LTC3878EGN#PBF [Linear]
LTC3878 - Fast, Wide Operating Range No RSENSE Step-Down DC/DC Controller; Package: SSOP; Pins: 16; Temperature Range: -40°C to 85°C;
| 型号： | LTC3878EGN#PBF |
| 厂家： | 
Linear |
| 描述： | LTC3878 - Fast, Wide Operating Range No RSENSE Step-Down DC/DC Controller; Package: SSOP; Pins: 16; Temperature Range: -40°C to 85°C 开关 光电二极管 |
| 文件： | 总26页 (文件大小：390K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3878
Fast,Wide Operating Range
No R
Step-Down
SENSE
DC/DC Controller
DescripTion
FeaTures
Theꢀ LTC®3878ꢀ isꢀ aꢀ synchronousꢀ step-downꢀ switchingꢀ
DC/DCꢀcontrollerꢀoptimizedꢀforꢀhighꢀswitchingꢀfrequencyꢀ
andꢀfastꢀtransientꢀresponse.ꢀTheꢀconstantꢀon-timeꢀvalleyꢀ
currentꢀmodeꢀarchitectureꢀallowsꢀforꢀaꢀwideꢀinputꢀrange,ꢀ
includingꢀveryꢀlowꢀdutyꢀfactorꢀoperation.ꢀNoꢀexternalꢀsenseꢀ
resistorꢀorꢀslopeꢀcompensationꢀisꢀrequired.ꢀTheꢀLTC3878ꢀ
isꢀpinꢀcompatibleꢀwithꢀtheꢀLTC1778ꢀinꢀapplicationsꢀthatꢀdoꢀ
n
ꢀ WideꢀV ꢀRange:ꢀ4Vꢀtoꢀ38Vꢀ
IN
n
n
n
n
n
n
n
n
n
n
n
n
n
n
ꢀ ±±1ꢀ0.8VꢀVoltageꢀReference
ꢀ ExtremelyꢀFastꢀTransientꢀResponse
ꢀ t
ꢀ NoꢀR
:ꢀ43nsꢀꢀ
ON(MIN)
™ꢀValleyꢀCurrentꢀModeꢀControl
SENSE
ꢀ StableꢀwithꢀLowꢀESRꢀCeramicꢀC
OUT
ꢀ SupportsꢀSmoothꢀStart-UpꢀIntoꢀPre-BiasedꢀOutput
ꢀ OptimizedꢀforꢀHighꢀStep-DownꢀRatios
notꢀuseꢀEXTV whileꢀofferingꢀbetterꢀefficiency.ꢀConsultꢀ
CCꢀ
withꢀtheꢀfactoryꢀtoꢀverifyꢀcompatibility.
ꢀ PinꢀCompatibleꢀwithꢀtheꢀLTC±778ꢀ(NoꢀEXTV ꢀPin)
CC
ꢀ PowerꢀGoodꢀOutputꢀVoltageꢀMonitor
ꢀ DualꢀN-ChannelꢀMOSFETꢀSynchronousꢀDrive
ꢀ AdjustableꢀSwitchingꢀFrequency
Operatingꢀfrequencyꢀisꢀsetꢀbyꢀanꢀexternalꢀresistorꢀandꢀ
compensatedꢀforꢀvariationsꢀinꢀV ꢀtoꢀofferꢀexcellentꢀlineꢀ
IN
stability.ꢀ Discontinuousꢀ modeꢀ operationꢀ providesꢀ highꢀ
efficiencyꢀduringꢀlightꢀloadꢀconditions.ꢀAꢀforcedꢀcontinu-
ousꢀcontrolꢀpinꢀallowsꢀtheꢀuserꢀtoꢀreduceꢀnoiseꢀandꢀRFꢀ
interference.ꢀSafetyꢀfeaturesꢀincludeꢀoutputꢀovervoltageꢀ
protectionꢀandꢀprogrammableꢀcurrentꢀlimitꢀwithꢀfoldback.ꢀ
Soft-startꢀcapabilityꢀforꢀsupplyꢀsequencingꢀisꢀaccomplishedꢀ
throughꢀanꢀexternalꢀtimingꢀcapacitor.ꢀTheꢀcurrentꢀlimitꢀisꢀ
userꢀprogrammable.
ꢀ ProgrammableꢀCurrentꢀLimitꢀwithꢀFoldback
ꢀ OutputꢀOvervoltageꢀProtection
ꢀ Smallꢀ16-PinꢀNarrowꢀSSOPꢀPackage
applicaTions
n
ꢀ DistributedꢀPowerꢀSystems
n
ꢀ EmbeddedꢀComputing
n
ꢀ CommunicationsꢀInfrastructure
TheꢀLTC3878ꢀallowsꢀoperationꢀfromꢀ4Vꢀtoꢀ38Vꢀatꢀtheꢀinputꢀ
L,ꢀLT,ꢀLTC,ꢀLTM,ꢀLinearꢀTechnologyꢀandꢀtheꢀLinearꢀlogoꢀareꢀregisteredꢀtrademarksꢀofꢀLinearꢀ
andꢀfromꢀ0.8Vꢀtoꢀ90%ꢀV ꢀatꢀtheꢀoutput.ꢀTheꢀLTC3878ꢀisꢀ
IN
TechnologyꢀCorporation.ꢀNoꢀR
ꢀisꢀaꢀtrademarkꢀofꢀLinearꢀTechnologyꢀCorporation.ꢀꢀ
SENSE
Allꢀotherꢀtrademarksꢀareꢀtheꢀpropertyꢀofꢀtheirꢀrespectiveꢀowners.ꢀꢀ
availableꢀinꢀaꢀsmallꢀ16-pinꢀnarrowꢀSSOPꢀpackage.
ProtectedꢀbyꢀU.S.ꢀPatents,ꢀincludingꢀ5481178,ꢀ6100678,ꢀ6580258,ꢀ5847554,ꢀ6304066.
Typical applicaTion
EfficiencyꢀvsꢀLoadꢀCurrent
HighꢀEfficiencyꢀStep-DownꢀConverter
100
432k
DISCONTINUOUS
90
0.1µF
I
ON
MODE
V
IN
80
RUN/SS
V
IN
4.5V TO 28V
220pF
RJK0305
0.56µH
TG
70
10µF
12.1k
LTC3878
CONTINUOUS
60
I
TH
SW
V
1.2V
15A
OUT
MODE
SGND BOOST
FCB
50
0.22µF
40
30
INTV
CC
330µF
s2
RJK0330
BG
4.7µF
V
V
= 12V
IN
OUT
20
10
0
= 1.2V
PGND
PGOOD
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
V
5.11k
10k
RNG
V
FB
0.01
0.1
1
10
100
LOAD CURRENT (A)
3878 G07
3878 TA01a
3878fa
ꢀ
FCB,ꢀV ꢀVoltages.................... –0.3VꢀtoꢀINTV ꢀ+ꢀ0.3V
ꢀ
StorageꢀTemperatureꢀRange................... –65°Cꢀtoꢀ150°C
I ꢀVoltage
................................................. –0.3Vꢀtoꢀ40V
RUN/SS
PGOOD
1
2
3
4
5
6
7
8
LTC3878
absoluTe MaxiMuM raTings
pin conFiguraTion
(Noteꢀ±)
TOP VIEW
InputꢀSupplyꢀVoltageꢀ(V )ꢀ......................... –0.3Vꢀtoꢀ40V
ON
IN
16 BOOST
15
14
13
12
11
10
9
TG
BOOSTꢀVoltageꢀ.......................................... –0.3Vꢀtoꢀ46V
V
RNG
SW
SWꢀVoltageꢀ................................................... –5Vꢀtoꢀ40V
FCB
PGND
BG
INTV ,ꢀ(BOOST-SW),ꢀRUN/SS,ꢀ
CC
I
TH
PGOODꢀVoltagesꢀ.......................................... –0.3Vꢀtoꢀ6V
SGND
INTV
CC
RNG
CC
I
ON
V
IN
V ,ꢀI ꢀVoltagesꢀ....................................... –0.3Vꢀtoꢀ2.7V
FB TH
V
NC
FB
OperatingꢀTemperatureꢀRangeꢀ(Noteꢀ4).... –40°Cꢀtoꢀ85°C
GN PACKAGE
16-LEAD PLASTIC SSOP NARROW
JunctionꢀTemperatureꢀ(Noteꢀ2)ꢀ............................. 125°C
T
ꢀ=ꢀ125°C,ꢀθ ꢀ=ꢀ110°C/W
JA
JMAX
LeadꢀTemperatureꢀ(Soldering,ꢀ10ꢀsec)ꢀ.................. 300°C
orDer inForMaTion
LEADꢀFREEꢀFINISH
LTC3878EGN#PBF
LTC3878IGN#PBF
TAPEꢀANDꢀREEL
PARTꢀMARKING*
3878
PACKAGEꢀDESCRIPTION
16-LeadꢀPlasticꢀSSOP
16-LeadꢀPlasticꢀSSOP
TEMPERATUREꢀRANGE
–40°Cꢀtoꢀ85°Cꢀ(Noteꢀ4)
–40°Cꢀtoꢀ85°Cꢀ(Noteꢀ4)
LTC3878EGN#TRPBF
LTC3878IGN#TRPBF
3878
ConsultꢀLTCꢀMarketingꢀforꢀpartsꢀspecifiedꢀwithꢀwiderꢀoperatingꢀtemperatureꢀranges.ꢀꢀ*Theꢀtemperatureꢀgradeꢀisꢀidentifiedꢀbyꢀaꢀlabelꢀonꢀtheꢀshippingꢀcontainer.
ConsultꢀLTCꢀMarketingꢀforꢀinformationꢀonꢀnon-standardꢀleadꢀbasedꢀfinishꢀparts.
Forꢀmoreꢀinformationꢀonꢀleadꢀfreeꢀpartꢀmarking,ꢀgoꢀto:ꢀhttp://www.linear.com/leadfree/ꢀꢀ
Forꢀmoreꢀinformationꢀonꢀtapeꢀandꢀreelꢀspecifications,ꢀgoꢀto:ꢀhttp://www.linear.com/tapeandreel/
elecTrical characTerisTics Theꢀlꢀdenotesꢀtheꢀspecificationsꢀwhichꢀapplyꢀoverꢀtheꢀfullꢀoperatingꢀ
ꢀ
temperatureꢀrange,ꢀotherwiseꢀspecificationsꢀareꢀatꢀTAꢀ=ꢀ25°C.ꢀVINꢀ=ꢀ±5Vꢀunlessꢀotherwiseꢀnoted.ꢀ
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
MainꢀControlꢀLoop
InputꢀOperatingꢀVoltageꢀRange
4
38
V
I
InputꢀDCꢀSupplyꢀCurrentꢀ
ꢀꢀꢀNormalꢀ
ꢀꢀꢀShutdownꢀSupplyꢀCurrent
ꢀ
ꢀ
ꢀ
µAꢀ
µA
Q
1500ꢀ
18
2000ꢀ
35
l
l
V
FeedbackꢀReferenceꢀVoltage
FeedbackꢀVoltageꢀLineꢀRegulation
FeedbackꢀVoltageꢀLoadꢀRegulation
FeedbackꢀInputꢀCurrent
ErrorꢀAmplifierꢀTransconductance
FCBꢀThreshold
I
ꢀ=ꢀ1.2Vꢀ(Noteꢀ3)
0.792
0.8
0.002
–0.05
–5
0.808
V
%/V
%
FBREF
TH
V ꢀ=ꢀ4Vꢀtoꢀ38V,ꢀI ꢀ=ꢀ1.2Vꢀ(Noteꢀ3)
IN
TH
I
TH
ꢀ=ꢀ0.5Vꢀtoꢀ1.9Vꢀ(Noteꢀ3)
–0.3
50
I
FB
V
ꢀ=ꢀ0.8V
FB
nA
g
m(EA)
I
TH
ꢀ=ꢀ1.2Vꢀ(Noteꢀ3)
1.4
1.7
2
mS
V
V
FCB
0.76
0.8
0.84
1
FCBꢀPinꢀCurrent
V
ꢀ=ꢀ0.8V
FCB
0
µA
t
t
On-Time
I
I
ꢀ=ꢀ30µAꢀ
ꢀ=ꢀ15µA
198ꢀ
396
233ꢀ
466
268ꢀ
536
nsꢀ
ns
ON
ON
ON
MinimumꢀOn-Time
I
ON
ꢀ=ꢀ180µA
43
75
ns
ON(MIN)
3878fa
ꢁ
LTC3878
elecTrical characTerisTics Theꢀlꢀdenotesꢀtheꢀspecificationsꢀwhichꢀapplyꢀoverꢀtheꢀfullꢀoperatingꢀ
ꢀ
temperatureꢀrange,ꢀotherwiseꢀspecificationsꢀareꢀatꢀTAꢀ=ꢀ25°C.ꢀVINꢀ=ꢀ±5Vꢀunlessꢀotherwiseꢀnoted.ꢀ
SYMBOL
PARAMETER
CONDITIONS
ꢀ=ꢀ30µA
MIN
TYP
MAX
UNITS
t
MinimumꢀOff-Time
I
220
300
ns
OFF(MIN)
ON
l
l
l
V
V
V
ValleyꢀCurrentꢀSenseꢀThresholdꢀ
V
V
V
ꢀ=ꢀ1V,ꢀV ꢀ=ꢀ0.76Vꢀ
108ꢀ
74ꢀ
133ꢀ
93ꢀ
165ꢀ
119ꢀ
224
mVꢀ
mVꢀ
mV
SENSE(MAX)
SENSE(MIN)
RUN/SS
RNG
RNG
RNG
FB
V
ꢀ–ꢀV
ꢀ
ꢀ=ꢀ0V,ꢀV ꢀ=ꢀ0.76Vꢀ
PGND
SW
FB
PeakꢀCurrentꢀ=ꢀValleyꢀ+ꢀRipple
ꢀ=ꢀINTV ,ꢀV ꢀ=ꢀ0.76V
152
186
CC FB
MinimumꢀCurrentꢀSenseꢀThresholdꢀ
V
RNG
V
RNG
V
RNG
ꢀ=ꢀ1V,ꢀV ꢀ=ꢀ0.84Vꢀ
–67ꢀ
–47ꢀ
–93
mVꢀ
mVꢀ
mV
FB
V
ꢀ–ꢀV
ꢀ
ꢀ=ꢀ0V,ꢀV ꢀ=ꢀ0.84Vꢀ
FB
PGND
SW
ForcedꢀContinuousꢀOperation
RUN/SSꢀPinꢀOnꢀThreshold
Soft-StartꢀChargingꢀCurrent
ꢀ=ꢀINTV ,ꢀV ꢀ=ꢀ0.84V
CC FB
V
V
ꢀRising
ꢀ=ꢀ0V
1.4
1.5
–1.2
3.3
3.6
2.5
1.2
2.5
0.7
20
1.6
V
µA
V
RUN/SS
RUN/SS
l
l
INTV
INTV
INTV ꢀUndervoltageꢀLockout
Falling
3.9
4
CC(UVLO)
CC
INTV ꢀUndervoltageꢀLockoutꢀRelease
Rising
V
CC(UVLOR)
CC
TGꢀDriverꢀPull-UpꢀOn-Resistance
TGꢀDriverꢀPull-DownꢀOn-Resistance
BGꢀDriverꢀPull-UpꢀOn-Resistance
BGꢀDriverꢀPull-DownꢀOn-Resistance
TGꢀRiseꢀTime
TGꢀHigh
TGꢀLow
BGꢀHigh
BGꢀLow
Ω
Ω
Ω
Ω
C
LOAD
C
LOAD
C
LOAD
C
LOAD
C
LOAD
ꢀ=ꢀ3300pFꢀ(Noteꢀ5)
ns
ns
ns
ns
ns
TGꢀFallꢀTime
ꢀ=ꢀ3300pFꢀ(Noteꢀ5)
20
BGꢀRiseꢀTime
ꢀ=ꢀ3300pFꢀ(Noteꢀ5)
20
BGꢀFallꢀTime
ꢀ=ꢀ3300pFꢀ(Noteꢀ5)
20
TG/BGꢀt
TG/BGꢀt
TopꢀGateꢀOffꢀtoꢀBottomꢀGateꢀOnꢀDelayꢀ
SynchronousꢀSwitch-OnꢀDelayꢀTime
ꢀ=ꢀ3300pfꢀEachꢀDriverꢀ(Noteꢀ5)
15
1D
2D
BottomꢀGateꢀOffꢀtoꢀTopꢀGateꢀOnꢀDelayꢀ
SynchronousꢀSwitch-OnꢀDelayꢀTime
C
LOAD
ꢀ=ꢀ3300pfꢀEachꢀDriverꢀ(Noteꢀ5)
15
ns
InternalꢀV ꢀRegulator
CC
InternalꢀV ꢀVoltage
6Vꢀ<ꢀV ꢀ<ꢀ38V
5.15
5.3
5.45
2
V
CC
IN
InternalꢀV ꢀLoadꢀRegulation
I
ꢀ=ꢀ0mAꢀtoꢀ20mA
CC
–0.1
%
CC
PGOODꢀOutput
PGOODꢀUpperꢀThreshold
PGOODꢀLowerꢀThreshold
PGOODꢀHysteresis
V
V
V
ꢀRising
5.5
7.5
–7.5
2
9.5
–9.5
3.5
%
%
%
V
FB
ꢀFalling
FB
–5.5
ꢀReturning
FB
PGOODꢀLowꢀVoltage
PGOODꢀTurn-OnꢀDelay
I
ꢀ=ꢀ5mA
0.15
12
0.4
PGOOD
µs
Noteꢀ±:ꢀStressesꢀbeyondꢀthoseꢀlistedꢀunderꢀAbsoluteꢀMaximumꢀRatingsꢀ
mayꢀcauseꢀpermanentꢀdamageꢀtoꢀtheꢀdevice.ꢀExposureꢀtoꢀanyꢀAbsoluteꢀ
MaximumꢀRatingꢀconditionꢀforꢀextendedꢀperiodsꢀmayꢀaffectꢀdeviceꢀ
reliabilityꢀandꢀlifetime.
