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LT3761

3761f

Typical applicaTion

FeaTures

applicaTions

DescripTion

60V

LED Controller with

Internal PWM Generator

The LT

3761 is a DC/DC controller designed to operate as

a constant-current source and constant-voltage regulator.

It features a programmable internal PWM dimming signal.

The LT3761 is ideally suited for driving high current LEDs,

but also has features to make it suitable for charging bat-

teries and supercapacitors. The fixed frequency, current

mode architecture results in stable operation over a wide

range of supply and output voltages. A voltage feedback

pin serves as the input for several LED protection features,

and also makes it possible for the converter to operate

as a constant-voltage source. A frequency adjust pin

allows the user to program the frequency from 100kHz

to 1MHz to optimize efficiency, performance or external

component size.

The LT3761 senses output current at the high side or at the

low side of the load. The PWM input can be configured to

self-oscillate at fixed frequency with duty ratio program-

mable from 4% to 96%. When driven by an external

signal, the PWM input provides LED dimming ratios of

up to 3000:1. The CTRL input provides additional analog

dimming capability.

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

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, 7321203.

94% Efficient Boost LED Driver for Automotive

Headlamp with 25:1 Internal PWM Dimming

PWM Dimming Waveforms at

Various DIM Voltage Settings

3000:1 True Color PWM™ Dimming for LEDs

Wide V

Range: 4.5V to 60V

Rail-to-Rail Current Sense Range: 0V to 80V

Programmable PWM Dimming Signal Generator

Constant Current (±3%) and Constant-Voltage

(±2%) Regulation

Analog Dimming

Drives LEDs in Boost, SEPIC, Inverting, Buck Mode,

Buck-Boost Mode, or Flyback Configuration

Output Short-Circuit Protected Boost

Open LED Protection and Reporting

Adjustable Switching Frequency: 100kHz to 1MHz

Programmable V

UVLO with Hysteresis

C/10 Indication for Battery Chargers

Low Shutdown Current: <1µA

Thermally Enhanced 16-Lead MSOP Package

High Voltage LED Strings >100V with Ground

Referred Current Sense

Grounded Anode LEDs

Battery and SuperCap Chargers

Accurate Current Limited Voltage Regulators

LT3761

10µH

RTV

INTV

EN/UVLO

REF

SENSE

100k

INTV

499k

2.2µF

×2

4.7nF

0.01µF

47nF

300Hz

2.2µF

×4

8V TO

60V

124k

90.9k

5.1k

28.7k

350kHz

1µF

140k

CTRL

10mΩ

16.9k

60W

LED

STRING

3761 TA01a

OPENLED

DIM/SSDIM

PWM

GND

ISP

ISN

GATE

PWMOUT

0.25Ω

LED

1A/DIV

0.5ms/DIV

DIM

= 7.7V

PWM

= 96%

DIM

= 4V

PWM

= 50%

DIM

= 1.5V

PWM

= 10%

DIM

= 0.4V

PWM

= 4.3%

3761 TA01b

Summary of content (28 pages)

