LT3754IUHPBF_技术文档

LT3754

× 50mA

16-Channel

LED Driver

FEATURES

DESCRIPTION

The LT^®3754 is a 16-channel LED driver with a step-up

DC/DC controller capable of driving up to 45V of LEDs.

Eachchannelcontainsanaccuratecurrentsinkwith±±.ꢀ8

currentmatching.Channelsfollowamasterprogrammable

current to allow between 10mA to 50mA of LED current

per string. Channels can be paralleled for higher LED

n

Up to 45V of LEDs × 50mA, 16-Channel LED Driver

n

Wide Input Range : 6V to 40V

n

±2.8% LED Current Matching at 20mA (Typ 0.ꢀ%ꢁ

n

Up to 3000:1 True Color PWM™ Dimming Range

n

Single Resistor Sets LED Current (10mA to 50mAꢁ

n

LED Current Regulated Even for PV > V

IN

OUT

n

current. Output voltage adapts to variations in LED V for

Output Adapts to LED V for Optimum Efﬁciency

F

optimum efﬁciency and open LED faults do not affect the

Fault Flag + Protection for Open LED Strings

operation of connected LED strings.

Protection for LED Pin to V

Short

OUT

Parallel Channels for Higher LED Current

Programmable LED Current Derating vs Temperature

Accurate Undervoltage Lockout Threshold with

Programmable Hysteresis

TheLT3754allowsaPWMdimmingrangeupto3000:1and

an analog dimming range up to ±5:1. Operating frequency

can be programmed from 100kHz up to 1MHz using a

single resistor or synchronized to an external clock.

n

Programmable Frequency (100kHz to 1MHz)

Synchronizable to an External Clock

Additional features include: programmable maximum

V

OUT

for open LED protection, a fault ﬂag for open LED,

APPLICATIONS

programmable LED current derating vs temperature,

micropower shutdown and internal soft-start. The LT3754

is available in a thermally enhanced 5mm × 5mm 3±-pin

QFN Package.

n

Automotive, Notebook and TV Monitor Backlighting

L, LT, LTC and LTM, Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. True Color PWM is a trademark of Linear Technology Corporation.

All other trademarks are the property of their respective owners. Protected by U.S. Patents,

including 7199560, 73±1±03.

TYPICAL APPLICATION

Worst-Case Channels LED Current Matching

(Normalized to 16-channel Averageꢁ

92% Efﬁcient, 36W Backlight LED Driver

PV

IN

±4V

4.7μF

0.ꢀ

10μH

UP TO 45V OF LEDs PER STRING

V

IN

10V

5s

±.±μF

V

IN

0.4

0.0

INTV

GATE

CC

4.7μF

499k

• • • •

SENSE

SHDN/UVLO

0.015Ω

40.±k

•

CTRL

V

OUT

–0.4

–0.ꢀ

LT3754

PWM

REF

LED1

LED±

•

R

= 14.7k (I(LED) = ±0mA)

ISET

16 CHANNELS

•

–50 –±5

0

±5

50 75

100 1±5

±0k

LED15

JUNCTION TEMERATURE (°C)

T

SET

LED16

3754 TA01

V

IN

100k

30.9k 11k

±0k

FAULT

OVP

SET

GND RT

I

V

SYNC

SET

C

39.±k 5.76k

10k

±.±nF

3754f

1

LT3754

ABSOLUTE MAXIMUM RATINGS

PIN CONFIGURATION

(Note 1ꢁ

TOP VIEW

V

, LED1-16 .........................................................60V

OUT

V , SHDN/UVLO, FAULT...........................................40V

IN

INTV ......................................................................13V

CC

3± 31 30 ±9 ±ꢀ ±7 ±6 ±5

INTV above V ...................................................+0.3V

CC

IN

LED1

LED±

LED3

LED4

LED5

LED6

LED7

LEDꢀ

LED16

LED15

LED14

1

±

3

4

5

6

7

ꢀ

±4

±3

±±

PWM, CTRL, SYNC .....................................................6V

V ...............................................................................3V

C

REF

±1 LED13

±0 LED1±

V

, RT, I , T , OVP .......................................±V

SET SET SET

33

SENSE......................................................................0.4V

19

1ꢀ

17

LED11

LED10

LED9

Operating Junction Temperature Range

(Notes ±,3)...........................................−40 °C to 1±5 °C

Storage Temperature Range.................−65 °C to 150 °C

9

10 11 1± 13 14 15 16

UH PACKAGE

3±-LEAD (5mm s 5mm) PLASTIC QFN

T

= 1±5°C, θ = 34°C/W, θ = 3°C/W

JA JC

JMAX

EXPOSED PAD (PIN 33) IS GND, MUST BE SOLDERED TO PCB

ORDER INFORMATION

LEAD FREE FINISH

LT3754EUH#PBF

LT3754IUH#PBF

TAPE AND REEL

PART MARKING*

3754

PACKAGE DESCRIPTION

TEMPERATURE RANGE

LT3754EUH#TRPBF

LT3754IUH#TRPBF

–40°C to 1±5°C

3±-Lead (5mm × 5mm) Plastic QFN

3754

Consult LTC Marketing for parts speciﬁed with wider operating temperature ranges. *The temperature grade is identiﬁed by a label on the shipping container.

Consult LTC Marketing for information on non-standard lead based ﬁnish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel speciﬁcations, go to: http://www.linear.com/tapeandreel/

3754f

2

LT3754

ELECTRICAL CHARACTERISTICS ^Thel denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= V_OUT= 6V, R_ISET= 14.ꢀk unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

INPUT BIAS, REFERENCE

Minimum Operational V (To Allow GATE Switching)

V = 1.5V

IN

C

l

V

= INTV (Shorted)

4.±

5.5

4.5

6.0

V

IN

CC

≠ INTV

CC

Operational V

V

= INTV (Shorted)

4.5

6

14

40

V

IN

CC

≠ INTV

CC

V

IN

Quiescent Current

CTRL = 0.1V, PWM = 0V

4.±

9.5

5.7

1±

mA

CTRL = 0.1V, PWM = 1.5V, (Not Switching)

LED

= 1.±V

1–16

V

Shutdown Current (V ≠ INTV ) (Not Shorted)

SHDN/UVLO = 0V, V =6V

0.1

±

μA

IN

CC

IN

SHDN/UVLO = 0V, V = 40V

10

IN

Shutdown Current (V = INTV (Shorted))

SHDN/UVLO = 0V, V = INTV = 4.5V

10

±0

40

μA

IN

CC

IN

CC

SHDN/UVLO = 0V, V = INTV = 13V

IN

l

SHDN/UVLO Threshold (Micropower) (Falling) (V

SHDN/UVLO Threshold (UVLO) (Falling)

)

SD

I

< ±0μA

VIN

0.3

0.7

V

1.414

1.476

1.53ꢀ

3.±

(Stop Switching) (V

)

UV

l

SHDN/UVLO Pin Current

SHDN/UVLO = V - 50mV

SHDN/UVLO = V + 50mV

1.6

±.4

0

μA

UV

V

REF

V

REF

V

REF

Voltage

I

= 0μA

1.450

1.4ꢀ5

0.01

±

1.5±4

0.05

V

8/V

mV

VREF

Line Regulation

Load Regulation

= 0μA, 6V < V < 40V

IN

0 < I

< 150μA (Max)

VREF

OSCILLATOR

l

Frequency: f

(100kHz)

(1MHz)

RT = 5±3k

RT = 39.±k

9±

101

1

11±

1.10

0.±

kHz

MHz

8/V

V

OSC

0.90

f

(1MHz) Line Regulation

RT = 39.±k, 6V < V < 40V

0.1

1.6

OSC

IN

RT Pin Voltage

RT = 39.±k

Minimum Off-Time

Minimum On-Time

(Note 5)

170

190

±50

nS

SYNC Input High Threshold

SYNC Input Low Threshold

SYNC Input Current

±.±

V

0.6

SYNC = 0V

SYNC = 5V

0

±5

μA

SYNC Frequency Range

RT = 5±3k

RT = 39.±k

0.1±

1.±

1.5

MHz

LINEAR REGULATOR (INTV

ꢁ

CC

INTV Regulation Voltage

V

= 1±V

IN

6.65

7

7.35

V

mV

V

CC

I

= 10mA

±50

3.ꢀ

3.4

57

Dropout (V − INTV

)

CC

INTVCC

IN

(Start Switching)

(Stop Switching)

INTV UVLO (+)

CC

V

INTV UVLO (−)

CC

l

INTV Current Limit

44

mA

CC

OVP/ LED ERROR AMPLIFIERS

Transconductance (OVP)

Voltage Gain (OVP)

ΔI = ±±.5μA

VC

4

5

μmhos

V/V

Transconductance (LED)

ΔI = ±±.5μA

VC

33

μmhos

3754f

3

LT3754

ELECTRICAL CHARACTERISTICS ^Thel denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= V_OUT= 6V, R_ISET= 14.ꢀk unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