Noteꢀ4:ꢀTheꢀLTC3878Eꢀisꢀguaranteedꢀtoꢀmeetꢀspecificationsꢀfromꢀ
0°Cꢀtoꢀ85°C.ꢀSpecificationsꢀoverꢀtheꢀ–40°Cꢀtoꢀ85°Cꢀoperatingꢀtemperatureꢀ
rangeꢀareꢀassuredꢀbyꢀdesign,ꢀcharacterizationꢀandꢀcorrelationꢀwithꢀ
statisticalꢀprocessꢀcontrols.ꢀTheꢀLTC3878Iꢀisꢀguaranteedꢀtoꢀmeetꢀ
specificationsꢀoverꢀtheꢀfullꢀ–40°Cꢀtoꢀ85°Cꢀoperatingꢀtemperatureꢀrange.
Noteꢀ2:ꢀT ꢀisꢀcalculatedꢀfromꢀtheꢀambientꢀtemperatureꢀT ꢀandꢀpowerꢀ
J
A
dissipationꢀP ꢀasꢀfollows:
Noteꢀ5:ꢀRiseꢀandꢀfallꢀtimeꢀareꢀmeasuredꢀusingꢀ10%ꢀandꢀ90%ꢀlevels.ꢀDelayꢀ
D
timesꢀareꢀmeasuredꢀusingꢀ50%ꢀlevels.
ꢀ
T ꢀ=ꢀT ꢀ+ꢀ(P ꢀ•ꢀ110°C/W)
J A D
Noteꢀ3:ꢀTheꢀLTC3878ꢀisꢀtestedꢀinꢀaꢀfeedbackꢀloopꢀthatꢀadjustsꢀV ꢀtoꢀ
FB
achieveꢀaꢀspecifiedꢀerrorꢀamplifierꢀoutputꢀvoltageꢀ(I ).
TH
3878fa
ꢂ
LTC3878
Typical perForMance characTerisTics
TransientꢀResponseꢀFCMꢀ
(ForcedꢀContinuousꢀMode)
TransientꢀResponseꢀFCMꢀ
PositiveꢀLoadꢀStep
TransientꢀResponseꢀFCMꢀ
NegativeꢀLoadꢀStep
V
V
SW
20V/DIV
SW
20V/DIV
V
(AC)
OUT
V
(AC)
OUT
50mV/DIV
V
(AC)
OUT
50mV/DIV
50mV/DIV
I
L
I
L
I
L
10A/DIV
10A/DIV
10A/DIV
I
I
LOAD
10A/DIV
I
LOAD
10A/DIV
LOAD
10A/DIV
3878 G03
3878 G01
3878 G02
5µs/DIV
50µs/DIV
5µs/DIV
LOAD STEP 10A TO 0A
LOAD STEP 0A TO 10A TO 0A
LOAD STEP 0A TO 10A
V
V
= 12V
V
V
= 12V
V
V
= 12V
IN
OUT
IN
OUT
IN
OUT
= 1.2V
= 1.2V
= 1.2V
FCB = 0V
FCB = 0V
FCB = 0V
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
TransientꢀResponseꢀDCMꢀ
(DiscontinuousꢀMode)
NormalꢀStart-Up,ꢀRUN/SSꢀ
ReleaseꢀfromꢀZero
Start-UpꢀVINꢀCycledꢀLowꢀandꢀHigh
V
IN
RUN/SS
2V/DIV
10V/DIV
V
(AC)
OUT
RUN/SS
5V/DIV
50mV/DIV
I
I
I
L
L
L
10A/DIV
10A/DIV
10A/DIV
V
OUT
V
I
OUT
LOAD
500mV/DIV
1V/DIV
10A/DIV
3878 G04
3878 G05
3878 G06
50µs/DIV
LOAD STEP 1A TO 11A TO 1A
50ms/DIV
100ms/DIV
V
V
= 12V
V
V
= 12V
IN
OUT
IN
OUT
= 1.2V
= 1.2V
FCB = 0V
FCB = 0V
V
V
= 12V
IN
OUT
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
= 1.2V
FCB = INTV
CC
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
EfficiencyꢀvsꢀLoadꢀCurrent
EfficiencyꢀvsꢀInputꢀVoltage
FrequencyꢀvsꢀInputꢀVoltage
100
90
80
70
60
50
40
30
20
10
0
420
410
400
390
380
370
360
350
340
330
320
310
300
100
95
90
85
80
75
70
65
60
55
50
DISCONTINUOUS
MODE
15A
15A CCM
CONTINUOUS
MODE
1A DCM
1A CCM
0A
V
V
= 12V
IN
OUT
V
OUT
= 1.2V
= 1.2V
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
SW FREQ = 400kHz
FIGURE 7 CIRCUIT
V
= 1.2V
OUT
FIGURE 7 CIRCUIT
0.01
0.1
1
10
100
4
12
16
(V)
20 24
28
8
4
8
16
(V)
20
24
28
12
LOAD CURRENT (A)
V
V
IN
IN
3878 G07
3878 G09
3878 G08
3878fa
ꢃ
LTC3878
Typical perForMance characTerisTics
FrequencyꢀvsꢀLoadꢀCurrent
LoadꢀRegulationꢀFCM
ITHꢀVoltageꢀvsꢀLoadꢀCurrent
2.5
2.3
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5
410
370
330
290
250
210
170
130
90
0
–0.02
–0.04
–0.06
CONTINUOUS MODE
V
V
= 15V
IN
OUT
= 1.2V
FIGURE 7 CIRCUIT
DISCONTINUOUS MODE
–0.08
–0.10
V
= 15V
OUT
IN
V
= 1.2V
–0.12
–0.14
–0.16
FIGURE 7 CIRCUIT
V
V
= 15V
IN
OUT
CONTINUOUS MODE
DISCONTINUOUS MODE
= 1.2V
50
FIGURE 7 CIRCUIT
10
0
5
10
LOAD CURRENT (A)
15
20
25
5
10
0
2
4
6
8
10
12
14
0
15
I
(A)
LOAD CURRENT (A)
LOAD
3878 G12
3878 G10
3878 G11
CurrentꢀSenseꢀVoltageꢀ
vsꢀITHꢀVoltage
On-TimeꢀvsꢀIONꢀCurrent
CurrentꢀLimitꢀFoldback
300
250
200
150
100
50
10000
1000
100
140
120
100
V
= 1V
FIGURE 7 CIRCUIT
RNG
FIGURE 7 CIRCUIT
80
60
40
20
0
0
V
V
V
V
V
= 0.2V
= 0.5V
= 1.0V
= 1.5V
= 2.0V
RNG
RNG
RNG
RNG
RNG
–50
–100
10
–150
1
10
CURRENT (µA)
100
0.2
0.4
(V)
0.8
0
0.6
0
2
2.5
0.5
1
1.5
I
I
VOLTAGE (V)
V
ON
TH
FB
3878 G14
3878 G15
3878 G13
MaximumꢀVDSꢀCurrentꢀSenseꢀ
ThresholdꢀvsꢀVRNGꢀVoltage
MaximumꢀVDSꢀCurrentꢀSenseꢀ
ThresholdꢀvsꢀRUN/SSꢀVoltage
FeedbackꢀReferenceꢀVoltageꢀvsꢀ
Temperature
140
120
100
80
300
250
0.808
0.806
0.804
0.802
0.800
0.798
0.796
0.794
0.792
200
150
100
60
40
50
0
20
0
30 50
TEMPERATURE (°C)
–50
–30
–10
10
70
90
110
2.5
RUN/SS VOLTAGE (V)
3.0
1.5
2.0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
VOLTAGE (V)
V
RNG
3878 G18
3878 G17
3878 G16
3878fa
ꢄ
LTC3878
Typical perForMance characTerisTics
ShutdownꢀCurrentꢀ
vsꢀInputꢀVoltage
ErrorꢀAmplifierꢀgmꢀvsꢀTemperature
QuiescentꢀCurrentꢀvsꢀINTVCC
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
35
30
25
20
15
10
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–50
0
50
100
4.5
5.0
6.0
4.0
6.5
7.0
5
10
15
20
25
30
35
40
5.5
INTV (V)
TEMPERATURE (°C)
INPUT VOLTAGE (V)
CC
3878 G19
3878 G21
3878 G20
INTVCCꢀLoadꢀRegulation
INTVCCꢀDropout
INTVCCꢀvsꢀINTVCCꢀILOAD
6
0
–0.2
–0.4
–0.6
–0.8
–1.0
–1.2
–1.4
–1.6
–1.8
–2.0
0
V
= 12V
V
= 4.5V
IN
V
IN
= 12V
IN
–200
5
4
INTV
I
INTV
LOAD
RISING
CC
LOAD
FALLING
CC
I
–400
–600
3
2
DO NOT EXCEED ≥50mA
CONTINUOUS
= 150mA,
–800
–1000
–1200
I
LIMIT
INTV > 0.7V
CC
LIMIT
1
0
I
= 22mA,
INTV < 0.7V
CC
0
10
20
30
40
50
0
10
20
30
40
50
0
50
100
(mA)
LOAD
150
200
INTV LOAD CURRENT (mA)
INTV LOAD CURRENT (mA)
I
CC
CC
3878 G22
3878 G23
3878 G24
RUN/SSꢀPinꢀCurrentꢀvsꢀ
Temperature
Efficiency:ꢀLTC3878ꢀvsꢀLTC±778
1.6
94
92
V
V
= 15V
IN
OUT
= 1.2V
FCB = INTV
CC
LTC3878
LTC1778
1.4
1.2
1.0
0.8
90
88
86
84
82
80
1.6%
Q
Q
= RJK0305DPB
= RJK0330DPB
T
B
L = PULSE PA0513.441NLT
f
V
V
= 300kHz
= 12V
= 1.2V
SW
IN
OUT
5
10
15
25
50
0
TEMPERATURE (°C)
0
20
–50
100
LOAD CURRENT (A)
3878 G25
3878 G26
3878fa
ꢅ
LTC3878
pin FuncTions
RUN/SSꢀ (Pinꢀ ±):ꢀ Runꢀ Controlꢀ andꢀ Soft-Startꢀ Input.ꢀ Aꢀ V ꢀ(Pinꢀ8):ꢀErrorꢀAmplifierꢀFeedbackꢀInput.ꢀThisꢀpinꢀcon-
FB
capacitorꢀtoꢀgroundꢀonꢀthisꢀpinꢀsetsꢀtheꢀrampꢀtimeꢀtoꢀfullꢀ nectsꢀtheꢀerrorꢀamplifierꢀtoꢀanꢀexternalꢀresistiveꢀdividerꢀ
outputꢀcurrentꢀ(approximatelyꢀ3s/µF)ꢀwhenꢀRUN/SSꢀisꢀ fromꢀV .ꢀ
OUT
open.ꢀTheꢀswitchingꢀoutputsꢀareꢀdisabledꢀwhenꢀbelowꢀ1.5V.ꢀ
NCꢀ(Pinꢀ9):ꢀForꢀfactoryꢀuseꢀonly.ꢀCanꢀbeꢀconnectedꢀtoꢀanyꢀ
voltageꢀequalꢀtoꢀorꢀlessꢀthanꢀINTV .
Theꢀdeviceꢀisꢀinꢀmicropowerꢀshutdownꢀwhenꢀunderꢀ0.7V.ꢀ
Ifꢀleftꢀopen,ꢀthereꢀisꢀanꢀinternalꢀ1.2µAꢀpull-upꢀcurrentꢀonꢀ
CC
V ꢀ(Pinꢀ±0):ꢀMainꢀInputꢀSupply.ꢀTheꢀsupplyꢀvoltageꢀcanꢀ
RUN/SS.ꢀINTV ꢀisꢀenabledꢀwhenꢀRUN/SSꢀexceedsꢀ0.7V.
IN
CC
rangeꢀfromꢀ4Vꢀtoꢀ38V.ꢀForꢀincreasedꢀnoiseꢀimmunityꢀde-
PGOODꢀ (Pinꢀ 2):ꢀ Powerꢀ Goodꢀ Output.ꢀ Thisꢀ open-drainꢀ
logicꢀoutputꢀisꢀpulledꢀtoꢀgroundꢀwhenꢀtheꢀoutputꢀvoltageꢀisꢀ
outsideꢀofꢀaꢀ 7.5%ꢀwindowꢀaroundꢀtheꢀregulationꢀpoint.
coupleꢀthisꢀpinꢀtoꢀPGNDꢀwithꢀanꢀRCꢀfilter.
INTV ꢀ (Pinꢀ ±±):ꢀ Internalꢀ 5.3Vꢀ Regulatorꢀ Output.ꢀ Theꢀ
CC
driverꢀandꢀcontrolꢀcircuitsꢀareꢀpoweredꢀfromꢀthisꢀvoltage.ꢀ
DecoupleꢀthisꢀpinꢀtoꢀPGNDꢀwithꢀaꢀminimumꢀofꢀ1µF,ꢀ10Vꢀ
X5RꢀorꢀX7Rꢀceramicꢀcapacitor.
V
ꢀ(Pinꢀ3):ꢀV ꢀSenseꢀVoltageꢀRangeꢀInput.ꢀTheꢀmaxi-
DS
RNG
mumꢀallowedꢀbottomꢀMOSFETꢀV ꢀsenseꢀvoltageꢀbetweenꢀ
DS
RNG
SWꢀandꢀPGNDꢀisꢀequalꢀtoꢀ(0.133)V .ꢀTheꢀvoltageꢀappliedꢀ
BGꢀ(Pinꢀ±2):ꢀBottomꢀGateꢀDrive.ꢀThisꢀpinꢀdrivesꢀtheꢀgateꢀ
toꢀV ꢀcanꢀbeꢀanyꢀvalueꢀbetweenꢀ0.2Vꢀandꢀ2V.ꢀIfꢀV ꢀisꢀ
RNG
RNG
ofꢀtheꢀbottomꢀN-ChannelꢀpowerꢀMOSFETꢀbetweenꢀPGNDꢀ
tiedꢀtoꢀSGND,ꢀtheꢀdeviceꢀoperatesꢀwithꢀaꢀmaximumꢀvalleyꢀ
andꢀINTV .
currentꢀsenseꢀthresholdꢀofꢀ93mVꢀtypical.ꢀIfꢀV ꢀisꢀtiedꢀ
CC
RNG
toꢀINTV ,ꢀtheꢀdeviceꢀoperatesꢀwithꢀaꢀmaximumꢀvalleyꢀ
CC
PGNDꢀ(Pinꢀ±3):ꢀPowerꢀGround.ꢀConnectꢀthisꢀpinꢀasꢀcloseꢀ
currentꢀsenseꢀthresholdꢀofꢀ186mVꢀtypical.
asꢀpracticalꢀtoꢀtheꢀsourceꢀofꢀtheꢀbottomꢀN-channelꢀpowerꢀ
MOSFET,ꢀtheꢀ(–)ꢀterminalꢀofꢀC
ꢀandꢀtheꢀ(–)ꢀterminalꢀ
FCBꢀ(Pinꢀ4):ꢀForcedꢀContinuousꢀInput.ꢀConnectꢀthisꢀpinꢀ
INTVCC
ofꢀC
.
toꢀINTV ꢀtoꢀenableꢀdiscontinuousꢀmodeꢀforꢀlightꢀloadꢀ
VIN
CC
operation.ꢀConnectꢀthisꢀpinꢀtoꢀSGNDꢀtoꢀforceꢀcontinuousꢀ
SWꢀ(Pinꢀ±4):ꢀSwitchꢀNode.ꢀTheꢀ(–)ꢀterminalꢀofꢀtheꢀbootstrapꢀ
modeꢀoperationꢀinꢀallꢀconditions.
capacitor,ꢀC ,ꢀconnectsꢀtoꢀthisꢀnode.ꢀThisꢀpinꢀswingsꢀfromꢀ
B
aꢀdiodeꢀvoltageꢀbelowꢀgroundꢀupꢀtoꢀV .
I ꢀ(Pinꢀ5):ꢀCurrentꢀControlꢀThresholdꢀandꢀErrorꢀAmplifierꢀ
TH
IN
CompensationꢀPoint.ꢀTheꢀcurrentꢀcomparatorꢀthresholdꢀ
increasesꢀwithꢀthisꢀcontrolꢀvoltage.ꢀTheꢀvoltageꢀrangesꢀ
fromꢀ0Vꢀtoꢀ2.4V,ꢀwithꢀ0.8Vꢀcorrespondingꢀtoꢀzeroꢀsenseꢀ
voltageꢀ(zeroꢀcurrent).
TGꢀ(Pinꢀ±5):ꢀTopꢀGateꢀDrive.ꢀThisꢀpinꢀdrivesꢀtheꢀgateꢀofꢀtheꢀ
topꢀN-channelꢀpowerꢀMOSFETꢀbetweenꢀSWꢀandꢀBOOST.
BOOSTꢀ(Pinꢀ±6):ꢀBoostedꢀFloatingꢀDriverꢀSupply.ꢀTheꢀ(+)ꢀ
terminalꢀofꢀtheꢀbootstrapꢀcapacitor,ꢀC ,ꢀconnectsꢀtoꢀthisꢀ
B
SGNDꢀ(Pinꢀ6):ꢀSignalꢀGround.ꢀAllꢀsmall-signalꢀcomponentsꢀ
shouldꢀbeꢀconnectedꢀtoꢀSGND.ꢀConnectꢀSGNDꢀtoꢀPGNDꢀ
usingꢀaꢀsingleꢀPCBꢀtrace.
node.ꢀThisꢀnodeꢀswingsꢀfromꢀ(INTV ꢀ–ꢀV
)ꢀtoꢀ
SCHOTTKY
CC
V ꢀ+ꢀ(INT ꢀ–ꢀV ).
