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LT3755/LT3755-1/LT3755-2

37551fd

Typical applicaTion

FeaTures

applicaTions

DescripTion

40V

, 75V

OUT

LED Controllers

The LT

3755, LT3755-1 and LT3755-2 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 LT3755/LT3755-1/LT3755-2 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.

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, 7321203.

50W White Automotive LED Headlamp Driver

3000:1 True Color PWM™ Dimming

Wide Input Voltage Range: 4.5V to 40V

Output Voltage Up to 75V

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 (LT3755-2)

Frequency Synchronization (LT3755-1)

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

Battery Chargers

Accurate Current Limited Voltage Regulators

LT3755-2

22µH

GNDV

INTV

SHDN/UVLO FB

REF

ISP

332k

100k

INTV

4.7µF

0.001µF

0.01µF

8V TO

40V

185k

10k

28.7k

375kHz

4.7µF

40.2k

CTRL

0.015Ω

0.1Ω

23.7k

4.7µF

50W

LED

STRING

37551 TA01a

OPENLED

PWM

ISN

GATE

SENSE

PWMOUT

Efﬁciency vs V

(V)

0 10

EFFICIENCY (%)

100

30 40

37551 TA01b

Summary of content (26 pages)

PAGE 1
LT3755/LT3755-1/LT3755-2 40VIN, 75VOUT LED Controllers 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: 4.
PAGE 2
LT3755/LT3755-1/LT3755-2 Absolute Maximum Ratings (Note 1) VIN.............................................................................40V SHDN/UVLO (Note 3).................................................40V ISP, ISN......................................................................75V INTVCC....................................................... VIN + 0.3V, 8V GATE, PWMOUT (Note 4).......................... INTVCC + 0.3V CTRL, PWM, OPENLED..............................................
PAGE 3
LT3755/LT3755-1/LT3755-2 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 = 1.15V, PWM = 0V VIN Operating IQ (Not Switching) PWM = 0V VREF Voltage 100µA ≤ IVREF ≤ 0µA VREF Line Regulation 4.
PAGE 4
LT3755/LT3755-1/LT3755-2 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 7 7.15 7.3 UNITS Linear Regulator INTVCC Regulation Voltage Dropout (VIN – INTVCC) IINTVCC = –10mA, VIN = 7V 350 INTVCC Undervoltage Lockout INTVCC Current Limit INTVCC Current in Shutdown 3.9 4.
PAGE 5
LT3755/LT3755-1/LT3755-2 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 20 0 VCTRL (V) 40 60 ISP VOLTAGE (V) 100 99 98 97 –50 –25 80 VREF Voltage vs Temperature 1.27 2.03 2.03 1.26 2.02 2.02 1.25 2.01 2.01 VREF (V) 2.04 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
LT3755/LT3755-1/LT3755-2 Typical Performance Characteristics SENSE Current Limit Threshold vs Temperature Quiescent Current vs VIN 110 SENSE THRESHOLD (mV) PWM = 0V VIN CURRENT (mA) 1.5 1.0 0.5 0 0 10 20 VIN (V) 30 1.28 105 100 95 90 –50 –25 40 0 2 30 38 7.3 36 34 0 125 25 50 75 DUTY CYCLE (%) 100 37551 G16 25 50 75 100 125 150 TEMPERATURE (°C) Gate Rise/Fall Time vs Capacitance 100 VCTRL = 2V 100 75 50 25 0 1.
PAGE 7
LT3755/LT3755-1/LT3755-2 Typical Performance Characteristics ISP/ISN Input Bias Current vs CTRL Voltage INTVCC Dropout Voltage vs Current, Temperature 0 40 TA = –45°C –0.5 30 LDO DROPOUT (V) INPUT BIAS CURRENT (µA) ISP ISN 20 10 0 TA = 25°C, unless otherwise noted. TA = 25°C TA = 125°C –1.0 TA = 150°C –1.5 –2.0 0 0.5 1 CTRL (V) 1.5 2 37551 G19 Pin Functions –2.
PAGE 8
LT3755/LT3755-1/LT3755-2 Pin Functions (MSOP/QFN) 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
LT3755/LT3755-1/LT3755-2 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 – 50k VREF VIN SCILMB Q2 140µA PWM 10µA AT FB = 1.
PAGE 10
LT3755/LT3755-1/LT3755-2 Operation The LT3755 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
LT3755/LT3755-1/LT3755-2 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
LT3755/LT3755-1/LT3755-2 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