45

MAX

UNITS

V/V

μA

μS

V

Voltage Gain (LED)

V Source Current (Out of Pin)

C

V = 1.5V, V

= 0.ꢀV, OVP = 1.5V

10

C

LEDx

SET

V Sink Current (OVP)

C

V = 1.5V, V

= 0.ꢀV, OVP = 0V

15

C

SET

V Sink Current (LED)

C

V = 1.5V, V

= 1.±V, OVP = 1.5V

9

C

SET

V Output High (clamp) (V

C

)

±.3

0.ꢀ

1.1

COH

V Output Low (clamp) (V

C

)

V

COL

V Switching Threshold (V

C

)

V

CSW

SENSE AMP

SENSE Input Current (Out of Pin)

SENSE Current Limit Threshold

Current Mode Gain

65

5±

6

μA

mV

V/V

mV

l

46

90

60

ΔV(V )/ΔV(SENSE)

C

SENSE Over Current Limit Threshold

LED CURRENT / CONTROL

100

110

I

Pin Voltage

CTRL = 1.5V

1.00

±0.±

±0.7

50.1

1.1

V

mA

8

SET

LEDx Current (±0mA) (R

= 14.7k)

V

= 1V, CTRL = 1.5V

= 1V, CTRL = 0.04V

19.±9

47.ꢀ5

±1.11

±±.ꢀ

ISET

LEDx

l

LEDx Current Matching (±0mA) (R

= 14.7k)

ISET

LEDx Current (50mA) (R

= 5.76k)

5±.35

mA

V

ISET

LED Pin Regulation Voltage

Threshold

T

630

mV

SET

ANALOG DIMMING

CTRL Input Current (Out of Pin)

CTRL = 1V

CTRL = 0.04V

40

50

±00

nA

LEDx Current (Dimming ±5:1)

PWM DIMMING

V

LEDx

= 1V, CTRL = 0.04V

0.ꢀ

mA

PWM Input Low Threshold

PWM Input High Threshold

PWM Input Current

0.7

1

V

1.1

1.4

PWM = 1.5V

PWM = 6V

6

±4

μA

V

Pin Current in PWM Mode V(V ) = 60V

PWM = 1.5V, V = 1V

LEDx

370

±0

μA

OUT

PWM = 0V, V

= 1V

LEDx

LEDx Leakage Current

(PWM = 0V)

V

= 1V, V

= 1±V

= 60V

0.1

1

±

μA

LEDx

OUT

= 50V, V

FAULT DIAGNOSTICS

FAULT Output Sink Current

LED1 = Open, V

= 0.3V

0.3

0.6

mA

V

FAULT

LED Short Threshold (V

)

V

= 1±V

= 60V

6

x

SH

OUT

(V

– V

)

OUT

LEDx

LED Open Detection Threshold

V

OUT

= 1±V

0.5

V

3754f

4

LT3754

ELECTRICAL CHARACTERISTICS ^Thel denotes the speciﬁcations which apply over the full operating

temperature range, otherwise speciﬁcations are at T_A= 25°C. V_IN= V_OUT= 6V, R_ISET=14.ꢀk unless otherwise noted.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

GATE DRIVER

GATE Driver Output Rise Time

GATE Driver Output Fall Time

GATE Output Low

V

= 1±V, C = 3300pF (Note 4)

30

nS

V

IN

L

= 1±V, C = 3300pF (Note 4)

IN

L

I

= 0μA

0.1

GATE

GATE Output High

INTV = V = 7V

CC IN

GATE

I

= 0μA

6.95

V

OUTPUT VOLTAGE

V

OUT

Over Voltage Protection (OVP) Regulation Voltage

OVP = 0.±±V

1±.5

57

V

SET

OVP = 1V

SET

OVP Input Current (Out of Pin)

OVP = 0.±±V, V =1±V

OUT

40

±00

nA

SET

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

LT3754I is guaranteed to meet performance speciﬁcations from

−40°C to 1±5°C junction temperature.

Note 3: For Maximum Operating Ambient Temperature, see

Thermal Calculations in the Applications Information section.

Note 2: The LT3754E is guaranteed to meet performance speciﬁcations

from 0°C to 1±5°C junction temperature. Speciﬁcations over the −40°C

to 1±5°C operating junction temperature range are assured by design,

characterization and correlation with statistical process controls. The

Note 4: GATE rise and fall times are measured between 108 and 908

of INTV voltage.

CC

Note 5: See Duty Cycle Considerations in the Applications Information.

T_A= 25°C, unless otherwise noted.

TYPICAL PERFORMANCE CHARACTERISTICS

Worst-Case Channels LED

Current Matching

LED Current

vs Junction Temperature

LED Current

vs CTRL Pin Voltage

(Normalized to 16-channel Averageꢁ

0.ꢀ

0.4

±1.00

±0.50

±0.00

19.50

19.00

55

50

45

40

35

30

±5

±0

15

10

5

R

=

R

= 14.7k

ISET

5.76k

7.3±k

9.76k

14.7k

±9.4k

0.0

–0.4

R

ISET

= 14.7k (I(LED) = ±0mA)

–0.ꢀ

0

–50 –±5

0

±5

50 75

100 1±5

–50 –±5

0

±5

50

75 100 1±5

0.00 0.±5 0.50 0.75 1.00 1.±5 1.50

JUNCTION TEMERATURE (°C)

CTRL (V)

3754 G01

3754 G0±

3754 G03

3754f

5

LT3754

TYPICAL PERFORMANCE CHARACTERISTICS ^T_A^{= 25°C, unless otherwise noted.}

LED Current Waveforms

SHDN/UVLO Threshold

3000:1 PWM Dimming (100Hzꢁ

V_REFvs Junction Temperature

vs Junction Temperature

1.5±5

1.505

1.5±5

1.505

1.4ꢀ5

1.465

1.445

I(LEDx)

±0mA/DIV

1.4ꢀ5

1.465

I(L)

1A/DIV

PWM

10V/DIV

3754 G04

5μs/DIV

1.445

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

JUNCTION TEMERATURE (°C)

JUNCTION TEMPERATURE (°C)

3754 G06

3754 G05

SHDN/UVLO Pin (Hysteresisꢁ

Current vs Junction Temperature

V_INShutdown Current

vs Junction Temperature

V_INQuiescent Current vs V_IN

±.ꢀ0

±.70

±.60

±.50

±.40

5

4

3

±

1

0

1±

10

ꢀ

V

= 6V, SHDN/UVLO = 0V

R

= 14.7k

IN

ISET

PWM = 1.5V, NO SWITCHING,

V(LED ) = 1.±V, CTRL = 0.1V

1-16

6

4

PWM = 0V, CTRL = 0.1V

±

±.30

±.±0

0

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

0

5

10 15 ±0 ±5 30 35 40

JUNCTION TEMPERATURE (°C)

V

(V)

IN

3754 G07

3754 G0ꢀ

3754 G09

V_CHigh Clamp, Active

and Low Clamp Levels

vs Junction Temperature

V_INQuiescent Current

vs Junction Temperature

Switching Frequency

vs Junction Temperature

15

10

1100

1050

±.5

±.0

1.5

1.0

0.5

V

C

HIGH CLAMP

V

IN

= 6V, R

= 14.7k, CTRL = 0.1V

ISET

V

ACTIVE (SWITCHING)

PWM = 1.5V, NO SWITCHING,

V(LED ) = 1.±V, CTRL = 0.1V

C

1000

950

1-16

RT = 39.±k

5

0

V

C

LOW CLAMP

PWM = 0V, CTRL = 0.1V

900

0.0

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

JUNCTION TEMPERATURE (°C)

3754 G10

3754 G11

3754 G1±

3754f

6

LT3754

TYPICAL PERFORMANCE CHARACTERISTICS ^T_A^{= 25°C, unless otherwise noted.}

INTV_CC

INTV_CC, UVLO(+ꢁ, UVLO(–ꢁ

vs Junction Temperature

vs Current, Junction Temperature

7.0

6.9

6.ꢀ

6.7

6.6

6.0

5.5

ꢀ

7

6

5

4

3

±

V

= 6V, PWM = 0V

V

= 1±V

I

= 10mA, ±0mA, 30mA

IN

LOAD

INTV (REGULATED)

CC

INTV UVLO(+)

CC

I

= 40mA

5.00

4.5

LOAD

50

I

= 10mA

= ±0mA

= 30mA

= 40mA

LOAD

INTV UVLO(–)

CC

V

= ꢀV, PWM = 0V

IN

–50 –±5

0

±5

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

JUNCTION TEMPERATURE (°C)

3754 G13

3754 G14

3754 G15

INTV_CCCurrent Limit

vs Junction Temperature

SENSE Threshold

vs Junction Temperature

Overvoltage Protection (OVPꢁ

Level vs Junction Temperature

60

55

50

60.0

57.5

55.0

5±.5

50.0

47.5

45.0

4±.5

40.0

70

60

50

40

30

±0

10

0

V

= 6V, INTV = 0V

CC

IN

OVP

= 1.0V

SET

INDUCTOR PEAK CURRENT THRESHOLD

(CYCLE-BY-CYCLE)