IN
VCC
SCHOTTKY
I ꢀ(Pinꢀ7):ꢀOn-TimeꢀCurrentꢀInput.ꢀTieꢀaꢀresistorꢀfromꢀ
ON
V ꢀtoꢀthisꢀpinꢀtoꢀsetꢀtheꢀone-shotꢀtimerꢀcurrentꢀandꢀthusꢀ
IN
theꢀswitchingꢀfrequency.
3878fa
ꢆ
LTC3878
FuncTional DiagraM
R
ON
V
IN
FCB
4
C
VIN
I
ON
V
IN
7
10
0.8V
REF
0.8V
–
+
5.3V
LDO
OST
0.7V
F
t
=
(10pF)
R
S
ON
I
ON
BOOST
16
Q
FCNT
C
C
R
TG
B
DSS
20k
15
MT
MB
+
–
+
–
SW
14
I
I
REV
CMP
V
OUT
SWITCH
LOGIC
C
OUT
D
INTV
CC
B
RUN
OV
11
BG
1.4V
0.7V
INTVCC
V
RNG
3
12
PGND
13
sV
RNG
3.3µA
1
240k
PGOOD
2
NEG
CLMP
PG
+
–
POS
FCNT
0.6V
0.86V
+
+
–
–
–
CLMP
2.4V
RUN
OV
UV
+
–
–
+
R2
R1
1.16V
160µA
V
–
FB
1.5V
8
CURRENT
LIMIT
SOFT-START
+
–
1.7mS
SGND
6
s4
EA
1.2µA
+
–
0.74V
+
–
–90mV
0.8V
3878 FD
I
TH
C
RUN/SS
C
SS
5
1
C1
R
C
3878fa
ꢇ
LTC3878
operaTion
LTC±778ꢀCompatibility
currentꢀisꢀdetectedꢀandꢀpreventedꢀbyꢀtheꢀcurrentꢀreversalꢀ
comparatorꢀ I ,ꢀ whichꢀ shutsꢀ offꢀ MB.ꢀ Bothꢀ switchesꢀ
REVꢀ
TheꢀLTC3878ꢀisꢀcompatibleꢀwithꢀtheꢀLTC1778ꢀinꢀapplica-
remainꢀoffꢀwithꢀtheꢀoutputꢀcapacitorꢀsupplyingꢀtheꢀloadꢀ
tionsꢀwhichꢀdoꢀnotꢀuseꢀtheꢀEXTV ꢀfunction.ꢀTheꢀLTC3878ꢀ
CC
currentꢀuntilꢀtheꢀEAꢀmovesꢀtheꢀI ꢀvoltageꢀaboveꢀtheꢀzeroꢀ
TH
offersꢀimprovedꢀgateꢀdriveꢀandꢀreducedꢀdeadꢀtime,ꢀwhichꢀ
currentꢀlevelꢀ(0.8V)ꢀtoꢀinitiateꢀanotherꢀswitchingꢀcycle.ꢀ
allowsꢀhigherꢀefficiencyꢀthanꢀtheꢀLTC1778.ꢀOnꢀtheꢀLTC1778ꢀ
WhenꢀtheꢀFCBꢀ(forcedꢀcontinuousꢀbar)ꢀpinꢀisꢀbelowꢀtheꢀ
Pinꢀ9ꢀisꢀEXTV ,ꢀbutꢀonꢀtheꢀLTC3878ꢀitꢀisꢀaꢀnoꢀconnect.ꢀ
CC
internalꢀFCBꢀthresholdꢀreference,ꢀV ,ꢀtheꢀregulatorꢀisꢀ
FCB
Theꢀotherꢀnotableꢀdifferenceꢀisꢀthatꢀtheꢀshutdownꢀlatch-
offꢀtimerꢀisꢀremoved.ꢀTheꢀLTC3878ꢀshouldꢀbeꢀaꢀdropꢀin,ꢀ
pin-for-pinꢀreplacementꢀinꢀmostꢀapplicationsꢀthatꢀdoꢀnotꢀ
forcedꢀtoꢀoperateꢀinꢀcontinuousꢀmodeꢀbyꢀdisablingꢀreversalꢀ
comparator,ꢀI ,ꢀtherebyꢀallowingꢀtheꢀinductorꢀcurrentꢀtoꢀ
REVꢀ
becomeꢀnegative.
useꢀEXTV .ꢀTheꢀLTC3878ꢀshouldꢀbeꢀtestedꢀandꢀverifiedꢀ
CC
inꢀeachꢀapplicationꢀwithoutꢀassumingꢀcompatibility.ꢀCon-
tactꢀaꢀLinearꢀapplicationsꢀexpertꢀtoꢀanswerꢀanyꢀquestionsꢀ
regardingꢀLTC3878/LTC1778ꢀcompatibility.
Theꢀcontinuousꢀmodeꢀoperatingꢀfrequencyꢀcanꢀbeꢀdeter-
minedꢀbyꢀdividingꢀtheꢀcalculatedꢀdutyꢀcycle,ꢀV /V ,ꢀ
OUT IN
byꢀ theꢀ fixedꢀ on-time.ꢀ Theꢀ OSTꢀ generatesꢀ anꢀ on-timeꢀ
proportionalꢀ toꢀ theꢀ idealꢀ dutyꢀ cycle,ꢀ thusꢀ holdingꢀ theꢀ
MainꢀControlꢀLoop
frequencyꢀapproximatelyꢀconstantꢀwithꢀchangesꢀinꢀV .ꢀ
IN
Theꢀnominalꢀfrequencyꢀcanꢀbeꢀadjustedꢀwithꢀanꢀexternalꢀ
TheꢀLTC3878ꢀisꢀaꢀvalleyꢀcurrentꢀmodeꢀcontrollerꢀICꢀforꢀ
useꢀinꢀDC/DCꢀstep-downꢀconverters.ꢀInꢀnormalꢀcontinu-
ousꢀoperation,ꢀtheꢀtopꢀMOSFETꢀisꢀturnedꢀonꢀforꢀaꢀfixedꢀ
intervalꢀdeterminedꢀbyꢀaꢀone-shotꢀtimer,ꢀOST.ꢀWhenꢀtheꢀ
topꢀMOSFETꢀisꢀturnedꢀoff,ꢀtheꢀbottomꢀMOSFETꢀisꢀturnedꢀ
onꢀuntilꢀtheꢀcurrentꢀcomparator,ꢀICMPꢀ,ꢀtrips,ꢀrestartingꢀ
theꢀone-shotꢀtimerꢀandꢀinitiatingꢀtheꢀnextꢀcycle.ꢀInductorꢀ
valleyꢀcurrentꢀisꢀmeasuredꢀbyꢀsensingꢀtheꢀvoltageꢀbetweenꢀ
theꢀPGNDꢀandꢀSWꢀpinsꢀusingꢀtheꢀbottomꢀMOSFETꢀon-
resistance.ꢀTheꢀvoltageꢀonꢀtheꢀITHꢀpinꢀsetsꢀtheꢀcompara-
torꢀthresholdꢀcorrespondingꢀtoꢀinductorꢀvalleyꢀcurrent.ꢀ
TheꢀerrorꢀamplifierꢀEAꢀadjustsꢀthisꢀvoltageꢀbyꢀcomparingꢀ
theꢀfeedbackꢀsignalꢀVFBꢀfromꢀtheꢀoutputꢀvoltageꢀtoꢀtheꢀ
feedbackꢀreferenceꢀvoltageꢀVFBREFꢀ.ꢀIncreasingꢀtheꢀloadꢀ
currentꢀcausesꢀaꢀdropꢀinꢀtheꢀfeedbackꢀvoltageꢀrelativeꢀ
toꢀtheꢀreference.ꢀTheꢀEAꢀsensesꢀtheꢀfeedbackꢀvoltageꢀ
dropꢀandꢀadjustsꢀtheꢀITHꢀvoltageꢀhigherꢀuntilꢀtheꢀaverageꢀ
inductorꢀcurrentꢀmatchesꢀtheꢀloadꢀcurrent.
resistor,ꢀR .ꢀ
ON
Foldbackꢀcurrentꢀlimitingꢀisꢀprovidedꢀtoꢀprotectꢀagainstꢀ
lowꢀimpedanceꢀshorts.ꢀIfꢀtheꢀcontrollerꢀisꢀinꢀcurrentꢀlimitꢀ
andꢀV ꢀdropsꢀtoꢀlessꢀ50%ꢀofꢀregulation,ꢀtheꢀcurrentꢀlimitꢀ
OUT
set-pointꢀ“foldsꢀback”ꢀtoꢀprogressivelyꢀlowerꢀvalues.ꢀToꢀ
recoverꢀfromꢀfoldbackꢀcurrentꢀlimit,ꢀtheꢀexcessiveꢀloadꢀorꢀ
lowꢀimpedanceꢀshortꢀneedsꢀtoꢀbeꢀremoved.
PullingꢀtheꢀRUN/SSꢀpinꢀlowꢀforcesꢀtheꢀcontrollerꢀintoꢀitsꢀ
shutdownꢀstate,ꢀturningꢀoffꢀbothꢀMTꢀandꢀMB.ꢀReleasingꢀ
theꢀpinꢀallowsꢀanꢀinternalꢀ1.2µAꢀcurrentꢀsourceꢀtoꢀchargeꢀ
upꢀanꢀexternalꢀsoft-startꢀcapacitor,ꢀCSS.ꢀWhenꢀtheꢀRUN/
SSꢀpinꢀisꢀlessꢀthanꢀ0.7V,ꢀtheꢀdeviceꢀisꢀinꢀtheꢀlowꢀpowerꢀ
shutdownꢀconditionꢀwithꢀaꢀnominalꢀbiasꢀcurrentꢀofꢀ18µA.ꢀ
WhenꢀRUN/SSꢀisꢀgreaterꢀthanꢀ0.7Vꢀandꢀlessꢀthanꢀ1.5V,ꢀ
INTVCCꢀandꢀallꢀinternalꢀcircuitryꢀareꢀenabledꢀwhileꢀMTꢀandꢀ
MBꢀareꢀforcedꢀoff.ꢀCurrent-limitedꢀsoft-startꢀbeginsꢀwhenꢀ
RUN/SSꢀexceedsꢀ1.5V.ꢀNormalꢀoperationꢀatꢀfullꢀcurrentꢀ
limitꢀisꢀachievedꢀatꢀapproximatelyꢀ3VꢀonꢀRUN/SS.ꢀFoldbackꢀ
currentꢀlimitꢀisꢀdefeatedꢀduringꢀsoft-start.
WithꢀDCꢀcurrentꢀloadsꢀlessꢀthanꢀ1/2ꢀofꢀtheꢀpeak-to-peakꢀ
rippleꢀtheꢀinductorꢀcurrentꢀcanꢀdropꢀtoꢀzeroꢀorꢀbecomeꢀ
negative.ꢀInꢀdiscontinuousꢀoperation,ꢀnegativeꢀinductorꢀ
3878fa
ꢈ
LTC3878
applicaTions inForMaTion
TheꢀbasicꢀLTC3878ꢀapplicationꢀcircuitꢀisꢀshownꢀonꢀtheꢀfirstꢀ
pageꢀofꢀthisꢀdataꢀsheet.ꢀExternalꢀcomponentꢀselectionꢀisꢀ
largelyꢀdeterminedꢀbyꢀmaximumꢀloadꢀcurrentꢀandꢀbeginsꢀ
withꢀtheꢀselectionꢀofꢀsenseꢀresistanceꢀandꢀpowerꢀMOSFETꢀ
switches.ꢀTheꢀLTC3878ꢀusesꢀtheꢀon-resistanceꢀofꢀtheꢀsyn-
chronousꢀpowerꢀMOSFETꢀtoꢀdetermineꢀtheꢀinductorꢀcurrent.ꢀ
Theꢀdesiredꢀrippleꢀcurrentꢀandꢀoperatingꢀfrequencyꢀlargelyꢀ
Theꢀgateꢀdriveꢀvoltagesꢀareꢀsetꢀbyꢀtheꢀ5.3VꢀINTV ꢀsupply.ꢀ
CC
Consequently,ꢀlogic-levelꢀthresholdꢀMOSFETsꢀmustꢀbeꢀusedꢀ
inꢀLTC3878ꢀapplications.ꢀIfꢀtheꢀinputꢀvoltageꢀisꢀexpectedꢀ
toꢀdropꢀbelowꢀ5V,ꢀthenꢀsub-logicꢀlevelꢀthresholdꢀMOSFETsꢀ
shouldꢀbeꢀconsidered.
UsingꢀtheꢀbottomꢀMOSFETꢀasꢀtheꢀcurrentꢀsenseꢀelementꢀ
requiresꢀparticularꢀattentionꢀbeꢀpaidꢀtoꢀitsꢀon-resistance.ꢀ
MOSFETꢀon-resistanceꢀisꢀtypicallyꢀspecifiedꢀwithꢀaꢀmaxi-
determinesꢀtheꢀinductorꢀvalue.ꢀFinally,ꢀC ꢀisꢀselectedꢀforꢀitsꢀ
IN
abilityꢀtoꢀhandleꢀtheꢀlargeꢀRMSꢀcurrentꢀintoꢀtheꢀconverter,ꢀ
mumꢀvalueꢀR
ꢀatꢀ25°C.ꢀInꢀthisꢀcaseꢀadditionalꢀ
DS(ON)(MAX)
andꢀC ꢀisꢀchosenꢀwithꢀlowꢀenoughꢀESRꢀtoꢀmeetꢀoutputꢀ
OUT
marginꢀisꢀrequiredꢀtoꢀaccommodateꢀtheꢀriseꢀinꢀMOSFETꢀ
voltageꢀrippleꢀandꢀtransientꢀspecifications.
on-resistanceꢀwithꢀtemperature.
MaximumꢀV ꢀSenseꢀVoltageꢀandꢀV ꢀPin
Max VDS Sense Voltage
DS
RNG
RDS(ON)(MAX)
=
IOUT • ρT
Inductorꢀ currentꢀ isꢀ measuredꢀ byꢀ sensingꢀ theꢀ bottomꢀ
MOSFETꢀ V ꢀ voltageꢀ thatꢀ appearsꢀ betweenꢀ theꢀ PGNDꢀ
ꢀ
DS
Theꢀ ρ ꢀ termꢀ isꢀ aꢀ normalizationꢀ factorꢀ (unityꢀ atꢀ 25°C)ꢀ
T
andꢀSWꢀpins.ꢀTheꢀmaximumꢀallowedꢀV ꢀsenseꢀvoltageꢀisꢀ
DS
accountingꢀforꢀtheꢀsignificantꢀvariationꢀinꢀon-resistanceꢀ
withꢀtemperature,ꢀtypicallyꢀaboutꢀ0.4%/°C,ꢀasꢀshownꢀinꢀ
Figureꢀ1.ꢀForꢀaꢀmaximumꢀjunctionꢀtemperatureꢀofꢀ100°Cꢀ
setꢀbyꢀtheꢀvoltageꢀappliedꢀtoꢀtheꢀV ꢀpinꢀandꢀisꢀapproxi-
RNG
matelyꢀequalꢀtoꢀ(0.133)V .ꢀTheꢀcurrentꢀmodeꢀcontrolꢀ
RNG
loopꢀdoesꢀnotꢀallowꢀtheꢀinductorꢀcurrentꢀvalleysꢀtoꢀexceedꢀ
usingꢀaꢀvalueꢀofꢀρ ꢀ=ꢀ1.3ꢀisꢀreasonable.
T
(0.133)V .ꢀInꢀpractice,ꢀoneꢀshouldꢀallowꢀmargin,ꢀtoꢀac-
RNG
TheꢀpowerꢀdissipatedꢀbyꢀtheꢀtopꢀandꢀbottomꢀMOSFETsꢀ
dependsꢀuponꢀtheirꢀrespectiveꢀdutyꢀcyclesꢀandꢀtheꢀloadꢀ
current.ꢀWhenꢀtheꢀLTC3878ꢀisꢀoperatingꢀinꢀcontinuousꢀ
mode,ꢀtheꢀdutyꢀcyclesꢀforꢀtheꢀMOSFETsꢀare:
countꢀforꢀvariationsꢀinꢀtheꢀLTC3878ꢀandꢀexternalꢀcomponentꢀ
values.ꢀAꢀgoodꢀguideꢀforꢀsettingꢀV ꢀis:
RNG
ꢀ V ꢀ=ꢀ7.5ꢀ•ꢀ(MaximumꢀV ꢀSenseꢀVoltage)
RNG
DS
AnꢀexternalꢀresistiveꢀdividerꢀfromꢀINTV ꢀcanꢀbeꢀusedꢀ
CC
VOUT
toꢀsetꢀtheꢀvoltageꢀonꢀtheꢀV ꢀpinꢀbetweenꢀ0.2Vꢀandꢀ2V,ꢀ
RNG
DTOP
DBOT
=
=
V
resultingꢀinꢀpeakꢀsenseꢀvoltagesꢀbetweenꢀ26.6mVꢀandꢀ
IN
266mV.ꢀTheꢀwideꢀpeakꢀvoltageꢀsenseꢀrangeꢀallowsꢀforꢀaꢀ
V – VOUT
IN
varietyꢀofꢀapplicationsꢀandꢀMOSFETꢀchoices.ꢀTheꢀV ꢀpinꢀ
RNG
V
ꢀ
IN
canꢀalsoꢀbeꢀtiedꢀtoꢀeitherꢀSGNDꢀorꢀINTV ꢀtoꢀforceꢀinternalꢀ
CC
defaults.ꢀWhenꢀV ꢀisꢀtiedꢀtoꢀSGND,ꢀtheꢀdeviceꢀoperatesꢀ
RNG
2.0
1.5
1.0
0.5
0
atꢀaꢀvalleyꢀcurrentꢀsenseꢀthresholdꢀofꢀ93mVꢀtypical.ꢀIfꢀV
ꢀ
RNG
isꢀtiedꢀtoꢀINTV ,ꢀtheꢀdeviceꢀoperatesꢀatꢀaꢀvalleyꢀcurrentꢀ
CC
senseꢀthresholdꢀofꢀ186mVꢀtypical.
