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LT3756/LT3756-1/LT3756-2

375612fb

Typical applicaTion

FeaTures

applicaTions

DescripTion

100V

, 100V

OUT

LED Controller

The LT

3756, LT3756-1 and LT3756-2 are DC/DC control-

lers designed to operate as a constant-current source for

driving high current LEDs. They drive a low side external

N-channel power MOSFET from an internal regulated 7.15V

supply. The ﬁxed frequency, current-mode architecture

results in stable operation over a wide range of supply

and output voltages. A ground referenced voltage FB 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 LT3756/LT3756-1/LT3756-2 sense output current at

the high side of the LED string. High side current sensing

is the most ﬂexible scheme for driving LEDs, allowing

boost, buck mode or buck-boost mode conﬁguration.

The PWM input provides LED dimming ratios of up to

3000:1, and the CTRL input provides additional analog

dimming capability.

94% Efﬁcient 30W White LED Headlamp Driver

3000:1 True Color PWM™ Dimming

Wide Input Voltage Range: 6V to 100V

Output Voltage Up to 100V

Constant-Current and Constant-Voltage Regulation

100mV High Side Current Sense

Drives LEDs in Boost, Buck Mode, Buck-Boost Mode,

SEPIC or Flyback Topology

Adjustable Frequency: 100kHz to 1MHz

Open LED Protection

Programmable Undervoltage Lockout with Hysteresis

Improved Open LED Status Pin (LT3756-2)

Frequency Synchronization (LT3756-1)

PWM Disconnect Switch Driver

CTRL Pin Provides Analog Dimming

Low Shutdown Current: <1µA

Programmable Soft-Start

Thermally Enhanced 16-Lead QFN (3mm × 3mm)

and MSOP Packages

High Power LED Applications

Current Limited Constant Voltage Applications

Battery Charging

LT3756-2

22µH

GNDV

INTV

SHDN/UVLO FB

REF

ISP

332k

100k

INTV

4.7µF

0.001µF

0.01µF

8V TO 60V

(100V TRANSIENT)

185k

10k

28.7k

375kHz

4.7µF

INTV

40.2k

CTRL

0.018Ω

0.27Ω

14k

370mA

4.7µF

30W

LED

STRING

375612 TA01a

OPENLED

PWM

ISN

GATE

SENSE

PWMOUT

Efﬁciency vs V

(V)

0 20

EFFICIENCY (%)

100

60 80

375612 TA01b

L, LT, LTC, 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 and 7321203.

Summary of content (24 pages)