PAGE 1
LT3761 60VIN LED Controller with Internal PWM Generator Features n n n n n n n n n n n n n n Description 3000:1 True Color PWM™ Dimming for LEDs Wide VIN Range: 4.
PAGE 2
LT3761 Absolute Maximum Ratings Pin Configuration (Note 1) VIN, EN/UVLO.............................................................60V ISP, ISN......................................................................80V INTVCC....................................................VIN + 0.3V, 9.6V GATE, PWMOUT................................................. (Note 2) CTRL, OPENLED.........................................................15V FB, PWM................................................................
PAGE 3
LT3761 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Error Amplifier Full-Scale ISP/ISN Current Sense Threshold (VISP-ISN) CTRL ≥ 1.2V, ISP = 48V CTRL ≥ 1.2V, ISN = 0V l l 242 243 250 257 258 268 mV mV 1/10th Scale ISP/ISN Current Sense Threshold (VISP-ISN) CTRL = 0.
PAGE 4
LT3761 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS 1.18 1.220 1.26 V Logic Inputs/Outputs EN/UVLO Threshold Voltage Falling l EN/UVLO Rising Hysteresis 20 EN/UVLO Input Low Voltage IVIN Drops Below 1µA EN/UVLO Pin Bias Current Low EN/UVLO = 1.
PAGE 5
LT3761 Typical Performance Characteristics VISP-ISN Threshold vs CTRL Voltage TA = 25°C, unless otherwise noted. VISP-ISN Threshold vs ISP Voltage 300 Full-Scale VISP-ISN Threshold vs Temperature 260 260 200 150 100 50 256 255 VISP -ISN (mV) VISP-ISN THRESHOLD (mV) VISP -ISN THRESHOLD (mV) 250 250 0.5 1 1.5 CTRL VOLTAGE (V) 240 2 0 20 3761 G01 250 ISP = 48V 300 1.265 250 1.260 200 1.250 1.245 0 3761 G03 VREF Source Current vs Temperature CTRL = 2V 150 CTRL = 0.
PAGE 6
LT3761 Typical Performance Characteristics SENSE Current Limit Threshold vs Temperature EN/UVLO Hysteresis Current vs Temperature 110 EN/UVLO Threshold vs Temperature 2.8 1.27 100 95 90 –50 –25 0 EN/UVLO THRESHOLD (V) 2.6 105 EN/UVLO CURRENT (µA) SENSE THRESHOLD (mV) TA = 25°C, unless otherwise noted. 2.4 2.2 2.0 1.8 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3761 G11 1.25 RISING 1.23 FALLING 1.21 1.
PAGE 7
LT3761 Typical Performance Characteristics DIM/SS Voltage vs Current, Temperature ISP/ISN Input Bias Current vs CTRL Voltage, ISP = 48V 1.30 0 INPUT BIAS CURRENT (µA) DIM/SS VOLTAGE (V) 1.25 TA = –45°C, 25°C 1.20 TA = 130°C 1.15 1.10 –10 0 10 20 30 DIM/SS CURRENT (µA) 40 80 60 40 0 50 ISN 0 0.5 3761 G19 PWMOUT Duty Ratio vs Temperature, IDIM/SS = 0µA 1 CTRL (V) 1.5 55 9.0 ISN –80 –120 ISP –160 –200 2 8.0 7.5 25 50 75 100 125 150 TEMPERATURE (°C) 3761 G22 1.
PAGE 8
LT3761 Pin Functions PWMOUT (Pin 1): Buffered Version of PWM Signal for Driving LED Load Disconnect NMOS or Level Shift. This pin also serves in a protection function for the FB overvoltage condition—will toggle if the FB input is greater than the FB regulation voltage (VFB) plus 60mV (typical). The PWMOUT pin is driven from INTVCC. Use of a FET with gate cut-off voltage higher than 1V is recommended. FB (Pin 2): Voltage Loop Feedback Pin.
PAGE 9
LT3761 Pin Functions between current sense inputs ISP and ISN is less than 25mV. To function, the pin requires an external pull-up resistor, usually to INTVCC. When the PWM input is low and the DC/DC converter is idle, the OPENLED condition is latched to the last valid state when the PWM input was high. When PWM input goes high again, the OPENLED pin will be updated.
PAGE 10
LT3761 Block Diagram 185µA VREF 1.3V – +A7 2.02V 0.8V + F3(IDIM/SS) F1(IDIM/SS) PWM 0.8V F2(IDIM/SS) 100mV + – CTRL 1V CLAMP ILED RLED ×1/4 – + S R PWMINT Q PWM LATCH + A2 – R Q S A1 – gm + CURRENT MODE COMPARATOR 10µA AT A1+ = A1– ISENSE A5 + gm – 1.25V 10µA AT FB = 1.25V CV EAMP VC IDIM_SS DETECT 100kHz TO 1MHz OSCILLATOR A6 + + – SHDN PWMINT FREQ PROG BANDGAP REFERENCE ISP > ISN + 0.6V T > 165°C GND ISP 12µA 1.