LT3755/LT3755-1/LT3755-2 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
LT3755/LT3755-1/LT3755-2 Applications Information 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. This gate drive current can be calculated as: IGATE = fSW • QG A low QG power MOSFET should always be used when operating at high input voltages, and the switching frequency should also be chosen carefully to ensure that the IC does not exceed a safe junction temperature.
PAGE 15
LT3755/LT3755-1/LT3755-2 Applications Information Therefore, a 10µF 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:  µF  CIN (µF) = ILED (A) • t SW (µs) • 4.7 •   A • µs  A 10µF input capacitor is an appropriate selection for a 400kHz buck mode converter with a 1A load.
PAGE 16
LT3755/LT3755-1/LT3755-2 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
LT3755/LT3755-1/LT3755-2 Applications Information 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. For typical LED applications, a 2.2nF compensation capacitor at VC is adequate, and a series resistor should always be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on the input supply to the converter.
PAGE 18
LT3755/LT3755-1/LT3755-2 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 “50W WHITE LED HEADLAMP DRIVER” SCHEMATIC LED+ 37551 F05 Figure 5.
PAGE 19
LT3755/LT3755-1/LT3755-2 Typical Applications 50W White LED Headlamp Driver VIN 8V TO 40V L1 22µH CIN 4.7µF R1 1M D1 SHDN/UVLO R2 187k 16.9k FB VREF 100k CSS 0.01µF RT 28.7k 375kHz RLED 0.1Ω LT3755-2 1A ISN 100k NTC RT1 INTVCC R4 23.7k ISP CTRL COUT 4.7µF R3 1M VIN M1 GATE SENSE OPENLED PWM SS RT PWMOUT VC GND INTVCC RC 10k CC 0.001µF L1: COILTRONICS DR127-220 M1: VISHAY SILICONIX SI7850DP D1: DIODES INC. PDS5100 M2: VISHAY SILICONIX SI2308DS RT1: MURATA NCP18WM1045 RSENSE 0.
PAGE 20
LT3755/LT3755-1/LT3755-2 Typical Applications Buck Mode 1.4A LED Driver VIN 15V TO 40V C1 1µF 1M ISP VIN 110k SHDN/UVLO 107k 0.068Ω CTRL 200k Q1 1.5k INTVCC 20k M3 LT3755-2 M2 PWMOUT PWM 100k 1.4A ISN FB VREF C3 4.7µF 200k 3 LUXEON K2 - WHITE 1k OPENLED L1 33µH SS D1 VIN 0.1µF 28.7k 375kHz VC GND INTVCC 47k 0.001µF C4 4.7µF M1 GATE RT SENSE 0.033Ω C2 4.
PAGE 21
LT3755/LT3755-1/LT3755-2 Typical Applications Buck Mode 500mA LED Driver for 20kHz PWM Dimming VIN 22V TO 36V 1M ISP VIN SHDN/UVLO 68.1k 0.2Ω ISN VREF CTRL 16V 0.22µF M2 PWMOUT 6V 4.7µF 2s 25V 1M LT3755-2 PWM INTVCC 4.7µF L1 3.3µH 2200pF SENSE VC GND 22k 0.1µF 470pF FB D1 M1 GATE OPENLED SS 500mA RT 0.033Ω 13k 800kHz 2.
PAGE 22
LT3755/LT3755-1/LT3755-2 Typical Applications 21W Buck-Boost Mode with 250:1 PWM Dimming and Open LED Protection L1 15µH VIN 8V TO 36V 0.1Ω D1 50V 2.2µF 2s M2 100V 2.2µF 2s M1 392k 21.5V 1A 1.5k 0.02Ω VIN Q1 100k 499k GATE SENSE VIN FB SHDN/UVLO 549k 93.1k VREF LT3755-2 PWMOUT CTRL 75.0k 20.0k 4.7µF Q2 PWM PWM 1k INTVCC ISP 100k ISN OPENLED SS VC 0.01µF GND 4.
PAGE 23
LT3755/LT3755-1/LT3755-2 Package Description MSE Package MSE Package 16-Lead Plastic MSOP, Exposed Die Pad 16-Lead , Exposed (Reference LTCPlastic DWG #MSOP 05-08-1667 RevDie A) Pad (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 24
LT3755/LT3755-1/LT3755-2 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 25
LT3755/LT3755-1/LT3755-2 Revision History (Revision history begins at Rev D) REV DATE DESCRIPTION PAGE NUMBER D 03/10 Revised Entire Data Sheet to Include H-Grade 1-26 37551fd 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 26
LT3755/LT3755-1/LT3755-2 Typical Application Buck-Boost LED Driver for Automotive VIN 6V TO 36V Efficiency vs VIN 100 4.7µF s2 LTC4440-5 VCC BOOST TG GND TS INP 330k D1 SHDN/UVLO VREF 383k M2 GATE 0.025Ω LT3755-2 47k 1M 100k OPENLED PWM SS ISN RT PWMOUT VC GND INTVCC 28.7k 375kHz 1% 88 84 4.7µF 40k 80 FB ISP 0.01µF 92 SENSE CTRL INTVCC D2 22µF VIN 1M 96 0.22µF M1 EFFICIENCY (%) INTVCC 10k 0.01µF 0.1Ω 0 10 20 VIN (V) 30 40 37551 TA05b 1A INTVCC 4.