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LT3956

3956f

Typical applicaTion

FeaTures

applicaTions

DescripTion

80V

, 80V

OUT

Constant-Current,

Constant-Voltage Converter

The LT

3956 is a DC/DC converter designed to operate as

a constant-current source and constant-voltage regulator.

It is ideally suited for driving high current LEDs. It features

an internal low side N-channel power MOSFET rated for

84V at 3.3A and driven 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 LT3956 senses 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 25W White LED Headlamp Driver

3000:1 True Color PWM

Dimming

Wide Input Voltage Range: 4.5V to 80V

Output Voltage Up to 80V

Internal 3.3A/84V Switch

Constant-Current and Constant-Voltage Regulation

250mV 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

Constant-Voltage Loop Status Pin

PWM Disconnect Switch Driver

CTRL Pin Adjusts High Side Current Sense Threshold

Low Shutdown Current: <1µA

Programmable Soft-Start

Available in the 36-Lead (5mm × 6mm) QFN Package

High Power LED

Battery Charger

Accurate Current Limited Voltage Regulator

LT3956

22µH

GNDV

INTV

EN/UVLO

PGND

REF

ISP

332k

100k

INTV

332k

2.2µF

s2

2.2µF

s5

4.7nF

, 6V TO 60V

(80V TRANSIENT)

47nF

100k

34k

28.7k

375kHz

4.7µF

40.2k

CTRL

16.2k

0.68Ω 370mA

INTV

25W LED STRING

3956 TA01a

VMODE

PWM

ISN

PWMOUT

Efﬁciency vs V

(V)

0 20

EFFICIENCY (%)

100

60 80

3956 TA01b

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 and 7321203.

Summary of content (20 pages)

