QFN 0506 REV B

0.25 p 0.05

0.50 BSC

0.200 REF

0.00 – 0.05

BOTTOM VIEW—EXPOSED PAD

NOTE:

1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X).

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE

MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION

ON THE TOP AND BOTTOM OF PACKAGE

38581fb

ꢂꢄ

LTC3858-1

package DescripTion

GN Package

28-Lead Plastic SSOP (Narrow .150 Inch)

(ReferenceꢀLTCꢀDWGꢀ#ꢀ05-08-1641)

.386 – .393*

(9.804 – 9.982)

.045 p.005

.033

(0.838)

REF

28 27 26 25 24 23 22 21 20 19 18 17 1615

.254 MIN

.150 – .165

.229 – .244

.150 – .157**

(5.817 – 6.198)

(3.810 – 3.988)

.0165 p.0015

.0250 BSC

1

2

3

4

5

6

7

8

9 10 11 12 13 14

RECOMMENDED SOLDER PAD LAYOUT

.015 p .004

(0.38 p 0.10)

.0532 – .0688

(1.35 – 1.75)

s 45o

.004 – .0098

(0.102 – 0.249)

.0075 – .0098

(0.19 – 0.25)

0o – 8o TYP

.016 – .050

(0.406 – 1.270)

.008 – .012

.0250

(0.635)

BSC

GN28 (SSOP) 0204

(0.203 – 0.305)

TYP

NOTE:

1. CONTROLLING DIMENSION: INCHES

INCHES

2. DIMENSIONS ARE IN

(MILLIMETERS)

3. DRAWING NOT TO SCALE

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

38581fb

ꢂꢅ

LTC3858-1

revision hisTory ^{(Revision history begins at Rev B)}

REV

DATE

DESCRIPTION

PAGE NUMBER

B

11/09 ChangeꢀtoꢀAbsoluteꢀMaximumꢀRatings

ChangeꢀtoꢀElectricalꢀCharacteristics

ChangeꢀtoꢀTypicalꢀPerformanceꢀCharacteristics

ChangeꢀtoꢀPinꢀFunctions

2

3,ꢀ4

6

8,ꢀ9

TextꢀChangesꢀtoꢀOperationꢀSection

TextꢀChangesꢀtoꢀApplicationsꢀInformationꢀSection

ChangeꢀtoꢀTableꢀ2

11,ꢀ12,ꢀ13

20,ꢀ21,ꢀ22,ꢀ23,ꢀ25

22

27

38

ChangeꢀtoꢀFigureꢀ10

ChangesꢀtoꢀRelatedꢀParts

38581fb

InformationꢀfurnishedꢀbyꢀLinearꢀTechnologyꢀCorporationꢀisꢀbelievedꢀtoꢀbeꢀaccurateꢀandꢀreliable.ꢀ

However,ꢀnoꢀresponsibilityꢀisꢀassumedꢀforꢀitsꢀuse.ꢀLinearꢀTechnologyꢀCorporationꢀmakesꢀnoꢀrepresenta-

tionꢀthatꢀtheꢀinterconnectionꢀofꢀitsꢀcircuitsꢀasꢀdescribedꢀhereinꢀwillꢀnotꢀinfringeꢀonꢀexistingꢀpatentꢀrights.

ꢂꢆ

LTC3858-1

relaTeD parTs

PART NUMBER

DESCRIPTION

COMMENTS

LTC3857/LTC3857-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀꢀ

Q

DC/DCꢀControllerꢀwithꢀ99%ꢀDutyꢀCycle

4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀI ꢀ=ꢀ50µA,

IN OUT Q

LTC3868/LTC3868-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀ

Q

DC/DCꢀControllerꢀwithꢀ99%ꢀDutyꢀCycle

4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ14V,ꢀI ꢀ=ꢀ170µA,

IN OUT Q

LTC3834/LTC3834-1 LowꢀI ,ꢀSynchronousꢀStep-DownꢀDC/DCꢀController

Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ140kHzꢀtoꢀ650kHz,ꢀ

4Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ10V,ꢀI ꢀ=ꢀ30µA,

Q

IN

OUT

Q

LTC3835/LTC3835-1 LowꢀI ,ꢀSynchronousꢀStep-DownꢀDC/DCꢀController

Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ140kHzꢀtoꢀ650kHz,ꢀ

4Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ10V,ꢀI ꢀ=ꢀ80µA,

Q

IN

OUT

Q

LT3845

LT3800

LTC3824

LowꢀI ,ꢀHighꢀVoltageꢀSynchronousꢀStep-Downꢀꢀ

AdjustableꢀFixedꢀOperatingꢀFrequencyꢀ100kHzꢀtoꢀ500kHz,ꢀ

4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ1.23Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀI ꢀ=ꢀ120µA,ꢀTSSOP-16