45

OVP

= 0.±±V

50

SET

40

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

75 100 1±5

JUNCTION TEMPERATURE (°C)

3754 G16

3754 G17

3754 G1ꢀ

V_OUT− V_(LEDxꢁShort Threshold

vs Junction Temperature

Minimum ON and OFF Times

vs Junction Temperature

GATE Rise/Fall Times

vs GATE Capacitance

7.00

6.75

±50

±±5

±00

175

150

1±5

100

1±0

100

ꢀ0

60

40

±0

0

V

= ꢀV, INTV = 7V, RT = 5±3k

C

= 3300pF

IN

CC

GATE

6.50

6.±5

MINIMUM ON-TIME

MINIMUM OFF-TIME

V

= 60V

= 1±V

OUT

FALL TIME

6.00

5.75

RISE TIME

5.50

5.±5

5.00

–50 –±5

0

±5

50

75 100 1±5

–50 –±5

0

±5

50

75 100 1±5

0

5

10

(nF)

15

±0

JUNCTION TEMPERATURE (°C)

C

L

3754 G1ꢀ

3754 G±0

3754 G±1

3754f

7

LT3754

PIN FUNCTIONS

LED (Pin1-8,1ꢀ-24ꢁ:16LEDDriverOutputs.Eachoutput

RT (Pin 25ꢁ: A resistor to ground programs switching

frequency f between 0.1MHz and 1MHz.

x

contains an open collector constant current sink. LED

OSC

currents are programmable from 10mA to 50mA using

V (Pin 26ꢁ: Output of Both Transconductance Error

C

a single resistor at the I

pin. Channel matching is

SET

Ampliﬁers for the Converter Regulation Loop. The most

±±8 with an absolute current accuracy of ±38. Connect

commonly used gm error ampliﬁer (LED) regulates V

OUT

the cathode of each LED string to an LED pin. Connect

to ensure no LED pin falls below 1V. The other gm error

the anode of each LED string to V . Channels can be

OUT

ampliﬁer (OVP) is activated if all LEDs fail open and a

paralleled for greater LED current or individually disabled

regulated maximum V

is required. Connect a resistor

OUT

(connect LED to V ).

OUT

and capacitor in series from the V pin to ground.

C

SENSE (Pin 9ꢁ: The Current Sense Input for the Control

Loop. Connect this pin to the sense resistor in the source

of the external power MOSFET.

PWM(Pin2ꢀꢁ:InputPinforPWMDimmingControl.Above

1.4V allows converter switching and below 0.7V disables

switching. The PWM signal can be driven from 0V to 6V.

GATE (Pin 10ꢁ: Drives the gate of an N-channel MOSFET

If unused, connect to V

REF.

from 0V to INTV .

CC

OVP

(Pin 28ꢁ: Programs maximum allowed V

OUT

SET

INTV (Pin 11ꢁ: A 7V LDO supply generated from V and

regulation level if all LEDs are open circuit.

CC

IN

used to power the GATE driver and some control circuitry.

CTRL (Pin 29ꢁ: CTRL pin voltage below 1V controls

Must be bypassed with a 4.7μF capacitor to GND.

maximum LED current. CTRL voltage can be set by a

V (Pin 12ꢁ: Input Supply Pin. Must be locally bypassed

resistordividerfromV ,V oranexternalvoltagesource.

IN

IN REF

with a 1μF capacitor to ground.

LED current derating versus temperature is achievable

if the voltage programmed at the CTRL pin has a negative

temperature coefﬁcient using an external resistor divider

SHDN/UVLO(Pin13ꢁ: TheSHDN/UVLOpinhasanaccurate

1.476Vthresholdandcanbeusedtoprogramanundervoltage

lockout (UVLO) threshold for system input supply using

a resistor divider from supply to ground. A ±.4μA pin

currenthysteresisallowsprogrammingofUVLOhysteresis.

SHDN/UVLO above 1.476V turns the part on and removes

a ±.4μA sink current from the pin. SHDN/UVLO < 0.7V

from V pin with temperature dependent resistance.

REF

T

(Pin 30ꢁ: Programs LT3754 junction temperature

SET

breakpoint past which LED current will begin to derate.

V

(Pin 31ꢁ: 1.4ꢀ5V Reference Output Pin. This pin

REF

can supply up to 150μA. Can be used to program CTRL,

reducesV current<±0μA.Iftheshutdownfunctionisnot

IN

T

SET

and OVP pin voltages using resistor dividers to

SET

required, it should be forced above 1.476V or connected

ground.

directly to V .

IN

I

(Pin32ꢁ:ResistortoGroundProgramsLEDpincurrent.

SET

FAULT (Pin 14ꢁ: Active low if any or all LED strings have

an open fault or if any/all LED pins have been shorted to

See Table 6 in the Applications Information Section.

Exposed Pad (Pin 33ꢁ: GND. The ground for the IC and

the converter. The package has an exposed pad (Pin 33)

underneath the IC which is the best path for heat out of the

package.Pin33shouldbesolderedtoacontinuouscopper

ground plane under the device to reduce die temperature

and increase the power capability of the LT3754.

V

. If fault(s) removed, FAULT ﬂag returns high. Fault

OUT

status is only updated during PWM high state and latched

during PWM low.

SYNC(Pin15ꢁ:Allowssynchronizationofboostconverter

switchingfrequencytoanexternalclock.RTresistorshould

be programmed for f

±08 below SYNC frequency. If

OSC

unused, connect to GND.

V

(Pin16ꢁ: Boosted Output Voltage of the Converter.

OUT

Connect a capacitor from this pin to ground. Connect the

anode of each LED (string) to V

.

OUT

3754f

8

LT3754

BLOCK DIAGRAM

1±

11

SHDN/UVLO

V

INTV

CC

7V(REGULATED)

UVLO(ꢀ) = 3.ꢀV, UVLO(ꢁ) = 3.4V

IN

13

+

R

S

1.476V

–

GATE

10

Q

600k

V

C

SYNC

15

EN

+

–

+

–

OSC

RT

SLOPE

±5

–

+

–

+

1.4ꢀ5V

5±mV

100mV

OVER

CURRENT

REF

1.4ꢀ5V

+

–

PEAK

4.±V(ꢀ)

3.7V(ꢁ)

CURRENT

SENSE

9

EN

HICCUP__MODE

V

OUT

INTV

CC_UV

IN_UV

SHDN_UV

16

+

–

6V

V

LEDx

1-ꢀ, 17-±4

PWM

EN

FAULT

SOFT

START

±7

31

14

V

REF

EN

LED

LOGIC

SS

1V

+

–

CTRL

±9

LED CURRENT

CONTROL

PWM

–

+

CHANNEL X

1.1V

56R

R

OVERVOLTAGE

AMP

–

+

LED AMP

+

–

V

PTAT

T

I

V

C

OVP

SET

EXPOSED PAD (GND)

SET

30

3±

33

±6

±ꢀ

3754 BD

Figure 1. LT3ꢀ54 Block Diagram

OPERATION

TheoperationoftheLT3754isbestunderstoodbyreferring

to the typical application circuit on the front page and the

Block Diagram in Figure 1. The LT3754 drives 16 strings

of LEDs by using a constant switching frequency, current

mode boost controller to generate a single output voltage

V

regulates to the lowest possible voltage allowable to

OUT

maintain regulated current in each LED string. Any OPEN

LED fault is indicated by the FAULT pin driven low without

effecting the operation of the connected LED strings.

The Block Diagram in Figure 1 illustrates the key functions

of the LT3754. It can be seen that two external supplies,

V

for the top (anode) of all LED strings. LED string

OUT

current is generated and controlled by connection of the

bottom LED in each string (cathode) to a current source

contained in each corresponding LED pin. Each LED pin

containsanaccuratecurrentsinktoground,programmable

V

REF

and INTV , are generated by the LT3754. The V

CC REF

pinprovidesaprecision1.4ꢀ5Voutputforusewithexternal

resistors to program the CTRL, OVP

and T

input

SET

pins. The INTV pin provides a regulated 7V output to

CC

between 10mA to 50mA using a single resistor at the I

SET

supply the gate driver for the boost controller GATE pin.

An accurate 1.476V threshold on the SHDN/UVLO pin

combinedwithaSHDN/UVLOpincurrenthysteresisallows

pin. LED channels can be paralleled to achieve higher LED

currents. For applications requiring less than 16 strings

of LEDs, channels can be paralleled or disabled (connect

a programmable resistor divider from V to SHDN/UVLO

IN

LED pin to V

before startup). For optimum efﬁciency,

OUT

3754f

9

LT3754

OPERATION

to deﬁne the turn on/off voltages for V . SHDN/UVLO pin

the LT3754 only allows MOSFET turn-on approximately

every ±ms. This hiccup mode signiﬁcantly reduces the

power rating required for the MOSFET.