PAGE 1
LT3956 80VIN, 80VOUT Constant-Current, Constant-Voltage Converter Description Features n n n n n n n n n n n n n n n n 3000:1 True Color PWMTM Dimming Wide Input Voltage Range: 4.5V to 80V Output Voltage Up to 80V Internal 3.
PAGE 2
LT3956 Absolute Maximum Ratings Pin Configuration (Note 1) VC CTRL VREF PWM VMODE SS RT TOP VIEW 36 35 34 33 32 31 30 NC 1 28 ISP EN/UVLO 2 27 ISN INTVCC 3 37 GND GND 4 25 FB 24 GND VIN 6 23 PWMOUT 38 SW SW 8 SW 9 21 SW 20 SW NC 10 PGND PGND PGND PGND PGND 12 13 14 15 16 17 PGND VIN, ISP, ISN...............................................................80V SW.............................................................................84V EN/UVLO (Note 3)........................
PAGE 3
LT3956 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 SW Pin Leakage SW = 48V SW Pin Current Limit MIN l SW Pin Voltage Drop I(SW) = 2A SS Pull-Up Current Current Out of Pin 3.3 TYP MAX 5 10 µA 3.9 4.
PAGE 4
LT3956 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 0.85 1.35 1.8 45 60 1.185 1.220 UNITS Logic Inputs/Outputs PWM Threshold Voltage l PWM Pin Resistance to GND EN/UVLO Threshold Voltage Falling l EN/UVLO Rising Hysteresis V kΩ 1.
PAGE 5
LT3956 Typical Performance Characteristics VREF Voltage vs Temperature 2.04 1.27 2.03 2.03 1.26 2.02 2.02 1.25 2.01 2.01 1.24 VREF (V) 2.04 1.23 2.00 1.99 2.00 1.99 1.22 1.98 1.98 1.21 1.97 1.97 50 25 0 75 TEMPERATURE (°C) –25 100 1.96 –50 125 3956 G04 10000 SWITCHING FREQUENCY (kHz) 500 1000 100 10 400 350 50 25 0 75 TEMPERATURE (°C) 100 20 40 VIN (V) 60 80 3956 G10 80 3956 G06 2.0 1.
PAGE 6
LT3956 Typical Performance Characteristics INTVCC Current Limit vs Temperature 12 20 10 18 8 6 4 2 0 0 800 600 400 200 SWITCHING FREQUENCY (kHz) NOT SWITCHING 7.3 16 14 50 25 0 75 TEMPERATURE (°C) –25 100 SW Pin Current Limit vs Duty Cycle 312.5 3.5 3.0 50 75 DUTY CYCLE (%) 100 0 VCTRL = 2V 250.0 –0.5 187.5 125.0 62.50 –40°C 25°C –1.0 –1.5 125°C –2.0 0 1.
PAGE 7
LT3956 Pin Functions NC: No Internal Connection. These pins may be left floating or connected to an adjacent pin. EN/UVLO: Shutdown and Undervoltage Detect Pin. An accurate 1.22V falling threshold with externally programmable hysteresis detects when power is OK to enable switching. Rising hysteresis is generated by the external resistor divider and an accurate internal 2.1µA pull-down current. Above the 1.24V (nominal) threshold (but below 6V), EN/UVLO input bias current is sub-µA.
PAGE 8
LT3956 Pin Functions VREF: Voltage Reference Output Pin (typically 2V). This pin drives a resistor divider for the CTRL pin, either for analog dimming or for temperature limit/compensation of LED load. Can supply up to 100μA. high. When PWM input goes high again, the VMODE pin will be updated. This pin may be used to report an open LED fault. Use a pull-up current less than 1mA. SS: Soft-Start Pin. This pin modulates oscillator frequency and compensation pin voltage (VC) clamp.
PAGE 9
LT3956 Operation The LT3956 is a constant-frequency, current mode converter with a low side N-channel MOSFET switch. The switch 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 10
LT3956 Applications Information INTVCC Regulator Bypassing and Operation LED Current Programming The INTVCC pin requires a capacitor for stable operation and to store the charge for the switch driver and PWMOUT pin 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 11
LT3956 Applications Information 20mV should not cause misoperation, but may lead to noticeable offset between the average value and the userprogrammed value. Output Current Capability An important consideration when using a switch with a fixed current limit is whether the regulator will be able to supply the load at the extremes of input and output voltage range. Several equations are provided to help determine this capability. Some margin to data sheet limits is included.
PAGE 12
LT3956 Applications Information Dimming Control Duty Cycle Considerations There are two methods to control the current source for dimming using the LT3956. One method uses the CTRL pin to adjust the current regulated in the LEDs. A second method uses the PWM pin to modulate the current source between zero and full current to achieve a precisely programmed average current.
PAGE 13
LT3956 Applications Information will be enabled after the device temperature drops 10°C. This function is intended to protect the device during momentary overload conditions. The major contributors to internal power dissipation are the current in the linear regulator to drive the switch, and the ohmic losses in the switch.
PAGE 14
LT3956 Applications Information Output Capacitor Selection The selection of the output capacitor depends on the load and converter configuration, i.e., step-up or step-down and the operating frequency. For LED applications, the equivalent resistance of the LED is typically low and the output filter capacitor should be sized to attenuate the current ripple. Use of an X7R type ceramic capacitor is recommended.
PAGE 15
LT3956 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 4.7nF 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 16
LT3956 Typical Applications 94% Efficient 25W White LED Headlamp Driver VIN 6V TO 60V (80V TRANSIENT) CVIN 2.2µF s2 L1 22µH R1 332k R2 100k VIN PGND VREF 100k CSS 47nF RS 0.68Ω VMODE PWM SS RT VC 370mA ISN 40.2k RT 28.7k 375kHz M1: VISHAY SILICONIX Si2328DS D1: DIODES INC PDS5100 L1: COILTRONICS DR125-220 C1, C2: MURATA GRM42-2x7R225 ISP LT3956 CTRL INTVCC COUT 2.2µF s5 SW EN/UVLO 332k D1 R3 1M FB R4 16.
PAGE 17
LT3956 Typical Applications Efficiency vs VIN Buck-Boost Mode LED Driver C1 4.7µF VIN 1M D1 SW EN/UVLO ISP 680mΩ LT3956 CTRL INTVCC 4.7µF 35V ISN 619k 24V LED STRING 350mA FB 100k 35.7k 300kHz 10k VMODE PGND PWM SS RT PWMOUT VC GND INTVCC 3.4k 92 88 84 80 0 4.7µF C4 10µF L1A 33µH ISN SW VIN D1 FB LT3956 VMODE VREF INTVCC 14k SS PWM 3956 TA04a C3 10µF 1M CTRL 10k C2 4.7µF L1B 536k 25k PWMOUT VC C1 10µF RT 10nF 1M 59k 2k GND 28.7k 375kHz 40.
PAGE 18
LT3956 Typical Applications Efficiency vs VIN 90% Efficient, 20W SEPIC LED Driver C4 2.2µF (50V) VIN 8V TO 50V C1 4.7µF 50V 1M 1:1 250k VIN EN/UVLO 185k VREF PGND CTRL ISP 25k INTVCC C3 10µF s2 35V L1B SW 100 D1 0.25Ω LT3956 96 EFFICIENCY (%) L1A 33µH 1A VMODE PWM SS RT VC 28.7k 375kHz 1M 20W LED STRING FB 56.2k PWMOUT GND INTVCC 10nF 80 0 10 20 30 VIN (V) 40 50 3956 TA05b CURRENT DERATED FOR VIN < 13V C2 4.7µF 10V 15k 0.
PAGE 19
LT3956 Package Description UHE Package Variation: UHE28MA 36-Lead Plastic QFN (5mm s 6mm) (Reference LTC DWG # 05-08-1836 Rev C) 28 27 25 24 23 21 20 0.70 p0.05 30 1.88 p 0.05 31 5.50 p 0.05 4.10 p 0.05 1.50 REF 3.00 p 0.05 32 33 16 3.00 p 0.05 0.12 p 0.05 34 17 1.53 p 0.05 15 14 PACKAGE OUTLINE 13 0.48 p 0.05 12 35 36 1 2 3 4 6 0.50 BSC 8 9 0.25 p0.05 10 2.00 REF 5.10 p 0.05 6.50 p 0.
PAGE 20
LT3956 Typical Application Buck Mode 1A LED Driver with High Dimming Ratio and Open LED Reporting Efficiency vs VIN 100 1M VIN 200k ISP EN/UVLO 750Ω 61.9k 0.1Ω 1A 200k Q2 ISN FB C2 4.7µF s5 20k M1 VREF 200k 30.1k CTRL 10k PWMOUT LT3956 Q1 6 WHITE LEDs 20W 1k INTVCC SW VMODE 0.1µF 28.7k 375kHz 47k INTVCC 2.2nF 4.