PowerꢀMOSFETꢀSelection
Theꢀ LTC3878ꢀ requiresꢀ twoꢀ externalꢀ N-channelꢀ powerꢀ
MOSFETs,ꢀoneꢀforꢀtheꢀtopꢀ(main)ꢀswitchꢀandꢀoneꢀforꢀtheꢀ
bottomꢀ(synchronous)ꢀswitch.ꢀImportantꢀparametersꢀforꢀ
theꢀpowerꢀMOSFETsꢀareꢀtheꢀbreakdownꢀvoltageꢀV
,ꢀ
BR(DSS)
50
100
–50
150
0
thresholdꢀ voltageꢀ V
,ꢀ on-resistanceꢀ R
,ꢀ re-
GS(TH)
DS(ON)
JUNCTION TEMPERATURE (°C)
verseꢀtransferꢀcapacitanceꢀC ꢀandꢀmaximumꢀcurrentꢀ
RSS
3878 F01
I
.
DS(MAX)
Figureꢀ±.ꢀRDS(ON)ꢀvsꢀTemperature
3878fa
ꢀ0
LTC3878
applicaTions inForMaTion
TheꢀresultingꢀpowerꢀdissipationꢀinꢀtheꢀMOSFETsꢀatꢀmaxi-
mumꢀoutputꢀcurrentꢀare:
V
IN
MILLER EFFECT
V
V
GS
PTOP =DTOP •IOUT(MAX)2 •ρτ(TOP) •RDS(ON)(MAX)
a
b
+
–
V
DS
I
2 OUT(MAX)
+
Q
IN
V
+V
C
GS
(
)
IN
MILLER
–
2
C
= (Q – Q )/V
B A DS
MILLER
3878 F02
DRTGHIGH
INTVCC – VMILLER VMILLER
PBOT =DBOT •IOUT(MAX)2 •ρτ(BOT) •RDS(ON)(MAX)
DRTGLOW
+
f
OSC
Figureꢀ2.ꢀGateꢀChargeꢀCharacteristic
V
2
BothꢀMOSFETsꢀhaveꢀI Rꢀpowerꢀloss,ꢀandꢀtheꢀtopꢀMOSFETꢀ
includesꢀanꢀadditionalꢀtermꢀforꢀtransitionꢀloss,ꢀwhichꢀareꢀ
ꢀ
highestꢀatꢀhighꢀinputꢀvoltages.ꢀForꢀV ꢀ<ꢀ20V,ꢀtheꢀhighꢀcur-
DR
ꢀisꢀpull-upꢀdriverꢀresistanceꢀandꢀDR
ꢀisꢀtheꢀ
IN
TGHIGH
TGLOW
rentꢀefficiencyꢀgenerallyꢀimprovesꢀwithꢀlargerꢀMOSFETs,ꢀ
TGꢀdriverꢀpull-downꢀresistance.ꢀV
ꢀisꢀtheꢀMillerꢀef-
MILLER
whileꢀforꢀV ꢀ>ꢀ20V,ꢀtheꢀtransitionꢀlossesꢀrapidlyꢀincreaseꢀ
fectꢀV ꢀvoltageꢀandꢀisꢀtakenꢀgraphicallyꢀfromꢀtheꢀpowerꢀ
MOSFETꢀdataꢀsheet.
IN
GS
toꢀtheꢀpointꢀthatꢀtheꢀuseꢀofꢀaꢀhigherꢀR
ꢀdeviceꢀwithꢀ
DS(ON)
lowerꢀ C
ꢀ actuallyꢀ providesꢀ higherꢀ efficiency.ꢀ Theꢀ
MILLER
MOSFETꢀinputꢀcapacitanceꢀisꢀaꢀcombinationꢀofꢀseveralꢀ
componentsꢀbutꢀcanꢀbeꢀtakenꢀfromꢀtheꢀtypicalꢀ“gateꢀcharge”ꢀ
curveꢀincludedꢀonꢀtheꢀmostꢀdataꢀsheetsꢀ(Figureꢀ2).ꢀTheꢀ
curveꢀisꢀgeneratedꢀbyꢀforcingꢀaꢀconstantꢀinputꢀcurrentꢀ
intoꢀtheꢀgateꢀofꢀaꢀcommonꢀsource,ꢀcurrentꢀsource,ꢀloadedꢀ
stageꢀandꢀthenꢀplottingꢀtheꢀgateꢀversusꢀtime.ꢀTheꢀinitialꢀ
slopeꢀisꢀtheꢀeffectꢀofꢀtheꢀgate-to-sourceꢀandꢀgate-to-drainꢀ
capacitance.ꢀTheꢀflatꢀportionꢀofꢀtheꢀcurveꢀisꢀtheꢀresultꢀofꢀtheꢀ
Millerꢀmultiplicationꢀeffectꢀofꢀtheꢀdrain-to-gateꢀcapacitanceꢀ
asꢀtheꢀdrainꢀdropsꢀtheꢀvoltageꢀacrossꢀtheꢀcurrentꢀsourceꢀ
load.ꢀTheꢀupperꢀslopingꢀlineꢀisꢀdueꢀtoꢀtheꢀdrain-to-gateꢀ
accumulationꢀcapacitanceꢀandꢀtheꢀgate-to-sourceꢀcapaci-
tance.ꢀTheꢀMillerꢀchargeꢀ(theꢀincreaseꢀinꢀcoulombsꢀonꢀtheꢀ
horizontalꢀaxisꢀfromꢀaꢀtoꢀbꢀwhileꢀtheꢀcurveꢀisꢀflat)ꢀisꢀspeci-
synchronousꢀMOSFETꢀlossesꢀareꢀgreatestꢀatꢀhighꢀinputꢀ
voltageꢀwhenꢀtheꢀtopꢀswitchꢀdutyꢀfactorꢀisꢀlowꢀorꢀduringꢀ
aꢀshort-circuitꢀwhenꢀtheꢀsynchronousꢀswitchꢀisꢀonꢀcloseꢀ
toꢀ100%ꢀofꢀtheꢀperiod.
OperatingꢀFrequency
Theꢀchoiceꢀofꢀoperatingꢀfrequencyꢀisꢀaꢀtradeoffꢀbetweenꢀ
efficiencyꢀandꢀcomponentꢀsize.ꢀLoweringꢀtheꢀoperatingꢀfre-
quencyꢀimprovesꢀefficiencyꢀbyꢀreducingꢀMOSFETꢀswitchingꢀ
lossesꢀbutꢀrequiresꢀlargerꢀinductanceꢀand/orꢀcapacitanceꢀ
toꢀmaintainꢀlowꢀoutputꢀrippleꢀvoltage.ꢀConversely,ꢀraisingꢀ
theꢀoperatingꢀfrequencyꢀdegradesꢀefficiencyꢀbutꢀreducesꢀ
componentꢀsize.ꢀ
fiedꢀfromꢀaꢀgivenꢀV ꢀdrainꢀvoltage,ꢀbutꢀcanꢀbeꢀadjustedꢀ
DS
TheꢀoperatingꢀfrequencyꢀofꢀLTC3878ꢀapplicationsꢀisꢀde-
terminedꢀimplicitlyꢀbyꢀtheꢀone-shotꢀtimerꢀthatꢀcontrolsꢀtheꢀ
forꢀdifferentꢀV ꢀvoltagesꢀbyꢀmultiplyingꢀbyꢀtheꢀratioꢀofꢀ
DS
theꢀapplicationꢀV ꢀtoꢀtheꢀcurveꢀspecifiedꢀV ꢀvalues.ꢀAꢀ
DS
DS
on-time,ꢀt ,ꢀofꢀtheꢀtopꢀMOSFETꢀswitch.ꢀTheꢀon-timeꢀisꢀ
ON
wayꢀtoꢀestimateꢀtheꢀC
ꢀtermꢀisꢀtoꢀtakeꢀtheꢀchangeꢀinꢀ
MILLER
setꢀbyꢀtheꢀcurrentꢀintoꢀtheꢀI ꢀpinꢀaccordingꢀto:
ON
gateꢀchargeꢀfromꢀpointsꢀaꢀandꢀbꢀorꢀtheꢀparameterꢀQ ꢀonꢀ
GD
0.7V
IION
aꢀmanufacturersꢀdataꢀsheetꢀandꢀdivideꢀbyꢀtheꢀspecifiedꢀ
tON
=
10pF
(
)
V ꢀtestꢀvoltage,ꢀV
.ꢀ
ꢀ
DS
DS(TEST)
QGD
VDS(TEST)
TyingꢀaꢀresistorꢀR ꢀfromꢀV ꢀtoꢀtheꢀI ꢀpinꢀyieldsꢀanꢀ
ON
IN
ON
CMILLER
=
on-timeꢀinverselyꢀproportionalꢀtoꢀV .ꢀForꢀaꢀstep-downꢀ
IN
ꢀ
converter,ꢀthisꢀresultsꢀinꢀpseudoꢀfixedꢀfrequencyꢀoperationꢀ
asꢀtheꢀinputꢀsupplyꢀvaries.
C
ꢀisꢀtheꢀmostꢀimportantꢀselectionꢀcriteriaꢀforꢀdeter-
MILLER
miningꢀtheꢀtransitionꢀlossꢀtermꢀinꢀtheꢀtopꢀMOSFETꢀbutꢀisꢀ
notꢀdirectlyꢀspecifiedꢀonꢀMOSFETꢀdataꢀsheets.
VOUT
fOP =
[Hz]
0.7V •RON 10pF
ꢀ
3878fa
ꢀꢀ
LTC3878
applicaTions inForMaTion
4
3
2
1
0
Figureꢀ3ꢀshowsꢀhowꢀR ꢀrelatesꢀtoꢀswitchingꢀfrequencyꢀ
ON
forꢀseveralꢀcommonꢀoutputꢀvoltages.ꢀ
Whenꢀdesigningꢀforꢀpseudoꢀfixedꢀfrequency,ꢀthereꢀisꢀsys-
DROPOUT
REGION
tematicꢀerrorꢀbecauseꢀtheꢀI ꢀpinꢀvoltageꢀisꢀapproximatelyꢀ
ON
0.7V,ꢀnotꢀzero.ꢀThisꢀcausesꢀtheꢀI ꢀcurrentꢀtoꢀbeꢀinverselyꢀ
ON
proportionalꢀtoꢀ(V ꢀ–ꢀ0.7V)ꢀandꢀnotꢀV .ꢀTheꢀI ꢀcurrentꢀ
IN
IN
ON
errorꢀincreasesꢀasꢀV ꢀdecreases.ꢀToꢀcorrectꢀthisꢀerror,ꢀanꢀ
IN
additionalꢀresistorꢀR ꢀcanꢀbeꢀconnectedꢀfromꢀtheꢀI ꢀ
ON2
ON
pinꢀtoꢀtheꢀ5.3VꢀINTV ꢀsupply.
CC
0
0.25
0.50
0.75
1
5.3V – 0.7V
3878 F04
RON2
=
RON
DUTY CYCLE (V /V
)
OUT IN
0.7V
ꢀ
Figureꢀ4.ꢀMaximumꢀSwitchingꢀFrequencyꢀvsꢀDutyꢀCycle
1000
V
OUT
= 12V
Likewise,ꢀtheꢀmaximumꢀfrequencyꢀofꢀoperationꢀisꢀdeter-
minedꢀbyꢀtheꢀfixedꢀon-time,ꢀt ,ꢀandꢀtheꢀminimumꢀoff-time,ꢀ
ON
V
= 1.5V
t
.ꢀTheꢀfixedꢀon-timeꢀisꢀdeterminedꢀbyꢀdividingꢀtheꢀ
OUT
V
OFF(MIN)
= 3.3V
dutyꢀfactorꢀbyꢀtheꢀnominalꢀfrequencyꢀofꢀoperation:ꢀ
OUT
V
OUT
= 5V
1
fMAX
=
[Hz]
VOUT
+ tOFF(MIN)
V • fOP
IN
ꢀ
100
100
1000
(kΩ)
10000
TheꢀLTC3878ꢀisꢀaꢀPFMꢀ(pulseꢀfrequencyꢀmode)ꢀregula-
torꢀwhereꢀpulseꢀdensityꢀisꢀmodulated,ꢀnotꢀpulseꢀwidth.ꢀ
Consequently,ꢀfrequencyꢀincreasesꢀwithꢀaꢀloadꢀstepꢀandꢀ
decreasesꢀwithꢀaꢀloadꢀrelease.ꢀTheꢀsteady-stateꢀoperatingꢀ
R
ON
3878 F03
Figureꢀ3.ꢀSwitchingꢀFrequencyꢀvsꢀRON
frequency,ꢀf ,ꢀshouldꢀbeꢀsetꢀsufficientlyꢀbelowꢀf
ꢀtoꢀ
OPꢀ
MAX
MinimumꢀOff-TimeꢀandꢀDropoutꢀOperation
Theꢀminimumꢀoff-time,ꢀt ,ꢀisꢀtheꢀshortestꢀtimeꢀ
allowꢀforꢀdeviceꢀtolerancesꢀandꢀtransientꢀresponse.
OFF(MIN)
InductorꢀValueꢀCalculation
requiredꢀforꢀtheꢀLTC3878ꢀtoꢀturnꢀonꢀtheꢀbottomꢀMOSFET,ꢀ
tripꢀtheꢀcurrentꢀcomparatorꢀandꢀthenꢀturnꢀoffꢀtheꢀbottomꢀ
MOSFET.ꢀThisꢀtimeꢀisꢀtypicallyꢀaboutꢀ220ns.ꢀTheꢀminimumꢀ
Givenꢀtheꢀdesiredꢀinputꢀandꢀoutputꢀvoltages,ꢀtheꢀinduc-
torꢀvalueꢀandꢀoperationꢀfrequencyꢀdetermineꢀtheꢀrippleꢀ
current:
off-timeꢀlimitꢀimposesꢀaꢀmaximumꢀdutyꢀcycleꢀofꢀt /ꢀ
ON
(t ꢀ+ꢀt
ON
).ꢀIfꢀtheꢀmaximumꢀdutyꢀcycleꢀisꢀreached,ꢀ
OFF(MIN)
dueꢀtoꢀaꢀdroopingꢀinputꢀvoltageꢀforꢀexample,ꢀthenꢀtheꢀ
outputꢀwillꢀdropꢀoutꢀofꢀregulation.ꢀTheꢀminimumꢀinputꢀ
voltageꢀtoꢀavoidꢀdropoutꢀis:
VOUT
VOUT
∆IL =
1–
f
•L
V
OP
IN
ꢀ
Lowerꢀrippleꢀcurrentꢀreducesꢀcoreꢀlossesꢀinꢀtheꢀinductor,ꢀ
ESRꢀlossesꢀinꢀtheꢀoutputꢀcapacitorsꢀandꢀoutputꢀvoltageꢀ
ripple.ꢀ Highestꢀ efficiencyꢀ operationꢀ isꢀ obtainedꢀ atꢀ lowꢀ
frequencyꢀwithꢀsmallꢀrippleꢀcurrent.ꢀHowever,ꢀachievingꢀ
thisꢀrequiresꢀaꢀlargeꢀinductor.ꢀThereꢀisꢀaꢀtrade-offꢀbetweenꢀ
componentꢀsize,ꢀefficiencyꢀandꢀoperatingꢀfrequency.
tON + tOFF(MIN)
V
IN(MIN) = VOUT
tON
ꢀ
Aꢀplotꢀofꢀmaximumꢀdutyꢀcycleꢀvs.ꢀfrequencyꢀisꢀshownꢀinꢀ
Figureꢀ4.
3878fa
ꢀꢁ
LTC3878
applicaTions inForMaTion
Aꢀreasonableꢀstartingꢀpointꢀisꢀtoꢀchooseꢀaꢀrippleꢀcurrentꢀ ThisꢀformulaꢀhasꢀaꢀmaximumꢀatꢀV ꢀ=ꢀ2V ,ꢀwhereꢀI
ꢀ
IN
OUT
RMS
thatꢀisꢀaboutꢀ40%ꢀofꢀI
.ꢀTheꢀlargestꢀrippleꢀcurrentꢀ =ꢀI
/2.ꢀThisꢀsimpleꢀworst-caseꢀconditionꢀisꢀcom-
OUT(MAX)
OUT(MAX)
occursꢀatꢀtheꢀhighestꢀV .ꢀToꢀguaranteeꢀthatꢀrippleꢀcurrentꢀ monlyꢀusedꢀforꢀdesignꢀbecauseꢀevenꢀsignificantꢀdeviationsꢀ
IN
doesꢀnotꢀexceedꢀaꢀspecifiedꢀmaximum,ꢀtheꢀinductanceꢀ doꢀnotꢀofferꢀmuchꢀrelief.ꢀNoteꢀthatꢀrippleꢀcurrentꢀratingsꢀ
shouldꢀbeꢀchosenꢀaccordingꢀto:
fromꢀcapacitorꢀmanufacturersꢀareꢀoftenꢀbasedꢀonꢀonlyꢀ
2000ꢀhoursꢀofꢀlife,ꢀwhichꢀmakesꢀitꢀadvisableꢀtoꢀde-rateꢀ
theꢀcapacitor.