IN

current switches from ±.4μA to 0μA when SHDN/UVLO

pin voltage exceeds 1.476V.

TheLT3754constantswitchingfrequencyisprogrammable

from 100kHz up to 1MHz using a single resistor at the RT

pin to ground. A SYNC pin is also provided to allow an

externalclocktodeﬁnetheconverterswitchingfrequency.

The GATE output provides a ±0.ꢀA peak gate drive for an

external N-channel power MOSFET to generate a boosted

LED current programming and dimming can be achieved

using the I , CTRL and PWM pins. A single resistor at

SET

the I

pin programs LED current. Analog dimming of

SET

LED brightness is achieved using the CTRL pin below 1V.

PWM dimming of LED brightness is achieved by control-

ling the duty cycle of the PWM pin.

outputvoltageV usingasingleinductor,Schottkydiode

OUT

For robust operation the LT3754 monitors system

conditions and performs soft start for startup after any of

and output capacitor. With LED strings connected from

V

to every LED pin, the lowest voltage on each LED pin

OUT

the following faults: V , SHDN or INTV voltages too low

IN

CC

is monitored and compared to an internal 1V reference.

is regulated to ensure the lowest LED pin voltage of

or MOSFET current too high. The LT3754, when entering

these faults, discharges an internal soft start node and

prevents switching at the GATE pin. When exiting these

faults the LT3754 ramps up an internal soft start node to

V

OUT

any connected LED string is maintained at 1V. If any of

the LED strings are open, the LT3754 will ignore the open

LED pin. If all of the LED strings are open V

charges up

OUT

controlV pinvoltageriseandhencecontrolMOSFETpeak

C

until a user programmable OVP (overvoltage protection)

level is reached. This programmable OVP level allows the

user to protect against LED damage when the LED strings

are opened and then reconnected.

switchcurrentrise.Inadditionthesoftstartperiodgradually

ramps up switching frequency from approximately 338

to 1008 of full scale.

The LT3754 monitors each LED pin voltage. If the LED

Since the LT3754 boost controller uses a current mode

string has an open fault (V(LED )<0.5V) the FAULT ﬂag

X

topology, the V pin voltage determines the peak current

C

is pulled low.

in the inductor of the converter and hence the duty cycle

of the GATE switching waveform. The basic loop uses a

pulse from an internal oscillator to set an RS ﬂip-ﬂop and

turn on the external power MOSFET. Current increases

For LED protection, the LT3754 CTRL pin allows an LED

current derating curve to be programmed versus the

ambient temperature of the LED strings. An NTC resistor

placed close to the LEDs decreases CTRL pin voltage and

hencedecreasesLEDcurrentasLEDambienttemperature

increases.

in the MOSFET and inductor until the V commanded

C

peak switch current is exceeded and the MOSFET is then

turned off. Inductor current is sensed during the GATE on

period by a sense resistor RS in the source of the external

N-channel power MOSFET. As with all current mode

converters, slope compensation is added to the control

path to ensure stability for duty cycles above 508. Any

over current fault condition in the MOSFET turns off the

MOSFETandtriggerssoftstartinternally.Inthisfaultmode

The LT3754 also allows it’s own junction temperature to

be monitored and regulated by derating LED currents

when a junction temperature programmed by the T

pin is exceeded.

SET

3754f

10

LT3754

APPLICATIONS INFORMATION

INTV Regulator Bypassing and Operation

Inductor

CC

The INTV pin is the output of an internal linear regulator

A list of inductor manufacturers is given in Table 1. How-

ever, there are many other manufacturers and inductors

that can be used. Consult each manufacturer for more

detailed information and their entire range of parts. Ferrite

cores should be used to obtain the best efﬁciency. Choose

an inductor that can handle the necessary peak current

without saturating. Also ensure that the inductor has a

CC

driven from V and is the supply for the LT3754 gate

IN

driver. The INTV pin should be bypassed with a 10V

CC

rated 4.7μF low ESR, X7R or X5R ceramic capacitor to

ensure stability and to provide enough charge for the gate

driver. For high enough V levels the INTV pin provides

IN

CC

a regulated 7V supply. Make sure INTV voltage does

CC

±

not exceed the V rating of the external MOSFET driven

low DCR (copper-wire resistance) to minimize I R power

GS

by the GATE pin. For low V levels the INTV level will

losses. Values between ±.±μH and 33μH will sufﬁce for

most applications. The typical inductor value required for

agivenapplication(assuming508inductorripplecurrent

for example) can be calculated as:

IN

CC

depend on V and the voltage drop of the regulator. The

IN

INTV regulator has an undervoltage lockout which

CC

prevents gate driver switching until INTV reaches 3.ꢀV

CC

and maintains switching until INTV falls below 3.4V.

CC

1

V_OUT

1

f_OSC

This feature prevents excessive power dissipation in the

1−

•

• V_IN

external MOSFET by ensuring a minimum gate drive level

V

to keep R

low. The INTV regulator has a current

CC

IN

DS(ON)

L =

limit of 44mA to limit power dissipation inside the I.C.

Thiscurrentlimitshouldbeconsideredwhenchoosingthe

N-channel power MOSFET and the switching frequency.

0.5• ^OUT•I_LEDx•16

V

IN

where:

The average current load on the INTV pin due to the

CC

LT3754 gate driver can be calculated as:

V

OUT

= (N • V ) + 1V

F

I

= Q • f

g OSC

INTVCC

(N = number of LEDs per string),

V = LED forward voltage drop,

where Q is the gate charge (at V = INTV ) speciﬁed

g

GS

CC

F

for the MOSFET and fosc is the switching frequency of the

I

= LED current per string

LEDx

LT3754 boost converter. It is possible to drive the INTV

CC

pin from a variety of external sources in order to remove

Example: For a 1±W LED driver application requiring 16

strings of 10 LEDs each driven with ±0mA, and choos-

power dissipation from the LT3754 and/or to remove the

INTV current limitation of 44mA. An external supply for

ing V = 1±V, V

= (3.75V • 10) + 1V = 3ꢀ.5V, I

=

CC

IN

OUT

LEDx

INTV should never exceed the V pin voltage or the

±0mA and f

= 1MHz the value for L is calculated as

IN

OSC

maximum INTV pin rating of 13V. If INTV is shorted

CC

IN

CC

1

3.2

1

to the V pin, V operational range is 4.5V to 13V.

IN

(1−

)•

•12V

10⁶

L =

= 16.5μH

0.5•3.2•20mA •16

3754f

11

LT3754

APPLICATIONS INFORMATION

Table 1. Inductor Manufacturers

Schottky Rectiﬁer

MANUFACTURER

Sumida

PHONE NUMBER

40ꢀ-3±1-9660

605-ꢀꢀ6-43ꢀ5

40±-563-6ꢀ66

ꢀ47-639-6400

561-99ꢀ-4100

WEB

The external diode for the LT3754 boost converter must

be a Schottky diode, with low forward voltage drop

and fast switching speed. Table 3 lists several Schottky

manufacturers. The diodes average current rating must

exceedtheapplication’saverageoutputcurrent.Thediode’s

maximum reverse voltage must exceed the maximum

output voltage of the application. For PWM dimming

applicationsbeawareofthereverseleakageoftheSchottky

diode.Lowerleakagecurrentwilldraintheoutputcapacitor

less during PWM low periods, allowing for higher PWM

dimming ratios. The companies below offer Schottky

diodes with high voltage and current ratings.

www.sumida.com

www.we-online.com

www.vishay.com

www.coilcraft.com

www.cooperet.com

Würth Elektronik

Vishay

Coilcraft

Coiltronics

Input Capacitor

TheinputcapacitoroftheLT3754boostconverterwillsup-

plythetransientinputcurrentofthepowerinductor.Values

between±.±μFand10μFwillworkwellfortheLT3754. Use

only X5R or X7R ceramic capacitors to minimize variation

over voltage and temperature. If inductor input voltage is

requiredtooperateneartheminimumallowedoperational

Table 3. Schottky Rectiﬁer Manufacturers

MANUFACTURER

Diodes, Inc.

PHONE NUMBER

ꢀ05-446-4ꢀ00

ꢀꢀꢀ-743-7ꢀ±6

631-360-±±±±

40±-563-6ꢀ66

WEB

V for the I.C., a larger capacitor value may be required.

www.microsemi.com

www.onsemi.com

www.zetex.com

www.vishay.com

IN

This is to prevent excessive input voltage ripple causing

On Semiconductor

Zetex

dips below the minimum operating input voltage.

Vishay Siliconix

Output Capacitor

LowESRceramiccapacitorsshouldbeusedattheLT3754

converter output to minimize output ripple voltage. Use

only X5R or X7R dielectrics as these materials retain their

capacitance over wider voltage and temperature ranges

thanotherdielectrics.Theoutputcapacitancerequirements

for several LED driver application circuits are shown in

Power MOSFET Selection

Several MOSFET vendors are listed in Table 4. Consult the

factory applications department for other recommended

MOSFETs. The power MOSFET selected should have a

V

rating which exceeds the maximum Overvoltage

DS

Protection (OVP) level programmed for the application.