PAGE 11
LT3761 Operation The LT3761 is a constant-frequency, current mode controller with a low side NMOS gate driver. The GATE pin and PWMOUT pin drivers and other chip loads are powered from INTVCC, which is an internally regulated supply. In the discussion that follows it will be helpful to refer to the Block Diagram of the IC.
PAGE 12
LT3761 Applications Information INTVCC Regulator Bypassing and Operation The INTVCC pin requires a capacitor for stable operation and to store the charge for the large GATE switching currents. Choose a 10V rated low ESR, X7R ceramic capacitor for best performance. A 1μF capacitor will be adequate for many applications. Place the capacitor close to the IC to minimize the trace length to the INTVCC pin and also to the IC ground.
PAGE 13
LT3761 Applications Information LED current to zero, although relative accuracy decreases with the decreasing voltage sense threshold. When the CTRL pin voltage is less than 1V, the LED current is: ILED = VCTRL − 100mV RLED • 4 50mV should not cause mis-operation, but may lead to noticeable offset between the current regulation and the user-programmed value. Programming Output Voltage (Constant Voltage Regulation) or Open LED/Overvoltage Threshold When the CTRL pin voltage is between 1V and 1.
PAGE 14
LT3761 Applications Information ISP/ISN Short-Circuit Protection Feature The ISP/ISN pins have a protection feature independent of their LED current sense feature. The purpose of this feature is to prevent the development of excessive currents that could damage the power components or the load. The action threshold (VISP-ISN > 600mV, typical) is above the default LED current sense threshold, so that no interference will occur with current regulation.
PAGE 15
LT3761 Applications Information PWM Dimming Signal Generator Programming the Switching Frequency The LT3761 features a PWM dimming signal generator with programmable duty cycle. The frequency of the square wave signal at PWMOUT is set by a capacitor CPWM from the PWM pin to GND according to the equation: The RT frequency adjust pin allows the user to program the switching frequency (fSW) from 100kHz to 1MHz to optimize efficiency/performance or external component size.
PAGE 16
LT3761 Applications Information 300 CGATE = 3300pF 250 MINIMUM ON-TIME TIME (ns) 200 MINIMUM OFF-TIME 150 100 50 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3761 F05 Figure 5. Typical Minimum On and Off GATE Pulse Width vs Temperature Besides the limitation by the minimum off-time, it is also recommended to choose the maximum duty cycle below 95%.
PAGE 17
LT3761 Applications Information For monitoring the LED string voltage, if the open LED clamp voltage is programmed correctly using the FB resistor divider then the FB pin should not exceed 1.18V when LEDs are connected. If the OPENLED pulldown is asserted and the PWM pin transitions low, the pulldown will continue to be asserted until the next rising edge of PWM even if FB falls below the OPENLED threshold. ILED ISP RLED OPENLED 25mV + – ISN C10 COMPARATOR – + S Q 1mA R 1.
PAGE 18
LT3761 Applications Information Soft-Start Capacitor Selection For many applications, it is important to minimize the inrush current at start-up. The built-in soft-start circuit significantly reduces the start-up current spike and output voltage overshoot. Connect a capacitor from the DIM/SS pin to GND to use this feature. The soft-start interval is set by the softstart capacitor selection according to the equation: TSS = CSS • 1.
PAGE 19