VOUT
OP • ∆I
VOUT
L =
1–
f
V
IL(MAX)
IN(MAX)
TheꢀselectionꢀofꢀC ꢀisꢀprimarilyꢀdeterminedꢀbyꢀtheꢀESRꢀ
OUT
ꢀ
requiredꢀtoꢀminimizeꢀvoltageꢀrippleꢀandꢀloadꢀstepꢀtransients.ꢀ
OnceꢀtheꢀvalueꢀforꢀLꢀisꢀknown,ꢀtheꢀtypeꢀofꢀinductorꢀmustꢀ
beꢀselected.ꢀHighꢀefficiencyꢀconvertersꢀgenerallyꢀcannotꢀ
tolerateꢀtheꢀcoreꢀlossꢀofꢀlowꢀcostꢀpowderedꢀironꢀcores,ꢀ
forcingꢀtheꢀuseꢀofꢀmoreꢀexpensiveꢀferriteꢀmaterialsꢀsuchꢀ
asꢀmolypermalloyꢀorꢀKoolꢀMµꢀcores.ꢀAꢀvarietyꢀofꢀinductorsꢀ
designedꢀforꢀhighꢀcurrent,ꢀlowꢀvoltageꢀapplicationsꢀareꢀ
availableꢀfromꢀmanufacturersꢀsuchꢀasꢀSumida,ꢀPanasonic,ꢀ
Coiltronics,ꢀCoilcraft,ꢀToko,ꢀVishay,ꢀPulseꢀandꢀWurth.
Theꢀ∆V ꢀisꢀapproximatelyꢀboundedꢀby:
OUT
1
∆VOUT ≤ ∆IL ESR +
8 • fOP •C
OUT
ꢀ
Sinceꢀ∆I ꢀincreasesꢀwithꢀinputꢀvoltage,ꢀtheꢀoutputꢀrippleꢀ
L
isꢀhighestꢀatꢀmaximumꢀinputꢀvoltage.ꢀTypically,ꢀonceꢀtheꢀ
ESRꢀrequirementꢀisꢀsatisfied,ꢀtheꢀcapacitanceꢀisꢀadequateꢀ
forꢀfilteringꢀandꢀhasꢀtheꢀnecessaryꢀRMSꢀcurrentꢀrating.
InductorꢀCoreꢀSelection
Multipleꢀcapacitorsꢀplacedꢀinꢀparallelꢀmayꢀbeꢀneededꢀtoꢀ
meetꢀtheꢀESRꢀandꢀRMSꢀcurrentꢀhandlingꢀrequirements.ꢀ
Dryꢀ tantalum,ꢀ specialtyꢀ polymer,ꢀ aluminumꢀ electrolyticꢀ
andꢀceramicꢀcapacitorsꢀareꢀallꢀavailableꢀinꢀsurfaceꢀmountꢀ
packages.ꢀ Specialtyꢀ polymerꢀ capacitorsꢀ offerꢀ veryꢀ lowꢀ
ESRꢀbutꢀhaveꢀlowerꢀspecificꢀcapacitanceꢀthanꢀotherꢀtypes.ꢀ
Tantalumꢀcapacitorsꢀhaveꢀtheꢀhighestꢀspecificꢀcapacitanceꢀ
butꢀitꢀisꢀimportantꢀtoꢀonlyꢀuseꢀtypesꢀthatꢀhaveꢀbeenꢀsurgeꢀ
testedꢀforꢀuseꢀinꢀswitchingꢀpowerꢀsupplies.ꢀAluminumꢀ
electrolyticꢀ capacitorsꢀ haveꢀ significantlyꢀ higherꢀ ESR,ꢀ
butꢀcanꢀbeꢀusedꢀinꢀcost-sensitiveꢀapplicationsꢀprovidingꢀ
thatꢀconsiderationꢀisꢀgivenꢀtoꢀrippleꢀcurrentꢀratingsꢀandꢀ
long-termꢀreliability.ꢀCeramicꢀcapacitorsꢀhaveꢀexcellentꢀ
lowꢀESRꢀcharacteristicsꢀbutꢀcanꢀhaveꢀaꢀhighꢀvoltageꢀco-
efficientꢀandꢀaudibleꢀpiezoelectricꢀeffects.ꢀTheꢀhighꢀQꢀofꢀ
ceramicꢀcapacitorsꢀwithꢀtraceꢀinductanceꢀcanꢀalsoꢀleadꢀtoꢀ
significantꢀringing.ꢀWhenꢀusedꢀasꢀinputꢀcapacitors,ꢀcareꢀ
mustꢀbeꢀtakenꢀtoꢀensureꢀthatꢀringingꢀfromꢀinrushꢀcurrentsꢀ
andꢀswitchingꢀdoesꢀnotꢀposeꢀanꢀovervoltageꢀhazardꢀtoꢀtheꢀ
powerꢀswitchesꢀandꢀcontroller.ꢀToꢀdampenꢀinputꢀvoltageꢀ
transients,ꢀaddꢀaꢀsmallꢀ5µFꢀtoꢀ40µFꢀaluminumꢀelectrolyticꢀ
capacitorꢀwithꢀanꢀESRꢀinꢀtheꢀrangeꢀofꢀ0.5Ωꢀtoꢀ2Ω.ꢀHighꢀ
performanceꢀthough-holeꢀcapacitorsꢀmayꢀalsoꢀbeꢀused,ꢀ
Onceꢀtheꢀinductanceꢀvalueꢀisꢀdetermined,ꢀtheꢀtypeꢀofꢀin-
ductorꢀmustꢀbeꢀselected.ꢀCoreꢀlossꢀisꢀindependentꢀofꢀcoreꢀ
sizeꢀforꢀaꢀfixedꢀinductorꢀvalue,ꢀbutꢀitꢀisꢀveryꢀdependentꢀ
onꢀinductanceꢀselected.ꢀAsꢀinductanceꢀincreases,ꢀcoreꢀ
lossesꢀ goꢀ down.ꢀ Unfortunately,ꢀ increasedꢀ inductanceꢀ
requiresꢀmoreꢀturnsꢀofꢀwireꢀandꢀthereforeꢀcopperꢀlossesꢀ
willꢀincrease.
Ferriteꢀdesignsꢀhaveꢀveryꢀlowꢀcoreꢀlossꢀandꢀareꢀpreferredꢀ
atꢀhighꢀswitchingꢀfrequencies,ꢀsoꢀdesignꢀgoalsꢀcanꢀcon-
centrateꢀonꢀcopperꢀlossꢀandꢀpreventingꢀsaturation.ꢀFerriteꢀ
coreꢀmaterialꢀsaturatesꢀ“hard,”ꢀwhichꢀmeansꢀthatꢀinduc-
tanceꢀcollapsesꢀabruptlyꢀwhenꢀtheꢀpeakꢀdesignꢀcurrentꢀisꢀ
exceeded.ꢀThisꢀresultsꢀinꢀanꢀabruptꢀincreaseꢀinꢀinductorꢀ
rippleꢀcurrentꢀandꢀconsequentꢀoutputꢀvoltageꢀripple.ꢀDoꢀ
notꢀallowꢀtheꢀcoreꢀtoꢀsaturate!
C ꢀandꢀC ꢀSelection
IN
OUT
TheꢀinputꢀcapacitanceꢀC ꢀisꢀrequiredꢀtoꢀfilterꢀtheꢀsquareꢀ
IN
waveꢀcurrentꢀatꢀtheꢀdrainꢀofꢀtheꢀtopꢀMOSFET.ꢀUseꢀaꢀlowꢀESRꢀ
capacitorꢀsizedꢀtoꢀhandleꢀtheꢀmaximumꢀRMSꢀcurrent.
VOUT
V
VOUT
IN
IRMS ≅ IOUT(MAX)
•
•
– 1
butꢀanꢀadditionalꢀceramicꢀcapacitorꢀinꢀparallelꢀisꢀrecom
mendedꢀtoꢀreduceꢀtheꢀeffectꢀofꢀleadꢀinductance.
-
V
IN
ꢀ
3878fa
ꢀꢂ
LTC3878
applicaTions inForMaTion
TopꢀMOSFETꢀDriverꢀSupplyꢀ(C ,ꢀD )
DiscontinuousꢀModeꢀOperationꢀandꢀFCBꢀPin
B
B
Anꢀexternalꢀbootstrapꢀcapacitor,ꢀC ,ꢀconnectedꢀtoꢀtheꢀBOOSTꢀ TheꢀFCBꢀ(forcedꢀcontinuousꢀbar)ꢀpinꢀdeterminesꢀwhetherꢀ
B
pinꢀsuppliesꢀtheꢀgateꢀdriveꢀvoltageꢀforꢀtheꢀtopsideꢀMOSFET.ꢀ theꢀLTC3878ꢀoperatesꢀinꢀforcedꢀcontinuousꢀmodeꢀorꢀal-
ThisꢀcapacitorꢀisꢀchargedꢀthroughꢀdiodeꢀD ꢀfromꢀINTV ꢀ lowsꢀdiscontinuousꢀconductionꢀmode.ꢀTyingꢀthisꢀpinꢀaboveꢀ
B
CC
whenꢀtheꢀswitchꢀnodeꢀisꢀlow.ꢀWhenꢀtheꢀtopꢀMOSFETꢀturnsꢀ 0.8Vꢀenablesꢀdiscontinuousꢀoperation,ꢀwhereꢀtheꢀbottomꢀ
on,ꢀtheꢀswitchꢀnodeꢀrisesꢀtoꢀV ꢀandꢀtheꢀBOOSTꢀpinꢀrisesꢀ MOSFETꢀ turnsꢀ offꢀ whenꢀ theꢀ inductorꢀ currentꢀ reversesꢀ
IN
toꢀapproximatelyꢀV ꢀ+ꢀINTV .ꢀTheꢀboostꢀcapacitorꢀneedsꢀ polarity.ꢀTheꢀloadꢀcurrentꢀatꢀwhichꢀcurrentꢀreversesꢀandꢀ
IN
CC
toꢀstoreꢀapproximatelyꢀ100ꢀtimesꢀtheꢀgateꢀchargeꢀrequiredꢀ discontinuousꢀoperationꢀbeginsꢀdependsꢀonꢀtheꢀamplitudeꢀ
byꢀtheꢀtopꢀMOSFET.ꢀInꢀmostꢀapplicationsꢀ0.1µFꢀtoꢀ0.47µF,ꢀ ofꢀtheꢀinductorꢀrippleꢀcurrentꢀandꢀwillꢀvaryꢀwithꢀchangesꢀinꢀ
X5RꢀorꢀX7Rꢀdielectricꢀcapacitorꢀisꢀadequate.ꢀ
V .ꢀInꢀsteady-stateꢀoperation,ꢀdiscontinuousꢀconductionꢀ
IN
modeꢀoccursꢀforꢀDCꢀloadꢀcurrentsꢀlessꢀthanꢀ1/2ꢀtheꢀpeak-
to-peakꢀrippleꢀcurrent.ꢀTyingꢀtheꢀFCBꢀpinꢀbelowꢀtheꢀ0.8Vꢀ
thresholdꢀforcesꢀcontinuousꢀswitching,ꢀwhereꢀinductorꢀ
currentꢀisꢀallowedꢀtoꢀreverseꢀatꢀlightꢀloadsꢀandꢀmaintainꢀ
synchronousꢀswitching.
ItꢀisꢀrecommendedꢀthatꢀtheꢀBOOSTꢀcapacitorꢀbeꢀnoꢀlargerꢀ
thanꢀ10%ꢀofꢀtheꢀINTV ꢀcapacitorꢀC ,ꢀtoꢀensureꢀthatꢀ
CC
VCC
theꢀC ꢀcanꢀsupplyꢀtheꢀupperꢀMOSFETꢀgateꢀchargeꢀandꢀ
VCC
BOOSTꢀcapacitorꢀunderꢀallꢀoperatingꢀconditions.ꢀVariableꢀ
frequencyꢀinꢀresponseꢀtoꢀloadꢀstepsꢀoffersꢀsuperiorꢀtran-
sientꢀperformanceꢀbutꢀrequiresꢀhigherꢀinstantaneousꢀgateꢀ Inꢀadditionꢀtoꢀprovidingꢀaꢀlogicꢀinputꢀtoꢀforceꢀcontinuousꢀ
drive.ꢀGateꢀchargeꢀdemandsꢀareꢀgreatestꢀinꢀhighꢀfrequencyꢀ operation,ꢀtheꢀFCBꢀpinꢀprovidesꢀaꢀmeansꢀtoꢀmaintainꢀaꢀ
lowꢀdutyꢀfactorꢀapplicationsꢀunderꢀhighꢀdI/dtꢀloadꢀstepsꢀ flyꢀbackꢀwindingꢀoutputꢀwhenꢀtheꢀprimaryꢀisꢀoperatingꢀ
andꢀatꢀstart-up.
inꢀdiscontinuousꢀmode.ꢀTheꢀsecondaryꢀoutputꢀV
ꢀisꢀ
OUT2
normallyꢀsetꢀasꢀshownꢀinꢀFigureꢀ6ꢀbyꢀtheꢀturnsꢀratioꢀNꢀ
ofꢀtheꢀtransformer.ꢀHowever,ꢀifꢀtheꢀcontrollerꢀgoesꢀintoꢀ
discontinuousꢀmodeꢀandꢀhaltsꢀswitchingꢀdueꢀtoꢀaꢀlightꢀ
SettingꢀOutputꢀVoltage
TheꢀLTC3878ꢀoutputꢀvoltageꢀisꢀsetꢀbyꢀanꢀexternalꢀfeed-
backꢀresistiveꢀdividerꢀcarefullyꢀplacedꢀacrossꢀtheꢀoutput,ꢀ
asꢀshownꢀinꢀFigureꢀ5.ꢀTheꢀregulatedꢀoutputꢀvoltageꢀisꢀ
determinedꢀby:
primaryꢀloadꢀcurrent,ꢀthenꢀV
ꢀwillꢀdroop.ꢀAnꢀexternalꢀ
OUT2
resistorꢀdividerꢀfromꢀV
ꢀtoꢀtheꢀFCBꢀpinꢀsetsꢀaꢀminimumꢀ
OUT2
voltageꢀV
ꢀbelowꢀwhichꢀcontinuousꢀoperationꢀisꢀ
OUT2(MIN)
forcedꢀuntilꢀV
ꢀhasꢀrisenꢀaboveꢀitsꢀminimum.
OUT2
RB
VOUT = 0.8V 1+
R4
R3
R
A
VOUT2(MIN) = 0.8V 1+
ꢀ
ꢀ
Toꢀimproveꢀtheꢀtransientꢀresponse,ꢀaꢀfeed-forwardꢀca-
pacitor,ꢀC ,ꢀmayꢀbeꢀused.ꢀGreatꢀcareꢀshouldꢀbeꢀtakenꢀtoꢀ
FFꢀ
V
IN
routeꢀtheꢀV ꢀlineꢀawayꢀfromꢀnoiseꢀsources,ꢀsuchꢀasꢀtheꢀ
FB
C
IN
LTC3878
inductorꢀorꢀtheꢀSWꢀline.
1N4148
V
IN
V
OUT2
Si4884
Si4874
TG
SW
BG
V
OUT
•
C
OUT2
V
R4
R3
•
OUT
R
C
FF
LTC3878
B
FCB
C
OUT
V
FB
R
A
SGND PGND
3878 F06
3878 F05
Figureꢀ5.ꢀSettingꢀOutputꢀVoltage
Figureꢀ6.ꢀSecondaryꢀOutputꢀLoop
3878fa
ꢀꢃ
LTC3878
applicaTions inForMaTion
FaultꢀConditions:ꢀCurrentꢀLimitꢀandꢀFoldback
ApplicationsꢀusingꢀlargeꢀMOSFETsꢀwithꢀaꢀhighꢀinputꢀvoltageꢀ
andꢀhighꢀfrequencyꢀofꢀoperationꢀmayꢀcauseꢀtheꢀLTC3878ꢀ
toꢀexceedꢀitsꢀmaximumꢀjunctionꢀtemperatureꢀratingꢀorꢀ
RMSꢀcurrentꢀrating.ꢀInꢀcontinuousꢀmodeꢀoperation,ꢀthisꢀ
Theꢀmaximumꢀinductorꢀcurrentꢀisꢀinherentlyꢀlimitedꢀinꢀaꢀ
currentꢀmodeꢀcontrollerꢀbyꢀtheꢀmaximumꢀsenseꢀvoltage.ꢀ
InꢀtheꢀLTC3878,ꢀtheꢀmaximumꢀsenseꢀvoltageꢀisꢀcontrolledꢀ
currentꢀisꢀI
ꢀ=ꢀf (Q
+ꢀQ
).ꢀTheꢀjunctionꢀ
GATECHG
OP g(TOP)ꢀ
g(BOT)
byꢀtheꢀvoltageꢀonꢀtheꢀV ꢀpin.ꢀWithꢀvalleyꢀcurrentꢀmodeꢀ
RNG
temperatureꢀcanꢀbeꢀestimatedꢀfromꢀtheꢀequationsꢀgivenꢀ
inꢀNoteꢀ2ꢀofꢀtheꢀElectricalꢀCharacteristics.ꢀForꢀexample,ꢀ
withꢀaꢀ30Vꢀinputꢀsupply,ꢀtheꢀLTC3878ꢀisꢀlimitedꢀtoꢀlessꢀ
thanꢀ16.5mA:
control,ꢀtheꢀmaximumꢀsenseꢀvoltageꢀandꢀtheꢀsenseꢀre-
sistanceꢀdetermineꢀtheꢀmaximumꢀallowedꢀinductorꢀvalleyꢀ
current.ꢀTheꢀcorrespondingꢀoutputꢀcurrentꢀlimitꢀis:
VSNS(MAX)
1
2
ꢀ T ꢀ=ꢀ70°Cꢀ+ꢀ(16.5mA)(30)(110°C/W)ꢀ=ꢀ125°C
J
ILIMIT
=
+ • ∆IL
RDS(ON) • ρT
ꢀ
UsingꢀtheꢀINTV ꢀregulatorꢀtoꢀsupplyꢀexternalꢀloadsꢀgreaterꢀ
CC
thanꢀ5mAꢀisꢀdiscouraged.ꢀINTV ꢀisꢀdesignedꢀtoꢀsupplyꢀ
Theꢀcurrentꢀlimitꢀvalueꢀshouldꢀbeꢀcheckedꢀtoꢀensureꢀthatꢀ
ꢀ >ꢀ I .ꢀ Theꢀ currentꢀ limitꢀ valueꢀ shouldꢀ
CC
theꢀLTC3878ꢀwithꢀminimalꢀexternalꢀloading.ꢀWhenꢀusingꢀ
theꢀ regulatorꢀ toꢀ supplyꢀ largerꢀ externalꢀ loads,ꢀ carefullyꢀ
considerꢀallꢀoperatingꢀloadꢀconditions.ꢀDuringꢀloadꢀstepsꢀ
andꢀsoft-start,ꢀtransientꢀcurrentꢀrequirementsꢀsignificantlyꢀ
I
LIMIT(MIN)
OUT(MAX)
beꢀgreaterꢀthanꢀtheꢀinductorꢀcurrentꢀrequiredꢀtoꢀproduceꢀ
maximumꢀ outputꢀ powerꢀ atꢀ theꢀ worst-caseꢀ efficiency.ꢀ
Worst-caseꢀefficiencyꢀtypicallyꢀoccursꢀatꢀtheꢀhighestꢀV ꢀ
IN
exceedꢀtheꢀRMSꢀvalues.ꢀAdditionalꢀloadingꢀonꢀINTV ꢀtakesꢀ
andꢀhighestꢀambientꢀtemperature.ꢀItꢀisꢀimportantꢀtoꢀcheckꢀ
CC
awayꢀfromꢀtheꢀdriveꢀavailableꢀtoꢀsourceꢀgateꢀchargeꢀduringꢀ
forꢀconsistencyꢀbetweenꢀtheꢀassumedꢀMOSFETꢀjunctionꢀ
highꢀfrequencyꢀtransientꢀloadꢀsteps.
temperaturesꢀandꢀtheꢀresultingꢀvalueꢀofꢀI
theꢀMOSFETꢀswitches.