(See “Programming OVP level” in the Applications

Information section). The MOSFET should also have a

the Applications Information section for various I

,

LED

V , V , L and f

values. Some suggested capacitor

IN OUT

OSC

manufacturers are listed in Table ±.

low enough total gate charge Q (at 7V V ) and a low

g

OSC

GS

enough switching frequency (f ) to not exceed the

Table 2. Ceramic Capacitor Manufacturers

INTV regulator current limit, where loading on INTV

MANUFACTURER

TDK

PHONE NUMBER

516-535-±600

40ꢀ-9ꢀ6-04±4

ꢀ14-±37-1431

40ꢀ-573-4150

ꢀ43-44ꢀ-9411

WEB

CC

pin due to gate switching should obey,

www.tdk.com

www.kemet.com

www.murata.com

t-yuden.com

Kemet

I

= Q • f

≤ 44mA

OSC

GATE

g

Murata

Taiyo Yuden

AVX

www.avxcorp.com

3754f

12

LT3754

APPLICATIONS INFORMATION

In addition, the current drive required for GATE switching

52mV •0.7

I_L(PEAK)

RS≤

should also be kept low in the case of high V voltages

IN

(see“ThermalConsiderations”intheApplicationsInforma-

where

tion section). The R

of the MOSFET will determine

DS(ON)

d.c. power losses but will usually be less signiﬁcant

compared to switching losses. Be aware of the power

dissipation within the MOSFET by calculating d.c. and

switching losses and deciding if the thermal resistance

of the MOSFET package causes the junction temperature

to exceed maximum ratings.

⎛

⎞

⎛

⎝

⎠

⎛

⎝

⎞

⎠

1

1−D

0.5

2

I_L(PEAK)

=

•16 •I • 1+

_LEDx⎟ ⎜

⎜⎜

⎟

⎠

⎟

⎝

⎛

⎞

V_OUT(MAX)

⎜

⎝

⎟

⎠

D= MOSFET duty cycle=

V_OUT(MAX)= N• V

,

V_IN(MIN)

+ 1V

(

)

F(MAX)

Table 4. MOSFET Manufacturers

N= number of LEDs in each string,

MANUFACTURER

PHONE NUMBER

WEB

Vishay Siliconix

40±-563-6ꢀ66

www.vishay.com

www.irf.com

V_F(MAX)= maximum LED forward voltage drop,

V_IN(MIN)= minimum input voltage to the inductor,

International Rectiﬁer 310-±5±-7105

Fairchild 97±-910-ꢀ000

www.fairchildsemi.com

andthe0.5termrepresentsaninductorpeak-to-peakripple

current of 508 of average inductor current.

Power MOSFET: Current Sense Resistor

The LT3754 current mode boost converter controls peak

currentintheinductorbycontrollingpeakMOSFETcurrent

ineachswitchingcycle.TheLT3754monitorscurrentinthe

external N-channel power MOSFET by sensing the voltage

acrossasenseresistor(RS)connectedbetweenthesource

of the FET and the power ground in the application. The

length of these tracks should be minimized and a Kelvin

sense should be taken from the top of RS to the sense

pin. A 5±mV sense pin threshold combined with the value

of RS sets the maximum cycle-by-cycle peak MOSFET

current. The low 5±mV threshold improves efﬁciency and

determines the value for RS given by:

The scale factor of • 0.7 ensures the boost converter

can meet the peak inductor requirements of the loop by

accounting for the combined errors of the 50mV sense

threshold, I

, RS and circuit efﬁciency.

LEDx

Example: For a 1±W LED driver application requiring 16

strings of 10 LEDs each driven with ±0mA, and choosing

V

= ꢀV, V

= (4V • 10)+1V = 41V and I

IN(MIN)

OUT(MAX) LEDx

= ±0mA, the value for RS is chosen as:

52mV • 0.7

I_L(PEAK)

52mV • 0.7

RS ≤

≤

⎛

⎜

⎞

41

⎝ 8

• 16 • 0.02 • 1+ 0.25

⎟

(

)

⎠

52mV • 0.7

2.05

≤

≤ 17.7 mΩ

3754f

13

LT3754

APPLICATIONS INFORMATION

The power rating of RS should be selected to exceed

Soft Start

±

the I R losses in the resistor. The peak inductor current

To limit inductor inrush current and output voltage

during startup or recovery from a fault condition, the

LT3754 provides a soft start function. The LT3754

when entering these faults will discharge an internal

soft start node and prevent switching at the GATE

should be recalculated for the chosen RS value to ensure

the chosen inductor will not saturate.

Power MOSFET: Overcurrent and Hiccup Mode

For severe external faults which may cause the external

MOSFET to reach currents greater than the peak current

deﬁnedbyRSandthe5±mVsensepinthresholddescribed

above, the LT3754 has an overcurrent comparator which

triggers soft start and turns off the MOSFET driver for

currents exceeding,

pin for any of the following faults: V , SHDN/UVLO

IN

or INTV

voltages too low or MOSFET current

CC

toohigh(seethetimingdiagraminFigure±).Whenexiting

thesefaultstheLT3754rampsupaninternalsoftstartnode

atapproximately0.5V/mstocontrolV pinvoltageriseand

C

hence control MOSFET switch current rise. In addition the

soft start period gradually ramps up switching frequency

from approximately 338 to 1008 of full scale.

100mV

RS

I_{D(OVERCURRENT)}

=

The conditions required to exit all faults and allow a soft

start ramp of the VC pin are listed in Figure ±. An added

feature of the LT3754 is that it waits for the ﬁrst PWM pin

active high (minimum ±00ns pulse width) before it allows

In this fault mode the LT3754 only allows MOSFET turn

on for approximately 100ns every ±ms. This hiccup mode

signiﬁcantly reduces the power rating required for the

MOSFET.

GATE

V

C

V

0.5V/ms

C MIN

CLAMP

0.4V ꢀ V (V SWITCHING

BE

C

THRESHOLD)

0.1V ꢀ V

SS

(INTERNAL)

0.5V/ms

0.4V

0.1V

ANY OF THE FOLLOWING FAULTS

TRIGGERS SOFT START LATCH

WITH GATE TURNED OFF

IMMEDIATELY:

SOFT-START LATCH RESET REQUIRES

ALL CONDITIONS SATISFIED:

SOFT-START

LATCH SET:

SS (INTERNAL) < 0.2V, V_INr 4.2V,

SHDN > 1.476V, INTV > 3.8V,

CC

V

< 3.7V, SHDN < 1.476V,

IN

I

(EXTERNAL MOSFET) < 100mV/RS,

DSS

INTVCC < 3.4V

(EXTERNAL MOSFET) > 100mV/RS

PWM > 1.4V (FOR AT LEAST 200ns)

I

DSS

3754 F02

Figure 2. LT3ꢀ54 Fault Detection and Soft Start Timing for V_CPin and Internal SS Node

3754f

14

LT3754

APPLICATIONS INFORMATION

the soft start of VC pin to begin. This feature ensures that

during startup of the LT3754 the soft start ramp has not

timed out before PWM is asserted high. Without this ‘wait

pin. After part turn on, 0μA ﬂows from the SHDN/UVLO

pin. Calculation of the turn on/off thresholds for a system

input supply using the LT3754 SHDN/UVLO pin can be

made as follows :

forPWMhigh’feature,systemswhichapplyPWMafterV

IN

and SHDN/UVLO are valid, can potentially turn on without

soft start and experience high inductor currents during

wake up of the converter’s output voltage. It is important

to note that when PWM subsequently goes low, the soft

start ramp is not held at its present voltage but continues

to ramp upwards. If the soft start ramp voltage was held

every time PWM goes low, this would cause very slow

startup of LED displays for applications using very high

PWM Dimming ratios.

⎛

⎝

⎞

⎠

R1

R2

V_{SUPPLY OFF}= 1.476 1+

⎜

⎟

V_{SUPPLY ON}= V_{SUPPLY OFF}+ 2.4μA •R1

(

)

An open drain transistor can be added to the resistor

divider network at the SHDN/UVLO pin to independently

control the turn off of the LT3754.

Programming Switching Frequency

Shutdown and Programming Undervoltage Lockout

The switching frequency of the LT3754 boost converter

can be programmed between 100kHz and 1MHz using a

The LT3754 has an accurate 1.476V shutdown threshold

at the SHDN/UVLO pin. This threshold can be used in

conjunction with a resistor divider from the system input

supply to deﬁne an accurate undervoltage lockout (UVLO)

threshold for the system (Figure 3). An internal hysteresis

current at the SHDN/UVLO pin allows programming of

hysteresis voltage for this UVLO threshold. Just before

partturnon, aninternal±.4μAﬂowsfromtheSHDN/UVLO

single resistor (R ) connected from the RT pin to ground

T

(Figure 4). Connect the R resistor as close as possible to

T

the RT pin to minimize noise pick up and stray capacitance

(see “Circuit Layout Considerations” in the Applications

Information section). Table 5 shows the typical R values

T

required for a range of frequencies.