LT3761 Applications Information Schottky Rectifier Selection For buck mode, select a resistor according to: The power Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage as described in the section on power MOSFET selection. If using the PWM feature for dimming, it may be important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval.
PAGE 20
LT3761 Applications Information Use the equation for Buck-Boost when choosing an inductor value for SEPIC – if the SEPIC inductor is coupled, then the equation’s result can be used as is. If the SEPIC uses two uncoupled inductors, then each should have a inductance double the result of the equation. and secondary inductors. The CDC should be sized to limit its voltage ripple. The power loss on the CDC ESR reduces the LED driver efficiency.
PAGE 21
LT3761 Applications Information The ground terminal of the switch current sense resistor should Kelvin connect to the GND of the LT3761. Likewise, the ground terminal of the bypass capacitor for the INTVCC regulator should be placed near the GND of the switching path. Typically this requirement will result in the external switch being closest to the IC, along with the INTVCC bypass capacitor. The ground for the compensation network (VC) and other DC control signals (e.g.
PAGE 22
LT3761 Typical Applications 94% Efficient Boost LED Driver for Automotive Headlamp with 25:1 PWM Dimming L1 10µH CIN 2.2µF ×2 100V R1 499k R2 90.9k EN/UVLO VREF 1M VIN SENSE RSENSE 10mΩ LT3761 RLED 0.25Ω 100 R4 16.9k 60W LED STRING 92 1.2 88 1.0 OUTPUT CURRENT 84 RT 28.7k 350kHz RC 5.1k CC 4.7nF 1.4 EFFICIENCY OUTPUT CURRENT (A) FB 1.
PAGE 23
LT3761 Typical Applications Boost LED Driver with Output Short-Circuit Protection with Internally Generated PWM L1 10µH VIN 8V TO 60V 2.2µF ×2 100V 499k 90.9k INTVCC OPTION FOR INTERNALLY GENERATED PWM DIMMING DIM EN/UVLO VREF 1M VIN GATE D1 0.25Ω 2.2µF ×4 100V M1 SENSE LT3761 CTRL 10mΩ 1k 100k 2.4k ISP ISN OPENLED M2 1M 124k DIM/SS 10nF 150pF RT PWMOUT INTVCC 28.7k 350kHz 5.1k 4.7nF ILED 1A LED+ FB PWM VC 22nF 640Hz Q1 GND 140k 16.
PAGE 24
LT3761 Typical Applications 10W Grounded Anode Inverting LED Driver 3 100k EN/UVLO VIN GATE • 4.7µF 25V • VIN 5V TO 18V 2.2µF ×2 35V L1 4.7µH 1:1 1 10Ω 2 LT3761 GND OPENLED ISN FB ISP 0.1µF –10V CLAMP 10Ω M2 0.47µF DIM/SS VREF INTVCC CTRL VC 91mΩ 10mΩ PWM 1µF 4.7µF 10V SENSE 34k 0.01µF D1 M1 4 RT 59k 10nF LED– 2k 2.5A 10W LED PWMOUT 2k 28.
PAGE 25
LT3761 Typical Applications 40W SEPIC LED Driver C1 2.2µF ×2 50V 3 499k 90.9k EN/UVLO VREF 1M VIN INTVCC 1 GATE •2 C3 10µF 4 ×5 35V M1 SENSE LT3761 CTRL D1 • VIN 8V TO 40V C2 2.2µF ×2 50V L1 10µH 1:1 1M 8mΩ 100k 1.67A 49.9k GND 133k 0.15Ω FB ISP OPENLED ISN PWM DIM/SS PWMOUT VC RT INTVCC 10nF 10k 28.7k 350kHz 40W LED STRING INTVCC 1µF 4.
PAGE 26
LT3761 Typical Applications Boost LED Driver for 30kHz PWM Dimming L1 0.82µH VIN 8V TO 20V C1 10µF 25V 187k VIN VREF CTRL SENSE LT3761 6mΩ PWM 0.25Ω Boost PWM Dimming Waveform 1A VIN = 16V, VLED = 30V PWM 17.8k FB 100k OPENLED DIM/SS VC 0.1µF 499k GND INTVCC 1µF C2 10µF ×2 35V M1 GATE EN/UVLO 39.2k D1 ILED 0.5A/DIV ISP ISN PWMOUT 5µs/DIV RT 20.5k 500kHz 3k 4.
PAGE 27
LT3761 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev E) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.23 (.206) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 8 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 0.305 ±0.038 (.0120 ±.0015) TYP 16 0.50 (.0197) BSC 4.039 ±0.102 (.159 ±.
PAGE 28
LT3761 Typical Application HV Boost Efficiency and LED Current vs VIN 100 80W High Voltage Boost LED Driver with 25:1 Internally Generated PWM Dimming L1 22µH 187k 21k EN/UVLO VREF 590k VIN 20k COUT 0.22µF ×8 250V 1M 0805 80W LED STRING 135V MAX 8.66k FB 0.1µF 10nF 1.4kHz 2k 0.6 LED CURRENT 88 0.4 84 0.2 80 PWMOUT OPENLED DIM/SS ISP PWM ISN VC RT INTVCC 124k 92 12mΩ GND 100k DIM 0V TO 8V M1 SENSE LT3761 CTRL INTVCC GATE EFFICIENCY EFFICIENCY (%) 2.2µF ×2 100V 0.8 96 D1 39.