PAGE 1
LT3756/LT3756-1/LT3756-2 100VIN, 100VOUT LED Controller Features n n n n n n n n n n n n n n n n Description 3000:1 True Color PWM™ Dimming Wide Input Voltage Range: 6V to 100V Output Voltage Up to 100V Constant-Current and Constant-Voltage Regulation 100mV High Side Current Sense Drives LEDs in Boost, Buck Mode, Buck-Boost Mode, SEPIC or Flyback Topology Adjustable Frequency: 100kHz to 1MHz Open LED Protection Programmable Undervoltage Lockout with Hysteresis Improved Open LED Status Pin (LT3756-2) Freq
PAGE 2
LT3756/LT3756-1/LT3756-2 Absolute Maximum Ratings (Note 1) VIN...........................................................................100V SHDN/UVLO.............................................................100V ISP, ISN....................................................................100V INTVCC....................................................... VIN + 0.3V, 8V GATE, PWMOUT (Note 4)...........................INTVCC + 0.3V CTRL, PWM, OPENLED..............................................
PAGE 3
LT3756/LT3756-1/LT3756-2 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS VIN Minimum Operating Voltage VIN Tied to INTVCC VIN Shutdown IQ SHDN/UVLO = 0V, PWM = 0V SHDN/UVLO = 1.
PAGE 4
LT3756/LT3756-1/LT3756-2 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, CTRL = 2V, PWM = 5V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX UNITS Logic Inputs/Outputs PWM Input High Voltage l PWM Input Low Voltage l PWM Pin Resistance to GND 1.5 45 PWMOUT Output Low (VOL) 60 0 PWMOUT Output High (VOH) SHDN/UVLO Threshold Voltage Falling V 0.
PAGE 5
LT3756/LT3756-1/LT3756-2 Typical Performance Characteristics 103 100 102 80 60 40 20 0 0 0.5 1 101 100 99 98 97 2 1.5 103 VCTRL = 2V 0 20 40 60 ISP VOLTAGE (V) VCTRL (V) 375612 G01 99 98 97 –50 –25 VREF Voltage vs Temperature 1.27 2.03 2.03 1.26 2.02 2.02 1.25 2.01 2.01 2.04 VREF (V) 1.23 2.00 1.99 2.00 1.99 1.22 1.98 1.98 1.21 1.97 1.97 1.20 –50 –25 0 1.
PAGE 6
LT3756/LT3756-1/LT3756-2 Typical Performance Characteristics SENSE Current Limit Threshold vs Temperature Quiescent Current vs VIN 110 PWM = 0V SENSE THRESHOLD (mV) VIN CURRENT (mA) 1.5 1.0 0.5 0 0 20 40 60 VIN (V) 80 1.28 105 100 95 90 –50 –25 100 0 40 60 VIN (V) VINTVCC (V) 18 16 14 125 0 0 25 50 75 DUTY CYCLE (%) 7.
PAGE 7
LT3756/LT3756-1/LT3756-2 Typical Performance Characteristics INTVCC Dropout Voltage vs Current, Temperature ISP/ISN Input Bias Current vs CTRL Voltage 0 40 –0.5 LDO DROPOUT (V) INPUT BIAS CURRENT (µA) ISP 30 ISN 20 10 0 TA = 25°C, unless otherwise noted. –45°C –1.0 25°C –1.5 150°C 125°C –2.0 0 0.5 1 CTRL (V) 1.5 2 375612 G19 Pin Functions –2.
PAGE 8
LT3756/LT3756-1/LT3756-2 Pin Functions CTRL (Pin 6/Pin 16): Current Sense Threshold Adjustment Pin. Regulating threshold V(ISP – ISN) is 1/10th VCTRL plus an offset for 0V < VCTRL < 1V. For VCTRL > 1.2V the current sense threshold is constant at the full-scale value of 100mV. For 1V < VCTRL < 1.2V, the dependence of current sense threshold upon VCTRL transitions from a linear function to a constant value, reaching 98% of full-scale value by VCTRL = 1.1V. Do not leave this pin open.
PAGE 9
LT3756/LT3756-1/LT3756-2 Block Diagram SHDN/UVLO 1.22V – + A6 VC FB 1.3V 2.1µA A5 + gm – 1.25V SHORT-CIRCUIT DETECT + – + A10 – 150mV gm EAMP ISN + A1 – 5k ISP CTRL BUFFER CTRL 1.1V + + A3 – OVFB COMPARATOR 2V 170k R 7.15V INTVCC GATE Q DRIVER S PWM COMPARATOR 10µA AT A1+ = A1– ISENSE + – SENSE A4 FAULT LOGIC GND RAMP GENERATOR VC SSCLAMP – +A7 – LDO +A8 1.25V + A2 – Q2 140µA VIN SCILMB 50k VREF PWM 10µA AT FB = 1.
PAGE 10
LT3756/LT3756-1/LT3756-2 Operation The LT3756 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 11
LT3756/LT3756-1/LT3756-2 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 or X5R ceramic capacitor for best performance. A 4.7µF 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 12
LT3756/LT3756-1/LT3756-2 Applications Information When VCTRL is higher than 1.2V, the LED current is regulated to: ILED = 100mV RLED The LED current programming feature can increase total dimming range by a factor of 10. The CTRL pin should not be left open (tie to VREF if not used). The CTRL pin can also be used in conjunction with a thermistor to provide overtemperature protection for the LED load, or with a resistor divider to VIN to reduce output power and switching current when VIN is low.
PAGE 13