Q

DC/DCꢀController

IN

OUT

Q

LowꢀI ,ꢀHighꢀVoltageꢀSynchronousꢀStep-Downꢀꢀ

Fixedꢀ200kHzꢀOperatingꢀFrequency,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ1.23Vꢀ≤ꢀV ꢀ≤ꢀ36V,ꢀ

IN OUT

I ꢀ=ꢀ100µA,ꢀTSSOP-16

Q

DC/DCꢀController

LowꢀI ,ꢀHighꢀVoltageꢀDC/DCꢀController,ꢀ100%ꢀDutyꢀCycle SelectableꢀFixedꢀ200kHzꢀtoꢀ600kHzꢀOperatingꢀFrequency,ꢀ

Q

4Vꢀ≤ꢀV ꢀ≤ꢀ60V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀV ,ꢀI ꢀ=ꢀ40µA,ꢀMSOP-10E

IN

OUT

IN Q

LTC3850/LTC3850-1ꢀ Dualꢀ2-Phase,ꢀHighꢀEfﬁciencyꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ780kHz,ꢀ

LTC3850-2

DC/DCꢀControllers,ꢀR

ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀ 4Vꢀ≤ꢀV ꢀ≤ꢀ30V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V

SENSE IN OUT

Tracking

LTC3855

Dual,ꢀMultiphase,ꢀSynchronousꢀDC/DCꢀStep-Downꢀ

ControllerꢀwithꢀDiffampꢀandꢀDCRꢀTemperatureꢀ

Compensation

Phase-LockableꢀFixedꢀFrequencyꢀ250kHzꢀtoꢀ770kHz,ꢀꢀ

4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ12.5V

IN

OUT

LTC3853

TripleꢀOutput,ꢀMultiphaseꢀSynchronousꢀStep-Downꢀ

Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀ

4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀV ꢀUpꢀtoꢀ13.5V

DC/DCꢀController,ꢀR

Tracking

ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀ

SENSE

IN

OUT

LTC3854

LTC3775

SmallꢀFootprintꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ Fixedꢀ400kHzꢀOperatingꢀFrequency,ꢀ4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ

IN IN

DC/DCꢀController

0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀ2mmꢀ×ꢀ3mmꢀQFN-12,ꢀMSOP-12

OUT

HighꢀFrequencyꢀSynchronousꢀVoltageꢀModeꢀStep-Downꢀ FastꢀTransientꢀResponse,ꢀt

ꢀ=ꢀ30ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ

ON(MIN)

IN

DC/DCꢀController

0.6Vꢀ≤ꢀV ꢀ≤ꢀ0.8V ,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16

OUT IN

LTC3851A/ꢀ

LTC3851A-1

NoꢀR ™ꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀꢀ

SENSE

IN

DC/DCꢀController

4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16,ꢀ

IN

OUT

SSOP-16

LTC3878/LTC3879 NoꢀR

ꢀConstantꢀOn-TimeꢀSynchronousꢀStep-Downꢀ VeryꢀFastꢀTransientꢀResponse,ꢀt

ꢀ=ꢀ43ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ

SENSE

ON(MIN) IN

DC/DCꢀController

V

ꢀUpꢀ90%ꢀofꢀV ,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16,ꢀSSOP-16

OUT IN

LTM4600HV

10AꢀDC/DCꢀµModule^®ꢀCompleteꢀPowerꢀSupply

HighꢀEfﬁciency,ꢀCompactꢀSize,ꢀUltraFast™ꢀTransientꢀResponse,ꢀ

4.5Vꢀ≤ꢀV ꢀ≤ꢀ28V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5V,ꢀ15mmꢀ×ꢀ15mmꢀ×ꢀ2.8mm

IN

OUT

LTM4601AHV

12AꢀDC/DCꢀµModuleꢀCompleteꢀPowerꢀSupply

HighꢀEfﬁciency,ꢀCompactꢀSize,ꢀUltrafastꢀTransientꢀResponse,ꢀ

4.5Vꢀ≤ꢀV ꢀ≤ꢀ28V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5V,ꢀ15mmꢀ×ꢀ15mmꢀ×ꢀ2.8mm

IN

OUT

38581fb

LT 0110 REV B • PRINTED IN USA

Linear Technology Corporation

1630ꢀ McCarthyꢀ Blvd.,ꢀ Milpitas,ꢀ CAꢀ 95035-7417

ꢀ

ꢂꢇ

●

●ꢀ

 LINEAR TECHNOLOGY CORPORATION 2009

(408)ꢀ432-1900ꢀ ꢀFAX:ꢀ(408)ꢀ434-0507ꢀ www.linear.com


型号：	LTC3858IGN-1TRPBF
厂家：	Linear
描述：	Low IQ, Dual 2-Phase Synchronous Step-Down Controller 低IQ ，双两相同步降压型控制器控制器
文件：	总38页 (文件大小：585K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LTC3858IGN-1TRPBF [Linear]

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LTC3859

LTC3859

LTC3859A

LTC3859A

LTC3859AL

LTC3859EFE-PBF