1000

900

ꢀ00

700

600

500

V

SUPPLY

R1

R±

SHDN/UVLO

13

–

+

600k

1.476V

400

300

±00

OFF ON

100

0

100

±00

300

400

500

600

RT (kΩ)

3754 F04

3754 F03

Figure 3. Programming Undervoltage

Lockout (UVLOꢁ with Hysteresis

Figure 4. Switching Frequency vs RT

3754f

15

LT3754

APPLICATIONS INFORMATION

Selecting the optimum frequency depends on several

factors. Higher frequency allows reduction of inductor

size but efﬁciency drops due to higher switching losses.

Lower frequency allows higher operational duty cycles to

drive a larger number of LEDs per string from a low input

supply but require larger magnetics. In each application

the switching frequency can be tailored to provide the

optimum solution.

Table 6. LED Current vs. R_ISET(1% resistorsꢁ

R

ISET

(kΩꢁ

LED Current per CHANNEL (mAꢁ

10

±0

30

40

50

±9.4

14.7

9.76

7.3±

5.76

An extra 50ns should be added to these tested timings to

account for errors in the rise/fall times of the GATE and

DRAIN of the external MOSFET and the d.c. resistance of

the external MOSFET and inductor.

Table 5. Switching Frequency vs. RT (1% resistorsꢁ

SWITCHING FREQUENCY (kHzꢁ

RT (kΩꢁ

5±3

100

±00

300

400

500

600

700

ꢀ00

900

1000

±49

15ꢀ

Synchronizing to an external clock

115

The SYNC pin allows the LT3754 oscillator to be

synchronized to an external clock. The SYNC pin can be

driven from a logic level output, requiring less than 0.6V

for a logic level low and greater than ±.±V for a logic level

high. SYNC pin high or low periods should exists for at

least 100ns. If unused, the SYNC pin should be tied to

ground. To avoid loss of slope compensation during

synchronization, the free running oscillator frequency

90.9

73.±

60.4

51.1

44.±

39.±

Duty Cycle Considerations

(f ) of the LT3754 should be programmed to ꢀ08 of

OSC

the external clock frequency.

When designing the LT3754 LED driver for a given

application, the duty cycle requirements should be

considered and compared to the minimum/maximum

achievabledutycyclesfortheLT3754GATEpin.Ifrequired,

the LT3754 switching frequency can be programmed to a

lowervaluetomeetthedutycyclerequirements.Ingeneral,

the minimum/maximum GATE duty cycles required for a

particular application are given by:

Programming LED Current

ThecurrentsourcetogroundateachLEDpinisprogrammed

using a single resistor R

connected from the I pin

ISET

SET

to ground according to the following equation:

295

R_ISET

( )

A CTRL> 1.1V

I LED

(

≈

)

(

)

X

MIN Duty Cycle = GATE Minimum On-Time • Switching

See Table 6 for resistor values and corresponding

programmed LED.

Frequency f

OSC

MAX Duty Cycle = 1 – (GATE Minimum Off-Time •

Switching Frequency f

)

OSC

The typical values for LT3754 GATE pin minimum on and

off times versus temperature are shown in the Typical

Performance Characteristics. The range of GATE pin

minimum on time and off times are given in the electrical

speciﬁcations.

3754f

16

LT3754

APPLICATIONS INFORMATION

Analog Dimming

T

PWM

ON(PWM)

(= 1/f

)

PWM

T

TheLT3754allowsforLEDdimming(brightnessreduction)

byanalogdimmingorbyPWMdimming. Analogdimming

uses the CTRL pin voltage below 1V to reduce LED

brightness by reducing LED current. For CTRL pin voltage

below 1V, the current in each LED pin is given by:

PWM

INDUCTOR

CURRENT

295

R_ISET

I LED ≈CTRL •

0.04< CTRL < 1V

(

)

MAX I

LED

(

)

LED

CURRENT

X

3754 F05

For CTRL pin voltages below 40mV (greater than ±5:1

dimming) the LED current will approach zero current. The

CTRL pin voltage can be derived from a resistor divider

Figure 5. PWM Dimming Waveforms

from V

pin to ground or generated from an external

Some general guidelines for LED current dimming using

the PWM pin (see Figure 5):

REF

source. If analog dimming is not required, the pin can be

directly connected to the V pin. The only drawback of

REF

(1) PWM Dimming Ratio (PDR) = 1/(PWM Duty Cycle) =

analog dimming is that reducing LED current to reduce

the brightness of the LED also changes the perceived

color of the LED.

1/T

• f

ON(PWM) PWM

(±) Lower PWM frequency (f ) allows higher PWM

PWM

dimming ratios (Typically choose 100Hz to maximize PDR

and to avoid visible ﬂicker which can occur for display

systems with refresh rates at frequencies below ꢀ0Hz)

PWM Dimming

Many applications require an accurate control of the

brightnessoftheLED(s).Inaddition,beingabletomaintain

a constant color over the entire dimming range can be just

as critical. For constant color LED dimming the LT3754

providesaPWMpinandspecialinternalcircuitrytoachieve

uptoa3000:1widePWMdimmingrange. Thisisachieved

by operating the LED at it’s programmed current and then

controlling the on time of that LED current. The duty cycle

of the PWM pin controls the on time of each LED pin

current source (Figure 5). For maximum PWM dimming

ratios (low PWM duty cycles) it is important to be able to

turn LED currents on/off as quickly as possible. For PWM

low, the LT3754 turns off the boost converter, turns off

(3)Higherf valueimprovesPDR(allowslowerT

)

OSC

ON(PWM)

but will reduce efﬁciency and increase internal heating. In

general, minimum operational T = 3 • (1/f

)

OSC

ON(PWM)

(4) Lower inductor value improves PDR

(5) Higher output capacitor value improves PDR

(6)ChoosetheSchottkydiodefortheLT3754boostconverter

for minimum reverse leakage current.

See “LED Current vs PWM Duty Cycle” in the Typical

Performance Characteristics section.

all LED channel currents and disconnects the V pin and

C

internal V

resistor divider connected to the OVP error

OUT

ampliﬁer. This allows the part to quickly return to the last

state of operation when the PWM pin is returned high.

3754f

17

LT3754

APPLICATIONS INFORMATION

Programming LED Current Derating (Breakpoint and

Slopeꢁ versus LED Ambient Temperature (CTRL Pinꢁ

divider with temperature dependent resistance (Figures 7

and ꢀ). A variety of resistor networks and NTC resistors

with different temperature coefﬁcients can be used to

achieve the desired CTRL pin voltage behaviour versus

temperature. The current derating curve in Figure 6 uses

the resistor network shown in option C of Figure 7.

LED datasheets provide curves of maximum allowed

LED current versus ambient temperature to warn against

damaging of the LED (Figure 6). The LT3754 LED driver

improves the utilization and reliability of the LED(s) by

allowingtheprogrammingofanLEDcurrentderatingcurve

versustheambienttemperatureoftheLED(s).Withoutthe

ability to back off LED currents as temperature increases,

many LED drivers are limited to driving the LED(s) at 508

orlessoftheirmaximumratedcurrents.TheLT3754allows

the temperature breakpoint and the slope of LED current

versus ambient temperature to be programmed using a

simple resistor network shown in Figure 7.

Table 7 shows a list of NTC resistor manufacturers/

distributors. There are several other manufacturers

available and the chosen supplier should be contacted

for more detailed information. To use an NTC resistor to

monitor the ambient temperature of the LED(s) it should

be placed as close as possible to the LED(s). Since the

temperature dependency of an NTC resistor can be non-

linear over a wide range of temperatures it is important to

obtain a resistor’s exact values over temperature from the

manufacturer. Hand calculations of CTRL voltage can then

be performed at each given temperature and the resulting

CTRL voltage plotted versus temperature.

Without the ability to back off LED current as the ambient

temperatureoftheLED(s)increases,manyLEDdriversare

limited to driving the LED(s) at only 508 or less of their

maximum rated currents. This limitation requires more

LEDstoobtaintheintendedbrightnessfortheapplication.