ꢀwhichꢀheatsꢀ
LIMIT
Soft-StartꢀwithꢀtheꢀRUN/SSꢀPin
Cautionꢀshouldꢀbeꢀusedꢀwhenꢀsettingꢀtheꢀcurrentꢀlimitꢀbasedꢀ
onꢀtheꢀR ꢀofꢀtheꢀMOSFETs.ꢀTheꢀmaximumꢀcurrentꢀ
TheꢀRUN/SSꢀpinꢀbothꢀenablesꢀtheꢀLTC3878ꢀandꢀprovidesꢀaꢀ
meansꢀofꢀprogrammableꢀcurrentꢀlimitedꢀsoft-start.ꢀPullingꢀ
theꢀRUN/SSꢀpinꢀbelowꢀ0.7VꢀputsꢀtheꢀLTC3878ꢀintoꢀaꢀlowꢀ
DS(ON)
limitꢀisꢀdeterminedꢀbyꢀtheꢀminimumꢀMOSFETꢀon-resistance.ꢀ
Dataꢀsheetsꢀtypicallyꢀspecifyꢀnominalꢀandꢀmaximumꢀvaluesꢀ
quiescentꢀcurrentꢀshutdownꢀ(I ꢀ<ꢀ15µA).ꢀReleasingꢀtheꢀ
Q
forꢀR
ꢀbutꢀnotꢀaꢀminimum.ꢀAꢀreasonableꢀassumptionꢀ
DS(ON)
isꢀthatꢀtheꢀminimumꢀR
pinꢀallowsꢀanꢀinternalꢀ1.2µAꢀcurrentꢀsourceꢀtoꢀchargeꢀupꢀ
ꢀliesꢀtheꢀsameꢀamountꢀbelowꢀ
DS(ON)
theꢀexternalꢀtimingꢀcapacitorꢀC .ꢀIfꢀRUN/SSꢀhasꢀbeenꢀ
SS
theꢀtypicalꢀvalueꢀasꢀtheꢀmaximumꢀliesꢀaboveꢀit.ꢀConsultꢀtheꢀ
MOSFETꢀmanufacturerꢀforꢀfurtherꢀguidelines.
pulledꢀallꢀtheꢀwayꢀtoꢀground,ꢀthereꢀisꢀaꢀdelayꢀbeforeꢀstart-
ing.ꢀThisꢀdelayꢀisꢀcreatedꢀbyꢀchargingꢀC ꢀfromꢀgroundꢀ
SS
Toꢀfurtherꢀlimitꢀcurrentꢀinꢀtheꢀeventꢀofꢀaꢀshortꢀcircuitꢀtoꢀ
ground,ꢀtheꢀLTC3878ꢀincludesꢀfoldbackꢀcurrentꢀlimiting.ꢀ
Ifꢀtheꢀoutputꢀfallsꢀbyꢀmoreꢀthanꢀ50%,ꢀthenꢀtheꢀmaximumꢀ
senseꢀvoltageꢀisꢀprogressivelyꢀloweredꢀtoꢀaboutꢀone-sixthꢀ
ofꢀitsꢀfullꢀvalue.ꢀ
toꢀ1.5Vꢀthroughꢀaꢀ1.2µAꢀcurrentꢀsource.
1.5V
1.2µA
tDELAY
=
•CSS = 1.3s/µF C
(
)
SS
ꢀ
WhenꢀtheꢀvoltageꢀonꢀRUN/SSꢀreachesꢀ1.5V,ꢀtheꢀLTC3878ꢀ
beginsꢀtoꢀswitch.ꢀI ꢀisꢀclampedꢀtoꢀbeꢀnoꢀgreaterꢀthanꢀ
INTV ꢀRegulator
TH
CC
RUN/SSꢀ–ꢀ0.6V,ꢀandꢀtheꢀdeviceꢀbeginsꢀswitchingꢀwhenꢀ
AnꢀinternalꢀP-channelꢀlowꢀdropoutꢀregulatorꢀproducesꢀtheꢀ
5.3Vꢀsupplyꢀthatꢀpowersꢀtheꢀdriversꢀandꢀinternalꢀcircuitryꢀ
I ꢀexceedsꢀ0.9V.ꢀAsꢀtheꢀRUN/SSꢀvoltageꢀrisesꢀtoꢀ3V,ꢀtheꢀ
TH
clampꢀonꢀI ꢀincreasesꢀuntilꢀitꢀreachesꢀtheꢀfull-scaleꢀ2.4Vꢀ
TH
withinꢀtheꢀLTC3878.ꢀTheꢀINTV ꢀpinꢀcanꢀsupplyꢀupꢀtoꢀ50mAꢀ
CC
limitꢀafterꢀanꢀadditionalꢀdelayꢀofꢀ1.3s/µF.ꢀDuringꢀthisꢀtime,ꢀ
RMSꢀandꢀmustꢀbeꢀbypassedꢀtoꢀgroundꢀwithꢀaꢀminimumꢀofꢀ
1µFꢀlowꢀESRꢀtantalumꢀorꢀceramicꢀcapacitorꢀ(10V,ꢀX5Rꢀorꢀ
X7R).ꢀOutputꢀcapacitanceꢀgreaterꢀthanꢀ10µFꢀisꢀdiscouraged.ꢀ
Goodꢀbypassingꢀisꢀnecessaryꢀtoꢀsupplyꢀtheꢀhighꢀtransientꢀ
currentsꢀrequiredꢀbyꢀtheꢀMOSFETꢀgateꢀdrivers.
theꢀsoft-startꢀcurrentꢀlimitꢀisꢀsetꢀto:
RUN/SS – 0.6V – 0.8V
(
)
ILIMIT(SS) = ILIMIT
•
2.4V – 0.8V
ꢀ
3878fa
ꢀꢄ
LTC3878
applicaTions inForMaTion
RegulatorꢀoutputꢀcurrentꢀisꢀnegativeꢀwhenꢀI ꢀisꢀbetweenꢀ inputꢀvoltage,ꢀloadꢀcurrent,ꢀdriverꢀstrengthꢀandꢀMOSFETꢀ
TH
0Vꢀandꢀ0.8VꢀandꢀpositiveꢀwhenꢀI ꢀisꢀbetweenꢀ0.8Vꢀandꢀtheꢀ capacitance,ꢀamongꢀotherꢀfactors.ꢀTheꢀlossꢀisꢀsignificantꢀ
TH
maximumꢀfull-scaleꢀset-pointꢀofꢀ2.4V.ꢀInꢀnormalꢀoperatingꢀ atꢀinputꢀvoltagesꢀaboveꢀ20V.
conditionsꢀtheꢀRUN/SSꢀpinꢀwillꢀcontinueꢀtoꢀchargeꢀpositiveꢀ
3.ꢀINTV ꢀcurrent.ꢀThisꢀisꢀtheꢀsumꢀofꢀtheꢀMOSFETꢀdriverꢀ
CC
untilꢀtheꢀvoltageꢀisꢀequalꢀtoꢀINTV .ꢀ
CC
andꢀcontrolꢀcurrents.
INTV ꢀUndervoltageꢀLockout
4.ꢀC ꢀloss.ꢀTheꢀinputꢀcapacitorꢀhasꢀtheꢀdifficultꢀjobꢀofꢀfilter-
CC
IN
ingꢀtheꢀlargeꢀRMSꢀinputꢀcurrentꢀtoꢀtheꢀregulator.ꢀItꢀmustꢀhaveꢀ
WheneverꢀINTV ꢀdropsꢀbelowꢀapproximatelyꢀ3.4V,ꢀtheꢀ
CC
2
aꢀveryꢀlowꢀESRꢀtoꢀminimizeꢀtheꢀACꢀI Rꢀlossꢀandꢀsufficientꢀ
deviceꢀentersꢀundervoltageꢀlockoutꢀ(UVLO).ꢀInꢀaꢀUVLOꢀ
condition,ꢀtheꢀswitchingꢀoutputsꢀTGꢀandꢀBGꢀareꢀdisabled.ꢀ
Atꢀtheꢀsameꢀtime,ꢀtheꢀRUN/SSꢀpinꢀisꢀpulledꢀdownꢀfromꢀ
capacitanceꢀtoꢀpreventꢀtheꢀRMSꢀcurrentꢀfromꢀcausingꢀad-
ditionalꢀupstreamꢀlossesꢀinꢀfusesꢀorꢀbatteries.
INTV ꢀ toꢀ 0.8Vꢀ withꢀ aꢀ 3µAꢀ currentꢀ source.ꢀ Whenꢀ theꢀ Otherꢀlosses,ꢀwhichꢀincludeꢀtheꢀC ꢀESRꢀloss,ꢀbottomꢀ
CC
OUT
INTV ꢀUVLOꢀconditionꢀisꢀremoved,ꢀRUN/SSꢀrampsꢀfromꢀ MOSFETꢀreverseꢀrecoveryꢀlossꢀandꢀinductorꢀcoreꢀlossꢀ
CC
0.8Vꢀandꢀbeginsꢀaꢀnormalꢀcurrentꢀlimitedꢀsoft-start.ꢀThisꢀ generallyꢀaccountꢀforꢀlessꢀthanꢀ2%ꢀadditionalꢀloss.
featureꢀisꢀimportantꢀwhenꢀregulatorꢀstart-upꢀisꢀnotꢀiniti-
Whenꢀmakingꢀadjustmentsꢀtoꢀimproveꢀefficiency,ꢀtheꢀinputꢀ
atedꢀbyꢀapplyingꢀaꢀlogicꢀdriveꢀtoꢀRUN/SS.ꢀSoft-startꢀfromꢀ
currentꢀisꢀtheꢀbestꢀindicatorꢀofꢀchangesꢀinꢀefficiency.ꢀIfꢀyouꢀ
INTV ꢀUVLOꢀreleaseꢀgreatlyꢀreducesꢀtheꢀpossibilityꢀforꢀ
CC
makeꢀaꢀchangeꢀandꢀtheꢀinputꢀcurrentꢀdecreases,ꢀthenꢀtheꢀ
efficiencyꢀhasꢀincreased.ꢀIfꢀthereꢀisꢀnoꢀchangeꢀinꢀinputꢀ
currentꢀthereꢀisꢀnoꢀchangeꢀinꢀefficiency.
start-upꢀoscillationsꢀcausedꢀbyꢀtheꢀregulatorꢀstartingꢀupꢀ
atꢀINTV
ꢀandꢀthenꢀshuttingꢀdownꢀatꢀINTV
ꢀ
CC(UVLOR)
dueꢀtoꢀinrushꢀcurrent.
CC(UVLO)
CheckingꢀTransientꢀResponse
EfficiencyꢀConsiderations
Theꢀregulatorꢀloopꢀresponseꢀcanꢀbeꢀcheckedꢀbyꢀlookingꢀ
atꢀtheꢀloadꢀtransientꢀresponse.ꢀSwitchingꢀregulatorsꢀtakeꢀ
severalꢀcyclesꢀtoꢀrespondꢀtoꢀaꢀstepꢀinꢀloadꢀcurrent.ꢀWhenꢀ
Theꢀpercentꢀefficiencyꢀofꢀaꢀswitchingꢀregulatorꢀisꢀequalꢀtoꢀ
theꢀoutputꢀpowerꢀdividedꢀbyꢀtheꢀinputꢀpowerꢀtimesꢀ100%.ꢀ
Itꢀisꢀoftenꢀusefulꢀtoꢀanalyzeꢀindividualꢀlossesꢀtoꢀdetermineꢀ
whatꢀisꢀlimitingꢀtheꢀefficiencyꢀandꢀwhichꢀchangeꢀwouldꢀ
produceꢀtheꢀmostꢀimprovement.ꢀAlthoughꢀallꢀdissipativeꢀ
elementsꢀinꢀtheꢀcircuitꢀproduceꢀlosses,ꢀfourꢀmainꢀsourcesꢀ
accountꢀforꢀmostꢀofꢀtheꢀlossesꢀinꢀLTC3878ꢀcircuits.
aꢀloadꢀstepꢀoccurs,ꢀV ꢀimmediatelyꢀshiftsꢀbyꢀanꢀamountꢀ
OUT
equalꢀtoꢀ∆I
ꢀ(ESR),ꢀwhereꢀESRꢀisꢀtheꢀeffectiveꢀseriesꢀ
LOAD
resistanceꢀofꢀC .ꢀ∆I
ꢀalsoꢀbeginsꢀtoꢀchargeꢀorꢀdis-
OUT
LOAD
chargeꢀC ,ꢀgeneratingꢀaꢀfeedbackꢀerrorꢀsignalꢀusedꢀbyꢀtheꢀ
OUT
regulatorꢀtoꢀreturnꢀV ꢀtoꢀitsꢀsteady-stateꢀvalue.ꢀDuringꢀ
OUT
2
thisꢀrecoveryꢀtime,ꢀV ꢀcanꢀbeꢀmonitoredꢀforꢀovershootꢀ
1.ꢀDCꢀI Rꢀlosses.ꢀTheseꢀariseꢀfromꢀtheꢀresistancesꢀofꢀtheꢀ
OUT
orꢀringingꢀthatꢀwouldꢀindicateꢀaꢀstabilityꢀproblem.ꢀTheꢀI ꢀ
MOSFETs,ꢀinductorꢀandꢀPCꢀboardꢀtracesꢀandꢀcauseꢀtheꢀ
efficiencyꢀtoꢀdropꢀatꢀhighꢀoutputꢀcurrents.ꢀInꢀcontinuousꢀ
modeꢀtheꢀaverageꢀoutputꢀcurrentꢀflowsꢀthoughꢀtheꢀinductorꢀ
L,ꢀbutꢀisꢀchoppedꢀbetweenꢀtheꢀtopꢀandꢀbottomꢀMOSFETs.ꢀ
TH
pinꢀexternalꢀcomponentsꢀshownꢀinꢀtheꢀDesignꢀExampleꢀwillꢀ
provideꢀadequateꢀcompensationꢀforꢀmostꢀapplications.ꢀ
Aꢀroughꢀcompensationꢀcheckꢀcanꢀbeꢀmadeꢀbyꢀcalculatingꢀ
IfꢀtheꢀtwoꢀMOSFETsꢀhaveꢀapproximatelyꢀtheꢀsameꢀR
,ꢀ
DS(ON)
theꢀgainꢀcrossoverꢀfrequency,ꢀf .ꢀg
ꢀisꢀtheꢀerrorꢀ
GCO m(EA)
thenꢀtheꢀresistanceꢀofꢀoneꢀMOSFETꢀcanꢀsimplyꢀbyꢀsummedꢀ
amplifierꢀtransconductance,ꢀR ꢀisꢀtheꢀcompensationꢀre-
C
withꢀtheꢀresistancesꢀofꢀLꢀandꢀtheꢀboardꢀtracesꢀtoꢀobtainꢀ
sistorꢀandꢀfeedbackꢀdividerꢀattenuationꢀisꢀassumedꢀtoꢀbeꢀ
2
theꢀDCꢀI Rꢀloss.ꢀForꢀexample,ꢀifꢀR
L
ꢀ=ꢀ0.01Ωꢀandꢀ
DS(ON)
0.8V/V .ꢀThisꢀequationꢀassumesꢀthatꢀnoꢀfeed-forwardꢀ
OUT
R ꢀ=ꢀ0.005Ω,ꢀtheꢀlossꢀwillꢀrangeꢀfromꢀ15mWꢀtoꢀ1.5Wꢀasꢀ
theꢀoutputꢀcurrentꢀvariesꢀfromꢀ1Aꢀtoꢀ10A.