LT3756/LT3756-1/LT3756-2 Applications Information Programming the Switching Frequency The RT frequency adjust pin allows the user to program the switching frequency from 100kHz to 1MHz to optimize efficiency/performance or external component size. Higher frequency operation yields smaller component size but increases switching losses and gate driving current, and may not allow sufficiently high or low duty cycle operation.
PAGE 14
LT3756/LT3756-1/LT3756-2 Applications Information important when operating at high ambient temperatures. The majority of the power dissipation in the IC comes from the supply current needed to drive the gate capacitance of the external power MOSFET.
PAGE 15
LT3756/LT3756-1/LT3756-2 Applications Information Therefore, a 4.7µF capacitor is an appropriate selection for a 400kHz boost regulator with 12V input, 48V output and 1A load. With the same VIN voltage ripple of 100mV, the input capacitor for a buck converter can be estimated as follows: 4.7µF CIN (µF) =ILED (A) • t SW (µs) • A • µs A 10µF input capacitor is an appropriate selection for a 400kHz buck mode converter with a 1A load.
PAGE 16
LT3756/LT3756-1/LT3756-2 Applications Information Schottky Rectifier Selection The power Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage. If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with sufficiently low leakage current. Table 4 has some recommended component vendors.
PAGE 17
LT3756/LT3756-1/LT3756-2 Applications Information Loop Compensation The LT3756 uses an internal transconductance error amplifier whose VC output compensates the control loop. The external inductor, output capacitor and the compensation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop response and stability.
PAGE 18
LT3756/LT3756-1/LT3756-2 CTRL VREF OPENLED PWM Applications Information VIAS TO GROUND PLANE CSS RT R2 4 3 2 1 5 16 CC RC R1 6 15 CVCC 7 14 8 13 x VOUT VIA 9 10 11 12 L1 R3 x R4 5 4 1 6 3 M2 M1 7 2 8 1 3 LED– 2 RSENSE COUT COUT D1 CIN RLED VIN GND COMPONENT DESIGNATIONS REFER TO “30W WHITE LED HEADLAMP DRIVER WITH THERMAL DERATING” SCHEMATIC LED+ 375612 F05 Figure 5.
PAGE 19
LT3756/LT3756-1/LT3756-2 Typical Applications 30W White LED Headlamp Driver with Thermal Derating D1 L1, 22µH CIN 4.7µF R1 1M R3 1M VIN SHDN/UVLO R2 185k VREF 16.9k 100k FB RLED 0.27Ω CTRL ISN OPENLED PWM SS CSS 0.01µF RT VC RT 28.7k 375kHz M1 GATE SENSE COUT 4.7µF R4 14k ISP LT3756-2 100k NTC RT1 INTVCC 370mA 30W LED STRING RSENSE 0.018Ω PWMOUT GND INTVCC RC 10k CC 0.001µF CVCC 4.
PAGE 20
LT3756/LT3756-1/LT3756-2 Typical Applications Buck-Boost Mode LED Driver L1 68µH C1 4.7µF Efficiency vs VIN D1 1µF 100V VIN 1M SHDN/UVLO VREF 185k FB CTRL 0.1µF 35.7k 300kHz 90 VIN 13k 1Ω ISN M1 GATE 100k SENSE OPENLED PWM SS RT PWMOUT VC GND INTVCC 4700pF 24V TO 32V LED STRING 100mA 0.068Ω 80 70 60 50 1.5k C2 2.2µF 10V 39k 100 C3 4.
PAGE 21
LT3756/LT3756-1/LT3756-2 Package Description MSE Package MSE Package 16-Lead Plastic MSOP, Exposed Die Pad 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev A) (Reference LTC DWG # 05-08-1667 Rev A) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 p 0.102 (.112 p .004) 5.23 (.206) MIN 2.845 p 0.102 (.112 p .004) 0.889 p 0.127 (.035 p .005) 8 1 1.651 p 0.102 (.065 p .004) 1.651 p 0.102 3.20 – 3.45 (.065 p .004) (.126 – .136) 0.305 p 0.038 (.0120 p .0015) TYP 16 0.50 (.0197) BSC 4.
PAGE 22
LT3756/LT3756-1/LT3756-2 Package Description UD Package 16-Lead Plastic QFN (3mm × 3mm) (Reference LTC DWG # 05-08-1691) 0.70 ±0.05 3.50 ± 0.05 1.45 ± 0.05 2.10 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ± 0.10 (4 SIDES) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR 0.25 × 45° CHAMFER R = 0.115 TYP 0.75 ± 0.05 15 PIN 1 TOP MARK (NOTE 6) 16 0.40 ± 0.10 1 1.45 ± 0.10 (4-SIDES) 2 (UD16) QFN 0904 0.200 REF 0.00 – 0.05 NOTE: 1.
PAGE 23
LT3756/LT3756-1/LT3756-2 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 03/10 Revised Entire Data Sheet to Include H-Grade 1-24 375612fb 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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PAGE 24
LT3756/LT3756-1/LT3756-2 Typical Application Buck Mode 1A LED Driver with High Dimming Ratio and Open LED Reporting C1 1µF s2 1M VIN 200k ISP SHDN/UVLO 61.9k 1.5k 0.1Ω VREF Q1 PWMOUT LT3756-2 L1 33µH OPENLED 0.1µF 28.7k 375kHz 5 WHITE LEDs 20W 1k 100k GATE RT VC SENSE GND INTVCC 47k 96 92 88 84 80 D1 M1 SS C3 4.7µF s5 25V 20k M2 PWM 200k Q2 ISN FB CTRL INTVCC 1A 200k EFFICIENCY (%) VIN 24V TO 80V Efficiency vs VIN 100 VIN C4 4.