The LT3754 allows the LED(s) to be programmed for

maximum allowable current while still protecting the

LED(s) from excessive currents at high temperature. This

is achieved by programming a voltage at the CTRL pin

with a negative temperature coefﬁcient using a resistor

Table ꢀ. NTC Resistor Manufacturers

MANUFACTURER

Murata Electronics North America

TDK Corporation

WEB

www.murata.com

www.tdk.com

www.digikey.com

Digi-key

±50

31

REF

R1

±±5

LT3754

RESISTOR

OPTION A

29

CTRL

±00

175

R2

OPTION A TO D

LT3754

PROGRAMMED LED

CURRENT DERATING

150

CURVE

R

Y

1±5

100

R

NTC

R

X

NTC

X

NTC

–50 –±5

0

±5

50

75 100 1±5

T -TEMPERATURE (°C)

A

3754 F06

A

B

C

D

3754 F07

Figure 6. LED Current Derating vs LED Ambient Temperature

Figure ꢀ. Programming LED Current Derating Curve

vs Ambient Temperature (R_NTCLocated on LED PCBꢁ

3754f

18

LT3754

APPLICATIONS INFORMATION

1.50

Using the T Pin for Thermal Protection

SET

The LT3754 contains a special programmable thermal

regulationloopthatlimitstheinternaljunctiontemperature

of the part. Since the LT3754 topology consists of a single

boostcontrollerwithsixteenlinearcurrentsources,anyLED

string voltage mismatch will cause additional power to be

dissipatedinthepackage.Thistopologyprovidesexcellent

current matching between LED strings and allows a single

power stage to drive a large number of LEDs, but at the

priceofadditionalpowerdissipationinsidethepart(which

means a higher junction temperature). Being able to limit

the maximum junction temperature allows the beneﬁts of

this topology to be fully realized. This thermal regulation

featureprovidesimportantprotectionathighambienttem-

peratures, and allows a given application to be optimized

for typical, not worst-case, ambient temperatures with the

assurance that the LT3754 will automatically protect itself

and the LED strings under worst-case conditions.

1.±5

1.00

RESISTOR

OPTION A

0.75

0.50

0.±5

0

10 ±0 30 40 50 60 70 ꢀ0

- AMBIENT TEMPERATURE (°C)

T

A

3754 F0ꢀ

Figure 8. Programmed CTRL Voltage vs Temperature

IfcalculationofCTRLvoltageatvarioustemperaturesgives

a downward slope that is too strong, alternative resistor

networks can be chosen (B,C,D in Figure 7) which use

temperature independent resistance to reduce the effects

of the NTC resistor over temperature. Murata Electronics

provides a selection of NTC resistors with complete data

over a wide range of temperatures. In addition, a software

tool is available which allows the user to select from

different resistor networks and NTC resistor values and

then simulate the exact output voltage curve (CTRL pin

behavior) over temperature. Referred to on the website

as the ‘Murata Chip NTC Thermistor Output Voltage

Simulator’,userscanlogontowww.murata.com/designlib

and download the software followed by instructions for

The operation of the thermal loop is simple. As the ambi-

ent temperature increases, so does the internal junction

temperature of the part. Once the programmed maximum

junction temperature is reached, the LT3754 begins to

linearly reduce the LED current, as needed, to try and

maintain this temperature. This can only be achieved

when the ambient temperature stays below the desired

maximum junction temperature. If the ambient tempera-

ture continues to rise past the programmed maximum

junction temperature, the LEDs current will be reduced

to approximately 58 of the full LED current.

creatinganoutputvoltage‘V ’(LT3754CTRLpinvoltage)

from a speciﬁed V supply (LT3754 V pin voltage). At

OUT

CC

REF

WhilethisfeatureisintendedtodirectlyprotecttheLT3754,

it can also be used to derate the LED current at high

temperatures. Since there is a direct relationship between

theLEDtemperatureandLT3754junctiontemperature,the

TSET function also provides some LED current derating

at high temperatures.

any time during selection of circuit parameters the user

can access data on the chosen NTC resistor by clicking

on the link to the Murata catalog. For a detailed example

of hand calculations using an NTC type resistor divider

to program CTRL pin voltage, read the LT347ꢀ LED driver

data sheet section Programming LED Current Derating vs

Temperature under Applications Information.

3754f

19

LT3754

APPLICATIONS INFORMATION

Two external resistors program the maximum IC junction

Programming Overvoltage Protection (OVPꢁ level

temperature using a resistor divider from the V

pin,

REF

The LT3754 LED driver provides optimum protection

to the LEDs and the external MOSFET by providing a

programmable maximum regulated output voltage limit

as shown in Figure 9. Choose the ratio of R1 and R± for

the desired junction temperature. Figure 10 shows the

relationship of T voltage to junction temperature, and

SET

using the OVP pin. The Overvoltage Protection (OVP)

SET

Table ꢀ shows commonly used values for R1 and R±.

level is programmed as:

OVP (MAXIMUM REGULATED V ) = 57 • OVP

OUT

SET

31

V

REF

If every LED string fails open or the multiple string LED

displaybecomesdisconnectedtheLT3754LEDdriverloop

regulates to the programmed OVP level. The OVP level

should be programmed to a level high enough to regulate

the LED strings but low enough to prevent damage to the

power switch and to minimize the voltage across the LED

pinsuponreconnectionoftheLEDstrings.Recommended

OVP level is given by:

R±

R1

LT3754

30

T

SET

3754 F09

Figure 9. Programming the T_SETPin

950

900

ꢀ50

ꢀ00

750

700

650

600

550

500

OVP(RECOMMENDED) = 1.± • ((N • V ) + 1V)

F

where:

N = number of LEDs in each string,

V

PTAT

V = maximum LED forward voltage drop

F

and the scaling factor of 1.± accounts for variation in the

generation of OVP from OVP

logic requirements.

pin voltage and startup

SET

0

±5

50

150

75

100

1±5

Example:Foraconverteroperatingwith10LEDsperstring

at a maximum forward voltage of 4V per LED, the OVP

level should be programmed to:

JUNCTION TEMPERATURE (°C)

359ꢀ F10

Figure 10. Programing the T_SETPin Threshold

OVP(RECOMMENDED)= 1.2• (10 • 4)+ 1V = 49.2V

(

)

49.2

57

Table ꢀ. Resistor Values to Program Maximum IC Junction

Temperature (V_REF(Typicalꢁ = 1.485Vꢁ

For OVP= 49.2V, OVP_SET

=

= 0.863V

T (°Cꢁ

R1 (kꢁ

±4.9

R2 (kꢁ

±0

T

(Vꢁ

SET

J

The OVP pin voltage can be generated using a resistor

SET

100

115

130

0.ꢀ±4

0.ꢀ66

0.90±

divider from the REF pin.

±ꢀ.0

±0

30.9

±0

3754f

20

LT3754

APPLICATIONS INFORMATION

LED Open Circuit and PWM Dimming Ratios

more slowly during load transient conditions such as an

all-LEDs-openfault.AslowermovingV pinwilladdtoV

C

OUT

The LT3754 monitors each LED pin voltage to determine if

the LED string has an open fault (LED pin voltage < 0.5V).

IfanopenLEDfaultoccurs, theFAULTﬂagispulledlow. To

avoid false detection of faults during the initial converter

overshoot during an all-LEDs-open fault. An alternative

compensation approach is to place the dominant pole of

theconverterloopattheoutput.Thisrequiresanincreased

output capacitor value but will allow a much reduced Vc

startup when V

is low, the LT3754 ignores low LED

OUT

capacitor. The combination will allow V to move more

C

pin voltages until V

reaches 908 of its maximum

quickly and V

to move more slowly resulting in less

overshoot during an all-LEDs-open fault.

OUT

allowed OVP level. Once this condition is met, the LT3754

monitors all LED pins for open LED faults. To avoid false

detection of faults during PWM dimming edges (where

LED pins can possibly ring and trip fault detection levels)

the LT3754 only monitors/updates fault conditions during

PWMhigh(andonlyafterablankdurationof±μsfollowing

each PWM rising edge).

Thermal Considerations

TheinternalpowerdissipationoftheLT3754comesfrom3

main sources: V quiescent current (I total), V current

IN

Q

IN

for GATE switching (I

) and the LT3754 LED current

GATE

sources. Since the maximum operational V voltage is

IN

LED Short Circuit

40V, care should be taken when selecting the switching

frequency and type of external power MOSFET since the

A short circuit fault between the positive terminal of an

current required from V for GATE switching is given by,

IN

LED string (V ) and the negative terminal of the LED

OUT

string (LEDx pin) causes the channel to be disabled in

I

= f

• Qg

GATE

OSC

order to protect the internal current source. A resistive

where Q is the gate charge (at V = INTV ) speciﬁed

g

GS

CC

short is allowed as long as (V -V

) < 6V.

OUT LEDx

for the MOSFET and f

is the programmed switching

OSC

frequency for the LT3754. A low Q MOSFET should

g

Loop Compensation

always be used when operating the LT3754 from high V

IN

Be sure to check the stability of the loop with the LEDs

connected (LED regulation loop) and disconnected

(Overvoltage Protection (OVP) regulation loop). Various

applicationcircuitsareshowninthedatasheetwhichcovera

voltages. The internal junction temperature of the LT3754

can be estimated as:

T =T +[V •(I

+(f •Q ))+(16•I(LED )•1.1V)]

OSC g X

J

• θ

A

IN QTOTAL

JA

rangeofV ,V ,f ,outputpowerandinductorcurrent

IN OUT OSC

ripple values. For application requirements which deviate

where, T is the ambient temperature for the LT3754

A

from the circuits shown in the datasheet be sure to check

I

representstheV quiescentcurrentfortheLT3754

QTOTAL

IN

the stability of the ﬁnal application over the full V range,

(not switching, PWM = 1.5V and CTRL = 0.1V) - illustrated

IN

LED current range (if analog dimming) and temperature

in the Typical Characteristics Graphs – plus the base

range. Be aware that if the V pin components represent

currents of active channels (typically 16 • I(LED)/75). θ

JA

C

the dominant pole for the converter loop and they have

is the thermal resistance of the package (34°C/W for the

been adjusted to achieve stability, the V pin might move

5mm × 5mm QFN package).