compensationꢀisꢀusedꢀonꢀfeedbackꢀandꢀthatꢀC ꢀsetsꢀtheꢀ
OUT
dominantꢀoutputꢀpole.ꢀ
2.ꢀTransitionꢀloss.ꢀThisꢀlossꢀarisesꢀfromꢀtheꢀbriefꢀamountꢀ
ofꢀtimeꢀtheꢀtopꢀMOSFETꢀspendsꢀinꢀtheꢀsaturatedꢀregionꢀ
duringꢀ switchꢀ nodeꢀ transitions.ꢀ Itꢀ dependsꢀ uponꢀ theꢀ
ILIMIT
1
0.8
fGCO = gm(EA) •RC •
•
•
1.6 2 • π •COUT VOUT
ꢀ
3878fa
ꢀꢅ
LTC3878
applicaTions inForMaTion
Asꢀaꢀruleꢀofꢀthumbꢀtheꢀgainꢀcrossoverꢀfrequencyꢀshouldꢀbeꢀ Selectꢀtheꢀnearestꢀstandardꢀresistorꢀvalueꢀofꢀ432kꢀforꢀaꢀ
lessꢀthanꢀ20%ꢀofꢀtheꢀswitchingꢀfrequency.ꢀForꢀaꢀdetailedꢀ nominalꢀoperatingꢀfrequencyꢀofꢀ396kHz.ꢀSetꢀtheꢀinductorꢀ
explanationꢀofꢀswitchingꢀcontrolꢀloopꢀtheoryꢀseeꢀApplica-
tionꢀNoteꢀ76.
valueꢀtoꢀgiveꢀ35%ꢀrippleꢀcurrentꢀatꢀmaximumꢀV ꢀusingꢀ
IN
theꢀadjustedꢀoperatingꢀfrequency:
1.2V
1.2
28
HighꢀSwitchingꢀFrequencyꢀOperation
L =
1–
= 0.55µH
396kHz •0.35•15A
ꢀ
Specialꢀcareꢀshouldꢀbeꢀtakenꢀwhenꢀoperatingꢀatꢀswitchingꢀ
frequenciesꢀgreaterꢀthanꢀ800kHz.ꢀAtꢀhighꢀswitchingꢀfrequen-
ciesꢀthereꢀmayꢀbeꢀanꢀincreasedꢀsensitivityꢀtoꢀPCBꢀnoiseꢀ
whichꢀmayꢀresultꢀinꢀoff-timeꢀvariationꢀgreaterꢀthanꢀnormal.ꢀ
Thisꢀoff-timeꢀinstabilityꢀcanꢀbeꢀpreventedꢀinꢀseveralꢀways.ꢀ
First,ꢀcarefullyꢀfollowꢀtheꢀrecommendedꢀlayoutꢀtechniques.ꢀ
Second,ꢀuseꢀ2µFꢀorꢀmoreꢀofꢀX5RꢀorꢀX7Rꢀceramicꢀinputꢀ
capacitanceꢀperꢀAmpsꢀofꢀloadꢀcurrent.ꢀThird,ꢀifꢀnecessary,ꢀ
Selectꢀ0.56µHꢀwhichꢀisꢀtheꢀnearestꢀvalue.
Theꢀresultingꢀmaximumꢀrippleꢀcurrentꢀis:
1.2V
396kHz •0.56µH
1.2V
28V
∆IL =
1–
= 5.1A
ꢀ
Chooseꢀ theꢀ synchronousꢀ bottomꢀ MOSFETꢀ switchꢀ andꢀ
calculateꢀtheꢀV ꢀcurrentꢀlimitꢀset-point.ꢀToꢀcalculateꢀ
increaseꢀtheꢀbottomꢀMOSFETꢀrippleꢀvoltageꢀtoꢀ30mV
ꢀ
P-P
ꢀtypicalꢀ
RNG
orꢀgreater.ꢀThisꢀrippleꢀvoltageꢀisꢀequalꢀtoꢀR
DS(ON)
V
ꢀandꢀV ,ꢀtheꢀρτꢀtermꢀnormalizationꢀfactorꢀ(unityꢀ
RNG
DS
atꢀ25°Cꢀ•ꢀI
.
P-Pꢀ
atꢀ25°C)ꢀisꢀrequiredꢀtoꢀaccountꢀforꢀvariationꢀinꢀMOSFETꢀ
on-resistanceꢀwithꢀtemperature.ꢀChoosingꢀanꢀRJK0330ꢀ
DesignꢀExample
Figureꢀ7ꢀisꢀaꢀpowerꢀsupplyꢀdesignꢀexampleꢀwithꢀtheꢀfol-
lowingꢀspecifications:ꢀV ꢀ=ꢀ4.5Vꢀtoꢀ28Vꢀ(12Vꢀnominal),ꢀ
(R
ꢀ=ꢀ2.8mΩꢀ(nominal)ꢀ3.9mΩꢀ(maximum),ꢀV ꢀ=ꢀ
4.5V,ꢀθ ꢀ=ꢀ40°C/W)ꢀyieldsꢀaꢀdrainꢀsourceꢀvoltageꢀof:
DS(ON)
GS
JA
IN
ꢀ=ꢀ15Aꢀandꢀfꢀ=ꢀ400kHz.ꢀStartꢀ
OUT(MAX)
1
2
V
ꢀ=ꢀ1.2Vꢀ 5%,ꢀI
V = I
LIMIT
–
I
(
3.9mΩ ρτ
( )
)
OUT
DS
RIPPLE
byꢀcalculatingꢀtheꢀtimingꢀresistor,ꢀR :
ꢀ
ON
1.2V
RON
=
= 429k
0.7V •400kHz •10pF
ꢀ
V
IN
4.5V TO 28V
C
SS
D
B
C
IN1
+
C
IN2
0.1µF
CMDSH-3
10µF
50V
s3
100µF
50V
1
2
16
15
RUN/SS
LTC3878
PGOOD
BOOST
R1
R2
R
PG
C
B
10.0k 80.6k
100k
0.22µF
M1
TG
L1
0.56µH
RJK0305DPB
V
OUT
3
4
5
14
13
12
V
SW
PGND
BG
1.2V
15A
RNG
C
C1
R
220pF
C
FCB
C
OUT2
C
OUT1
12.1k
+
M2
47µF
6.3V
s2
330µF
2.5V
s2
I
TH
RJK0330DPB
C
VCC
C
C2
4.7µF
33pF
6
11
SGND
INV
CC
7
8
10
9
R
FB1
I
V
IN
ON
C
C
C
: UMK325BJ106MM s3
10.0k
IN1
V
NC
: SANYO 2R5TPE330M9 s2
FB
OUT1
OUT2
: MURATA GRM31CR60J476M s2
R
R
ON
FB2
L1: VISHAY IHLP4040DZ-11 0.56µH
5.11k 432k
3878 F07
Figureꢀ7.ꢀDesignꢀExample:ꢀ±.2V/±5Aꢀatꢀ400kHz
3878fa
ꢀꢆ
LTC3878
applicaTions inForMaTion
V
ꢀsetsꢀcurrentꢀlimitꢀbyꢀfixingꢀtheꢀmaximumꢀpeakꢀV ꢀ
SelectꢀC ꢀtoꢀgiveꢀanꢀRMSꢀcurrentꢀratingꢀgreaterꢀthanꢀ4Aꢀ
RNG
DS
IN
voltageꢀonꢀtheꢀbottomꢀMOSFETꢀswitch.ꢀAsꢀaꢀresult,ꢀtheꢀ
averageꢀDCꢀcurrentꢀlimitꢀincludesꢀsignificantꢀtemperatureꢀ
andꢀcomponentꢀvariability.ꢀDesignꢀtoꢀguaranteeꢀthatꢀtheꢀ
averageꢀDCꢀcurrentꢀlimitꢀwillꢀalwaysꢀexceedꢀtheꢀratedꢀoper-
atingꢀoutputꢀcurrentꢀbyꢀassumingꢀworst-caseꢀcomponentꢀ
toleranceꢀandꢀtemperature.ꢀ
atꢀ85°C.ꢀTheꢀoutputꢀcapacitorꢀC
ꢀisꢀchosenꢀforꢀaꢀlowꢀ
OUT1
ESRꢀofꢀ4.5mΩꢀtoꢀminimizeꢀoutputꢀvoltageꢀchangesꢀdueꢀtoꢀ
inductorꢀrippleꢀcurrentꢀandꢀloadꢀsteps.ꢀTheꢀoutputꢀvoltageꢀ
rippleꢀisꢀgivenꢀas:
∆VOUT(RIPPLE) = ∆IL(MAX) ESR
(
)
= 5.1•4.5mΩ= 23mV
ꢀ
Theꢀworst-caseꢀminimumꢀINTV ꢀisꢀ5.15V.ꢀTheꢀbottomꢀ
CC
MOSFETꢀworst-caseꢀR
ꢀisꢀ3.9mΩꢀandꢀtheꢀjunctionꢀ
DS(ON)
However,ꢀ aꢀ 0Aꢀ toꢀ 10Aꢀ loadꢀ stepꢀ willꢀ causeꢀ anꢀ outputꢀ
changeꢀofꢀupꢀto:
temperatureꢀisꢀ80°Cꢀaboveꢀaꢀ70°Cꢀambientꢀwithꢀρ
ꢀ=ꢀ
150°C
1.5.ꢀSetꢀT ꢀequalꢀtoꢀtheꢀminimumꢀspecificationꢀofꢀ15%ꢀ
ON
∆VOUT(STEP) = ∆ILOAD ESR
(
)
lowꢀandꢀtheꢀinductorꢀ15%ꢀhigh.ꢀ
=10A •4.5mΩ= 45mV
ByꢀsettingꢀI
ꢀequalꢀtoꢀ15Aꢀweꢀgetꢀ79mVꢀforꢀpeakꢀV ꢀ
ꢀ
LIMIT
DS
voltageꢀwhichꢀcorrespondsꢀtoꢀaꢀV ꢀequalꢀtoꢀ592mV:
RNG
Optionalꢀ2ꢀ×ꢀ47µFꢀceramicꢀoutputꢀcapacitorsꢀareꢀincludedꢀ
toꢀminimizeꢀtheꢀeffectꢀofꢀESRꢀandꢀESLꢀinꢀtheꢀoutputꢀrippleꢀ
andꢀtoꢀimproveꢀloadꢀstepꢀresponse.ꢀ
1
2
0.85 3.9mΩ
V = 15A – •5.1A •
•1.5
DS
5.15V
1.15
5.3V
PCꢀBoardꢀLayoutꢀChecklist
VRNG = 7.5• VDS
ꢀ
TheꢀLTC3878ꢀPCꢀboardꢀlayoutꢀcanꢀbeꢀdesignedꢀwithꢀorꢀ
withoutꢀaꢀgroundꢀplane.ꢀAꢀgroundꢀplaneꢀisꢀgenerallyꢀpre-
ferredꢀbasedꢀonꢀperformanceꢀandꢀnoiseꢀconcerns.
Verifyꢀ thatꢀ theꢀ calculatedꢀ nominalꢀ T ꢀ isꢀ lessꢀ thanꢀ theꢀ
assumedꢀworst-caseꢀT ꢀinꢀtheꢀbottomꢀMOSFET:
J
J
28V –1.2V
Whenꢀusingꢀaꢀgroundꢀplane,ꢀuseꢀaꢀdedicatedꢀgroundꢀplaneꢀ
layer.ꢀInꢀaddition,ꢀforꢀhighꢀcurrentꢀitꢀisꢀrecommendedꢀtoꢀ
useꢀaꢀmultilayerꢀboardꢀtoꢀhelpꢀwithꢀheatꢀsinkingꢀpowerꢀ
components.ꢀ
2
PBOT
=
15A •1.5•3.9mΩ=1.25W
(
)
28V
TJ = 70°C+1.25W •40°C/W =120°C
ꢀ
B
ecauseꢀ theꢀ topꢀ MOSFETꢀ isꢀ onꢀ forꢀ aꢀ shortꢀ time,ꢀ anꢀ
lꢀ Theꢀgroundꢀplaneꢀlayerꢀshouldꢀhaveꢀnoꢀtracesꢀandꢀbeꢀ
asꢀcloseꢀasꢀpossibleꢀtoꢀtheꢀroutingꢀlayerꢀconnectingꢀtheꢀ
powerꢀMOSFET’s.
RJK0305DPBꢀ(R
ꢀ=ꢀ10mΩꢀ(nominal)ꢀ13mΩꢀ(maxi-
DS(ON)
mum)ꢀ(C
4.5V,ꢀV
ꢀ=ꢀQ /10Vꢀ=ꢀ150pF,ꢀV
ꢀ=ꢀ5V),ꢀV ꢀ=ꢀ
MILLER
MILLER
GD
BOOST GS
ꢀ=ꢀ3V,ꢀθ ꢀ=ꢀ40°C/W)ꢀisꢀsufficient.ꢀCheckingꢀitsꢀ
JA
lꢀ PlaceꢀLTC3878ꢀPinsꢀ9ꢀtoꢀ16ꢀfacingꢀtheꢀpowerꢀcompo-
nents.ꢀKeepꢀcomponentsꢀconnectedꢀtoꢀPinꢀ1ꢀcloseꢀtoꢀ
LTC3878ꢀ(noiseꢀsensitiveꢀcomponents).
powerꢀdissipationꢀatꢀcurrentꢀlimitꢀwithꢀ=ꢀρ
ꢀ=ꢀ1.4:
100°C
1.2V
15A
2
2
2
PTOP
=
15A •1.4•13mΩ+ 28V
(
)
(
)
28V
150pF
lꢀ PlaceꢀC ,ꢀC ,ꢀMOSFETs,ꢀD ꢀandꢀinductorꢀallꢀinꢀoneꢀ
IN OUTꢀ
B
2.5Ω 1.2Ω
5V −3V 3V
compactꢀarea.ꢀItꢀmayꢀhelpꢀtoꢀhaveꢀsomeꢀcomponentsꢀ
+
400kHz
(
)
onꢀtheꢀbottomꢀsideꢀofꢀtheꢀboard.ꢀ
= 0.18W+0.58W = 0.65W
TJ = 70°C+0.76W •40°C/W =100°C
lꢀ Useꢀanꢀimmediateꢀviaꢀtoꢀconnectꢀcomponentsꢀtoꢀtheꢀ
groundꢀplaneꢀSGNDꢀandꢀPGNDꢀofꢀLTC3878.ꢀUseꢀseveralꢀ
largerꢀviasꢀforꢀpowerꢀcomponents.
ꢀ
Theꢀ junctionꢀ temperaturesꢀ willꢀ beꢀ significantlyꢀ lessꢀ atꢀ
nominalꢀ current,ꢀ butꢀ thisꢀ analysisꢀ showsꢀ thatꢀ carefulꢀ
attentionꢀtoꢀheatꢀsinkingꢀwillꢀbeꢀnecessary.
lꢀ Useꢀcompactꢀswitchꢀnodeꢀ(SW)ꢀplaneꢀtoꢀimproveꢀcool-
ingꢀofꢀtheꢀMOSFETsꢀandꢀtoꢀkeepꢀEMIꢀdown.ꢀ
3878fa
ꢀꢇ
LTC3878
applicaTions inForMaTion
lꢀ PlaceꢀM2ꢀasꢀcloseꢀtoꢀtheꢀcontrollerꢀasꢀpossible,ꢀkeepingꢀ
lꢀ UseꢀplanesꢀforꢀV ꢀandꢀV ꢀtoꢀmaintainꢀgoodꢀvoltageꢀ
IN
OUT
theꢀPGND,ꢀBGꢀandꢀSWꢀtracesꢀshort.
filteringꢀandꢀtoꢀkeepꢀpowerꢀlossesꢀlow.ꢀ
lꢀ KeepꢀtheꢀhighꢀdV/dTꢀSW,ꢀBOOSTꢀandꢀTGꢀnodesꢀawayꢀ
lꢀ Floodꢀallꢀunusedꢀareasꢀonꢀallꢀlayersꢀwithꢀcopper.ꢀFloodingꢀ
withꢀcopperꢀwillꢀreduceꢀtheꢀtemperatureꢀriseꢀofꢀpowerꢀ
component.ꢀYouꢀcanꢀconnectꢀtheꢀcopperꢀareasꢀtoꢀanyꢀ
fromꢀsensitiveꢀsmall-signalꢀnodes.ꢀ
lꢀ Connectꢀ theꢀ inputꢀ capacitor(s),ꢀ C ,ꢀ closeꢀ toꢀ theꢀ
IN
DCꢀnet.ꢀ(V ,ꢀV ,ꢀGNDꢀorꢀtoꢀanyꢀotherꢀDCꢀrailꢀinꢀyourꢀ
IN OUTꢀ
powerꢀMOSFETs.ꢀThisꢀcapacitorꢀcarriesꢀtheꢀMOSFETꢀACꢀ
system).
current.ꢀ
lꢀ PlaceꢀdecouplingꢀcapacitorꢀC ꢀnextꢀtoꢀtheꢀI ꢀandꢀSGNDꢀ
C2
TH
lꢀ ConnectꢀtheꢀINTV ꢀdecouplingꢀcapacitorꢀC ꢀcloselyꢀ
CC
VCC
pinsꢀwithꢀshort,ꢀdirectꢀtraceꢀconnections.
toꢀtheꢀINTV ꢀandꢀPGNDꢀpins.ꢀ
CC
Whenꢀlayingꢀoutꢀaꢀprintedꢀcircuitꢀboardꢀwithoutꢀaꢀgroundꢀ
plane,ꢀuseꢀtheꢀfollowingꢀchecklistꢀtoꢀensureꢀproperꢀoperationꢀ
ofꢀtheꢀcontroller.ꢀTheseꢀitemsꢀareꢀillustratedꢀinꢀFigureꢀ7.