C

3754f

21

LT3754

APPLICATIONS INFORMATION

Example : For a 1±W LED driver application requiring 16

copper ground plane underneath the device to reduce die

temperature and maximize the power capability of the IC.

Ananaloggroundisdownbondedtotheexposedpadnear

strings of 10 LEDs each driven with ±0mA, V = ±4V, f

IN

OSC

= 1MHz, Q (at 7V V ) = 15nC, I(LED ) = ±0mA, and an

g

GS

X

ꢀ5°C ambient temperature for the LT3754 IC, the LT3754

the RT and V pins. I , R and V components should

C SET T C

junction temperature can be approximated as:

be connected to an area of ground copper near these

pins. The OVP track should be kept away from fast

SET

T = ꢀ5°C + [±4 • (9.5mA + (16 • ±0mA/75) + (1MHz

J

moving signals and not loaded with an external capacitor.

GATE pin turn off currents escape through a downbond to

the exposed pad near the GATE pin. This area of copper

should be the power ground (PGND) connection for the

• 15nC)) + (16 •±0mA • 1.1V)] • 34

= ꢀ5°C + [(±4 • ±ꢀ.ꢀmA) + (3±0mA • 1.1V)] • 34

= ꢀ5°C + (0.691W + 0.35W) • 34

= ꢀ5°C + 35°C

inductor input capacitor, INTV capacitor and output

CC

capacitor. A separate bypass capacitor for the V pin of

IN

the IC may be required close the V pin and connected

T = 1±0°C

J

IN

to the copper area associated with analog ground. To

minimize MOSFET peak current sensing errors the sense

resistor(RS)shouldhaveKelvinconnectionstotheSENSE

pin and the power ground copper area near the pin. The

MOSFET drain rise and fall times are designed to be as

short as possible for maximum efﬁciency. To reduce the

effects of both radiated and conducted noise, the area of

the copper trace for the MOSFET drain should be kept as

small as possible. Use a ground plane under the switching

regulator to minimize interplane coupling. The Schottky

diode and output capacitor should be placed as close as

possible to the drain node to minimize this high switching

frequency path.

The exposed pad on the bottom of the package must be

soldered to the ground plane. The ground plane should

be connected to an internal copper ground plane with vias

placed directly under the package to spread out the heat

generated by the LT3754.

Circuit Layout Considerations

As with all switching regulators, careful attention must be

given to PCB layout and component placement to achieve

optimal thermal, electrical and noise performance. The

exposedpadoftheLT3754istheonlygroundconnectionfor

theIC.Theexposedpadshouldbesolderedtoacontinuous

3754f

22

LT3754

TYPICAL APPLICATIONS

3754f

23

LT3754

TYPICAL APPLICATIONS

3754f

24

LT3754

TYPICAL APPLICATIONS

31W LED Driver, 400kHz Boost, 3 Strings, 250mA Per String

L1

10μH

D1

UP TO 4±V OF LEDs PER STRING

V

IN

ꢀV TO 36V

10 s

4.7μF

50V

±.±μF

100V

V

IN

INTV

CC

M1

GATE

SENSE

4.7μF

10V

•

0.007Ω

100k

LT3754

1M

FAULT

V

OUT

SHDN/UVLO

LED16

±3±k

GND

LED1

LED±

SYNC

PWM

LED3

LED4

LED5

PWM DIMMING

LED6

LED7

ANALOG DIMMING

CTRL

REF

LEDꢀ

LED9

LED10

LED11

LED1±

LED13

LED14

LED15

±0k

T

SET

15k

OVP

SET

I

V

C

3754 TA06

RT

SET

30.9k ±3.±k

115k

5.76k

5.1k

4.7nF

L1: COOPER BUSSMANN HC9-100-R

M1: VISHAY SILICONIX Si7ꢀ50DP

D1: DIODES, INC. PDS560

3754f

25

LT3754

TYPICAL APPLICATIONS

14W LED Driver, ꢀ00kHz Boost, 4 Strings, 80mA Per String

L1

15μH

D1

UP TO 45V OF LEDs PER STRING

1±V

IN

10V TO 14V

4.7μF

±5V

V

IN

5 s ±.±μF

100V

INTV

CC

4.7μF

10V

M1

GATE

SENSE

V

•

IN

0.0±Ω

LT3754

1M

100k

FAULT

V

OUT

SHDN/UVLO

GND

LED1

LED±

SYNC

LED3

LED4

LED5

PWM

CTRL

REF

PWM DIMMING

LED6

LED7

ANALOG DIMMING

LEDꢀ

LED9

LED10

LED11

LED1±

LED13

LED14

LED15

LED16

±0k

T

SET

11k

OVP

I

V

C

RT

SET

3754 TA06

30.9k

±0k

60.4k

14.7k 7.5k

4.7nF

L1: SUMIDA CDRHꢀD3ꢀ

M1: VISHAY SILICONIX Si730ꢀDN

D1: DIODES, INC. DFLS160

3754f

26

LT3754

PACKAGE DESCRIPTION

DH Package

32-Lead Plastic QFN (5mm × 5mmꢁ

(Reference LTC DWG # 05-0ꢀ-1693)

0.70 p0.05

5.50 p0.05

4.10 p0.05

3.45 p 0.05

3.50 REF

(4 SIDES)

3.45 p 0.05

PACKAGE OUTLINE

0.±5 p 0.05

0.50 BSC

RECOMMENDED SOLDER PAD LAYOUT

APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED

BOTTOM VIEW—EXPOSED PAD

PIN 1 NOTCH R = 0.30 TYP

OR 0.35 s 45° CHAMFER

R = 0.05

TYP

0.00 – 0.05

R = 0.115

TYP

0.75 p 0.05

5.00 p 0.10

(4 SIDES)

31 3±

0.40 p 0.10

PIN 1

TOP MARK

(NOTE 6)

1

±

3.45 p 0.10

3.50 REF

(4-SIDES)

3.45 p 0.10

(UH3±) QFN 0406 REV D

0.±00 REF

0.±5 p 0.05

0.50 BSC

NOTE:

1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE

M0-±±0 VARIATION WHHD-(X) (TO BE APPROVED)

±. 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.±0mm 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

3754f

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-

tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.

27

LT3754

RELATED PARTS

PART NUMBER

DESCRIPTION

COMMENTS

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LT3755/LT3755-1 High Side 40V, 1MHz LED Controller with

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V

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High Side 36V, 1MHz LED Controller with

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OUT(MAX)

I

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1.3A, ±.5MHz High Current LED Driver

with 3,000:1 Dimming

V

SD

= 3.0V, V

= <1μA, 4mm × 4mm QFN-16

= 30V, V

= 45, 3,000:1 True Color PWM Dimming,

= 36V, 1,000:1 True Color PWM Dimming,

= 40V, 1,000:1 True Color PWM Dimming,

= 40V, 3,000:1 True Color PWM Dimming,

= 13.5V, 400:1 True Color PWM Dimming,

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= 36V, 1,000:1 True Color PWM Dimming,

IN(MIN)

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±.3A, ±.5MHz High Current LED Driver

with 3,000:1 Dimming

V

SD

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= <1μA, 4mm × 4mm QFN-16

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I

Dual 1.3A, ±MHz High Current LED Driver

V

SD

= ±.5V, V

= ±4V, V

IN(MAX)

IN(MIN)

I

= <1μA, 5mm × 3mm DFN, TSSOP-16E

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3,000:1 Dimming

V

SD

= ±.ꢀV, V

= <10μA, 5mm × 7mm QFN-10

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I

LT3496

Triple Output 750mA, ±.1 MHz High

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LT3475/LT3475-1 Dual 1.5A(I ), 36V, ±MHz, Step-Down

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LED Driver

I

= <1μA, TSSOP±0E

SD

LT3476

Quad Output 1.5A, ±MHz High Current

LED Driver with 1,000:1 Dimming

V

SD

= ±.ꢀV, V

= <10μA, 5mm × 7mm QFN-10

IN(MIN) IN(MAX)

I

3754f

LT 0809 • PRINTED IN USA

LinearTechnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

28

●

(40ꢀ) 43±-1900 FAX: (40ꢀ) 434-0507 www.linear.com


型号：	LT3754IUHPBF
厂家：	Linear
描述：	16-Channel × 50mA LED Driver 16通道× 50毫安LED驱动器驱动器
文件：	总28页 (文件大小：324K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LT3754IUHPBF [Linear]

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