lꢀ Connectꢀtheꢀtopꢀdriverꢀboostꢀcapacitor,ꢀC ,ꢀcloselyꢀtoꢀ
B
theꢀBOOSTꢀandꢀSWꢀpins.
lꢀ ConnectꢀtheꢀV ꢀpinꢀdecouplingꢀC ꢀcloselyꢀtoꢀtheꢀV ꢀ
IN
F
IN
lꢀ Segregateꢀtheꢀsignalꢀandꢀpowerꢀgrounds.ꢀAllꢀsmall-signalꢀ
componentsꢀshouldꢀreturnꢀtoꢀtheꢀSGNDꢀpinꢀatꢀoneꢀpoint.ꢀ
SGNDꢀandꢀPGNDꢀshouldꢀbeꢀtiedꢀtogetherꢀunderneathꢀ
theꢀICꢀandꢀthenꢀconnectꢀdirectlyꢀtoꢀtheꢀsourceꢀofꢀM2.ꢀ
andꢀPGNDꢀpins.ꢀ
C
C
B
SS
L
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
RUN/SS BOOST
PGOOD
TG
SW
D
B
V
+
RNG
M1
LTC3878
FCB
PGND
C
C
IN
C1
V
IN
R
C
M2
I
BG
TH
C
VCC
C
C2
SGND
INTV
CC
+
–
–
+
C
I
V
F
ON
IN
V
C
OUT
OUT
R1
R2
R
F
V
NC
FB
R
ON
3878 F08
BOLD LINES INDICATE HIGH CURRENT PATHS
Figureꢀ8.ꢀLTC3878ꢀLayoutꢀDiagramꢀWithoutꢀGroundꢀPlane
3878fa
ꢀꢈ
LTC3878
Typical applicaTions
4.5Vꢀtoꢀ±4VꢀInput,ꢀ±.2V/20AꢀOutputꢀatꢀ300kHz
V
IN
4.5V TO 14V
C
SS
D
B
C
IN1
+
C
0.1µF
IN2
CMDSH-3
10µF
16V
s2
180µF
16V
1
2
16
15
RUN/SS
BOOST
TG
R1
R2
R
PG
C
B
LTC3878
10.0k 57.6k
100k
0.22µF
M1
PGOOD
L1
0.44µH
RJK0305DPB
V
OUT
3
4
14
13
V
RNG
SW
1.2V
20A
C
OUT1
FCB
PGND
+
C
OUT2
330µF
2.5V
s3
C
C1
330pF
100µF
6.3V
s2
R
C
18k
5
6
12
11
M2
I
TH
BG
RJK0330DPB
C
VCC
C
C2
4.7µF
100pF
SGND
INTV
CC
7
8
10
9
R
FB1
I
V
IN
ON
C
C
C
: TDK C3225X5R1C106MT s2
: SANYO 2R5TPE330M9 s3
10.0k
C
IN1
OUT1
OUT2
R
F
F
V
NC
FB
0.1µF
1Ω
: MURATA GRM31CR60J107ME39 s2
R
R
ON
576k
FB2
L1: PULSE PA0513.441NLT
5.11k
3878 TA02
4.5Vꢀtoꢀ24VꢀInput,ꢀ±.8V/±0AꢀOutputꢀatꢀ500kHz
V
IN
4.5V TO 24V
C
SS
D
B
+
C
56µF
25V
C
10µF
25V
IN2
0.1µF
IN1
CMDSH-3
1
16
15
RUN/SS
BOOST
TG
R1
R2
R
PG
C
B
LTC3878
10.0k 95.3k
100k
0.22µF
2
M1
PGOOD
L1
0.8µH
FDS8690
V
OUT
3
4
14
13
V
RNG
SW
1.8V
10A
FCB
PGND
C
C1
1000pF
R
C
C
OUT1
330µF
2.5V
10k
+
C
OUT2
5
6
12
11
M2
FDS8670
I
TH
BG
100µF
6.3V
C
VCC
C
C2
4.7µF
100pF
SGND
INTV
CC
7
8
10
9
R
FB1
I
V
IN
ON
C
C
C
: TDK C3225X5R1E106MT
: SANYO 2R5TPE330M9
10.0k
C
IN1
OUT1
OUT2
R
F
F
V
NC
FB
0.1µF
2.2Ω
: MURATA GRM31CR60J107ME39
R
R
ON
FB2
L1: SUMIDA CDEP105NP-0R8MC-50
12.7k 511k
3878 TA03
3878fa
ꢁ0
LTC3878
Typical applicaTions
4.5Vꢀtoꢀ32VꢀInput,ꢀ±V/5AꢀOutputꢀatꢀ250kHz
V
IN
4.5V TO 32V
C
SS
D
B
+
C
22µF
35V
C
4.7µF
50V
IN2
0.1µF
IN1
ZLLS1000
1
2
16
15
RUN/SS
BOOST
TG
R
C
PG
B
LTC3878
100k
0.22µF
M1
PGOOD
L1
2.2µH
BSC093N04LS
V
OUT
3
4
5
14
13
12
V
SW
PGND
BG
1V
5A
RNG
R
C
FCB
13k
+
C
47µF
6.3V
C
OUT2
M2
OUT1
330µF
2.5V
I
TH
BSC093N04LS
C
VCC
C
C2
C
4.7µF
C1
100pF
1000pF
6
11
SGND
INTV
CC
7
8
10
9
R
FB1
I
V
IN
ON
C
C
C
: MURATA GRM32ER71H475K
: SANYO 2R5TPE330M9
10.0k
C
IN1
OUT1
OUT2
R
F
F
V
NC
FB
0.1µF
2.2Ω
: TDK C3216X5R0J476M
R
R
ON
FB2
L1: WURTH 744311220
2.55k 576k
3878 TA04
4.5Vꢀtoꢀ28VꢀInput,ꢀ2.5V/5AꢀOutputꢀatꢀ500kHz
V
IN
4.5V TO 28V
C
SS
D
B
+
C
C
4.7µF
50V
IN2
0.1µF
IN1
CMDSH-3
22µF
1
2
16
15
RUN/SS
BOOST
TG
35V
R1
R2
R
PG
C
B
LTC3878
10.0k 80.6k
100k
0.22µF
M1-1
PGOOD
L1
2.2µH
1/2 Si4816BDY
V
OUT
3
4
5
14
13
12
V
RNG
SW
PGND
BG
2.5V
5A
C
C1
R
1000pF
C
FCB
C
OUT
8.2k
M1-2
1/2 Si4816BDY
100µF
I
TH
C
6.3V
VCC
C
C2
4.7µF
s2
100pF
6
11
SGND
INTV
CC
7
8
10
9
R
FB1
I
V
IN
ON
10.0k
C
R
F
F
V
NC
FB
0.1µF
C
C
: MURATA GRM32ER71H475K
: MURATA GRM32ER60J107M
2.2Ω
IN1
OUT
R
R
ON
715k
FB2
L1: WURTH 744311220
21.5k
3878 TA05
3878fa
ꢁꢀ
LTC3878
Typical applicaTions
±3Vꢀtoꢀ32VꢀInput,ꢀ±2V/5AꢀOutputꢀatꢀ300kHz
V
IN
13V TO 32V
C
SS
D
B
+
+
C
22µF
35V
C
4.7µF
50V
IN2
0.1µF
IN1
ZLLS1000
1
2
16
15
RUN/SS
BOOST
TG
R
C
PG
B
LTC3878
100k
0.22µF
M1
PGOOD
L1
10µH
BSC093N04LS
V
OUT
3
4
5
14
13
12
V
SW
PGND
BG
12V
5A
RNG
R
C
FCB
20k
C
82µF
16V
OUT1
M2
I
TH
BSC093N04LS
C
VCC
C
C2
C
4.7µF
C1
100pF
1000pF
6
11
SGND
INTV
CC
7
8
10
9
R
FB1
I
V
IN
ON
10.0k
C
R
F
F
V
NC
FB
0.1µF
2.2Ω
C
C
: MURATA GRM32ER71H475K
IN1
R
R
ON
FB2
: SANYO 16SVPA82MAA
OUT1
140k 2.7M
2.7M
L1: IHLP5050FD01 10µH
3878 TA06
3878fa
ꢁꢁ
LTC3878
Typical applicaTions
Positive-to-NegativeꢀConverter,ꢀ–5V/5Aꢀatꢀ300kHz
V
IN
4.5V TO 20V
C
SS
D
B
+
+
C
82µF
25V
C
10µF
25V
IN2
0.1µF
IN1
CMDSH-3
V
I
IN OUT
1
2
16
15
RUN/SS
BOOST
TG
5V 5A
12V 7.7A
20V 9.1A
C
B
LTC3878
0.22µF
M1
PGOOD
L1
RJK0304DPB
2.2µH
3
4
5
14
13
12
V
SW
PGND
BG
RNG
R
C
FCB
C
C
OUT2
OUT1
+
15k
M2
120µF
10µF
10V
s4
I
TH
RJK0304DPB
6.3V
C
VCC
4.7µF
C
C2
C
s3
C1
100pF
V
–5V
5A
OUT
2200pF
6
11
SGND
INTV
CC
7
8
10
9
R
FB1
20.0k
I
ON
V
IN
C
F
R
C
C
C
: TDK C3225X5R1E106MT
F
IN1
OUT1
OUT2
V
FB
NC
0.1µF
2.2Ω
: KEMET A700D127M006ATE015 s3
: MURATA GRM31CR61A106KA01 s4
R
R
ON
FB2
L1: IHLP5050EZ-01 2.2µH
105k 2.4M
3878 TA07
3878fa
ꢁꢂ
LTC3878
package DescripTion
GNꢀPackage
±6-LeadꢀPlasticꢀSSOPꢀ(Narrowꢀ.±50ꢀInch)
(ReferenceꢀLTCꢀDWGꢀ#ꢀ05-08-1641)
.189 – .196*
(4.801 – 4.978)
.045 .005
.009
(0.229)
REF
16 15 14 13 12 11 10 9
.254 MIN
.150 – .165
.229 – .244
.150 – .157**
(5.817 – 6.198)
(3.810 – 3.988)
.0165 .0015
.0250 BSC
RECOMMENDED SOLDER PAD LAYOUT
1
2
3
4
5
6
7
8
.015 .004
(0.38 0.10)
s 45°
.0532 – .0688
(1.35 – 1.75)
.004 – .0098
(0.102 – 0.249)
.007 – .0098
(0.178 – 0.249)
0° – 8° TYP
.016 – .050
(0.406 – 1.270)
.0250
(0.635)
BSC
.008 – .012
GN16 (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
3878fa
ꢁꢃ
LTC3878
revision hisTory
REV
DATE
07/10 Updatedꢀtitle
UpdatedꢀFeatures
DESCTRIPTION
PAGEꢀNUMBER
A
1
1
EditedꢀTypicalꢀApplication
1
AddedꢀNoteꢀ4ꢀtoꢀOrderꢀInformationꢀsection
AddedꢀlabelsꢀtoꢀG22ꢀandꢀG24ꢀinꢀTypicalꢀPerformanceꢀCharacteristics
2
6
ModifiedꢀPinꢀ3ꢀV ꢀdescription
7
RNG
ModifiedꢀV ꢀdescription
10
RNG
ModifiedꢀR ꢀequation
DS(ON)
10
ModifiedꢀR ꢀdescriptionꢀinꢀApplicationsꢀInformation
12
ON
EditedꢀFigureꢀ7
17
EditedꢀDesignꢀExampleꢀsection
EditedꢀFigureꢀ8
18
19
EditedꢀTypicalꢀApplications
AddedꢀTypicalꢀApplication
UpdatedꢀRelatedꢀParts
20,ꢀ21,ꢀ22,ꢀ23
26
26
3878fa
InformationꢀfurnishedꢀbyꢀLinearꢀTechnologyꢀCorporationꢀisꢀbelievedꢀtoꢀbeꢀaccurateꢀandꢀreliable.ꢀ
However,ꢀnoꢀresponsibilityꢀisꢀassumedꢀforꢀitsꢀuse.ꢀLinearꢀTechnologyꢀCorporationꢀmakesꢀnoꢀrepresenta-
tionꢀthatꢀtheꢀinterconnectionꢀofꢀitsꢀcircuitsꢀasꢀdescribedꢀhereinꢀwillꢀnotꢀinfringeꢀonꢀexistingꢀpatentꢀrights.
ꢁꢄ
LTC3878
Typical applicaTion
4.5Vꢀtoꢀ±4VꢀInput,ꢀ0.9V/25AꢀOutputꢀatꢀ300kHz
V
IN
4.5V TO 14V
C
SS
D
B
C
IN1
+
C
IN2
0.1µF
CMDSH-3
10µF
16V
s4
180µF
16V
1
2
16
RUN/SS
PGOOD
BOOST
LTC3878
R1
R2
R
PG
C
B
10.0k 93.1k
100k
0.22µF
15
M1
TG
L1
0.26µH
SiR408DP
V
OUT
3
4
14
13
V
RNG
SW
0.9V
25A
FCB
PGND
C
C1
680pF
R
C
C
OUT2
C
OUT1
M2
SiR892DP
s2
7.5k
+
5
6
12
11
100µF
6.3V
s2
330µF
2.5V
s3
I
TH
BG
C
VCC
C
C2
4.7µF
100pF
SGND
INTV
CC
7
8
10
9
R
FB1
I
V
IN
ON
20.0k
C
R
F
C
C
C
: TDK C3225X5R1E106MT s4
F
IN1
V
NC
FB
0.1µF
2.2Ω
: SANYO 2R5TPE330M9 s3
OUT1
OUT2
R
R
ON
: MURATA GRM31CR60J107ME39 s2
FB2
2.49k 432k
L1: PULSE PA0513.261NLT
3878 TA08
relaTeD parTs
PARTꢀNUMBER
DESCRIPTION
COMMENTS
LTC3879
NoꢀR ꢀConstantꢀOn-TimeꢀSynchronousꢀStep-Downꢀꢀ
VeryꢀFastꢀTransientꢀResponse,ꢀt
ꢀ=ꢀ43ns,ꢀ4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ
ON(MIN) IN
SENSE
DC/DCꢀController
0.6Vꢀ≤ꢀV ꢀ≤ꢀ0.9V ,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16
OUT IN
LTC3854
SmallꢀFootprintꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ Fixedꢀ400kHzꢀOperatingꢀFrequencyꢀ4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ
IN IN
DC/DCꢀController
0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀ2mmꢀ×ꢀ3mmꢀQFN-12
OUT
LTC3851Aꢀ
LTC3851A-1
NoꢀR ꢀWideꢀV ꢀRangeꢀSynchronousꢀStep-Downꢀ
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀꢀ
SENSE
IN
DC/DCꢀController
4Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V,ꢀMSOP-16E,ꢀ3mmꢀ×ꢀ3mmꢀꢀ
IN
OUT
QFN-16,ꢀSSOP-16
LTC3775
HighꢀFrequencyꢀSynchronousꢀStep-DownꢀDC/DCꢀController FixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ1MHz,ꢀ4.5Vꢀ≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ
IN
0.6Vꢀ≤ꢀV ꢀ≤ꢀ0.8V ,ꢀ3mmꢀ×ꢀ3mmꢀQFN-16
OUT
IN
LTC3850/LTC3850-1ꢀ Dualꢀ2-Phase,ꢀHighꢀEfficiencyꢀSynchronousꢀStep-Downꢀ
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ780kHz,ꢀꢀ
4Vꢀ≤ꢀV ꢀ≤ꢀ30V,ꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ5.25V
LTC3850-2
DC/DCꢀControllers,ꢀR
ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀ
SENSE
IN
OUT
Tracking
LTC3853
TripleꢀOutput,ꢀMultiphaseꢀSynchronousꢀStep-DownꢀDC/DCꢀ Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ250kHzꢀtoꢀ750kHz,ꢀ
Controller,ꢀR ꢀorꢀDCRꢀCurrentꢀSensingꢀandꢀTracking 4Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀV ꢀUpꢀtoꢀ13.5V
SENSE
IN
OUT
LTC3857/LTC3857-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀꢀ
4V≤ꢀV ꢀ≤ꢀ38V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ24V,ꢀI ꢀ=ꢀ50µA,
Q
DC/DCꢀControllerꢀwithꢀ99%ꢀDutyꢀCycle
IN
OUT
Q
LTC3868/LTC3868-1 LowꢀI ,ꢀDualꢀOutputꢀ2-PhaseꢀSynchronousꢀStep-Downꢀ
Phase-LockableꢀFixedꢀOperatingꢀFrequencyꢀ50kHzꢀtoꢀ900kHz,ꢀꢀ
4V≤ꢀV ꢀ≤ꢀ24V,ꢀꢀ0.8Vꢀ≤ꢀV ꢀ≤ꢀ14V,ꢀI ꢀ=ꢀ170µA,
Q
DC/DCꢀControllerꢀwithꢀ99%ꢀDutyꢀCycle
IN
OUT
Q
3878fa
LT 0710 REV A • PRINTED IN USA
Linear Technology Corporation
1630ꢀ McCarthyꢀ Blvd.,ꢀ Milpitas,ꢀ CAꢀ 95035-7417
ꢀ
ꢁꢅ
●
ꢀ●ꢀ
LINEAR TECHNOLOGY CORPORATION 2009
(408)ꢀ432-1900ꢀ ꢀFAX:ꢀ(408)ꢀ434-0507ꢀ ꢀwww.linear.com
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LTC3880 - Dual Output PolyPhase Step-Down DC/DC Controller with Digital Power System Management; Package: QFN; Pins: 40; Temperature Range: -40°C to 85°C
Linear
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