Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - LED driver DC-DC voltage regulator QFN 38 Surface-mount

LT3476

3476fb

RGGB Lighting

Automotive and Avionic Lighting

TFT LCD Backlighting

Constant-Current Sources

ApplicAtions

typicAl ApplicAtion

High Current

Quad Output LED Driver

The LT

3476 is a quad output DC/DC converter designed

to operate as a constant-current source for driving high

current LEDs. A fixed frequency, current mode architecture

results in stable operation over a wide range of supply and

output voltages. A frequency adjust pin allows the user to

program switching frequency between 200kHz and 2MHz

to optimize efficiency and external component size.

The LT3476 senses output current at the high side of

the LED. High side current sensing is the most flexible

scheme for driving LEDs, allowing buck, boost or buck-

boost configurations. Each current monitor threshold is

trimmed to within 2.5% at the full scale of 105mV. With

an external sense resistor, the user programs the output

current range of each channel. Each of the four regulators

is independently operated by that channel’s PWM signal.

This PWM feature allows precise adjustment of the color

mixing or dimming ratio of the LED source. Dimming

ratios up to 1000:1 can be achieved.

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.

True Color PWM

Dimming Delivers Up to 5000:1

Dimming Ratio (In Boost Configuration)

LED Current Regulation with High-Side Sense

VADJ Pin Accurately Sets LED Current Sense

Threshold Over Range 10mV to 120mV

Four Independent Driver Channels with 1.5A, 36V

Internal NPN Switches

Frequency Adjust Pin: 200kHz to 2MHz

High Efficiency Conversion = Up to 96%

Open LED Protection

Low Shutdown Current < 10µA

Wide V

Range: 2.8V to 16V

Thermally Enhanced, 38-Lead, 5mm × 7mm

QFN Package

100W Quad 1A × 8 LED Driver

1000:1 PWM Dimming at 100Hz

10µH

2.2µF

1nF

21k

REF

CAP1-4

SW4

1.05V

SW1

CAP1

100mΩ

LED1

SW2 SW3

VC1-4

VADJ1-4

LED1-4

PWM1-4

0.22µF 0.22µF

UP TO

8 LEDS

2.8V TO 16V

33V

CAP2

100mΩ

LED2

GNDSHDN

LT3476

3476 TA01

SHDN

10µH

100k

2.2µF

× 4

4.99k

0.22µF 0.22µF

1A 1A 1A

10µH

CAP3

100mΩ

LED3

CAP4

100mΩ

LED4

10µH

PWM

5V/DIV

LED

500mA/DIV

5µs/DIV

3476 TA02

FeAtures Description

Summary of content (14 pages)

PAGE 1
LT3476 High Current Quad Output LED Driver Features n n n n n n n n n n Description True Color PWMTM Dimming Delivers Up to 5000:1 Dimming Ratio (In Boost Configuration) LED Current Regulation with High-Side Sense VADJ Pin Accurately Sets LED Current Sense Threshold Over Range 10mV to 120mV Four Independent Driver Channels with 1.5A, 36V Internal NPN Switches Frequency Adjust Pin: 200kHz to 2MHz High Efficiency Conversion = Up to 96% Open LED Protection Low Shutdown Current < 10µA Wide VIN Range: 2.
PAGE 2
LT3476 Absolute Maximum Ratings Pin Configuration (Note 1) VIN.............................................................................16V PWM1-4, SHDN.........................................................16V SW1-4, LED1-4, CAP1-4............................................36V REF, RT, VADJ1-4, VC1-4...............................................2V Operating Junction Temperature Range (Notes 2 and 3)....................................... –40°C to 125°C Maximum Junction Temperature................
PAGE 3
LT3476 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, (Note 3) otherwise specifications are at TA = 25°C. SW1-4 = 5V, VIN = 3.3V, SHDN = 3.3V, RT = 21k to GND, PWM1-4 = 3.3V, VADJ1-4 = REF, CAP1-4 = 5V, unless otherwise noted. PARAMETER CONDITIONS REF Line Regulation 2.8V ≤ VIN ≤ 16V MIN TYP MAX 0.003 UNITS %/V Quiescent Current in Shutdown SHDN = 0V 0.1 Quiescent Current Idle PWM1-4 = 0V 5.
PAGE 4
LT3476 Typical Performance Characteristics V(CAP-LED) Threshold vs VADJ 150 TA = 25°C, unless otherwise noted. Current Limit vs Duty Cycle 10000 2.5 TA = 25°C Oscillator Frequency vs RT 2 90 60 30 0 MINIMUM 1.5 FOSC (kHz) CURRENT LIMIT (A) V(CAP-LED) THRESHOLD (mV) TYPICAL 120 1 0.5 0 0.3 0.6 0.9 VADJ (V) 0 1.5 1.2 20 0 60 40 DUTY CYCLE (%) 80 3476 G01 1150 CURRENT LIMIT (A) 2 104 1.5 1 0.
PAGE 5
LT3476 Typical Performance Characteristics SHDN and PWM Pins Current vs Voltage 1.4 PIN THRESHOLD (V) PWM 1-4 80 60 40 36.0 CAP1-4 OVERVOLTAGE THRESHOLD (V) 1.6 100 CURRENT (µA) CAP Pins Overvoltage Threshold vs Temperature SHDN and PWM Pins Threshold vs Temperature 120 SHDN 1.2 1.0 0.8 0.6 20 0 TA = 25°C, unless otherwise noted. 0 4 8 12 16 PIN VOLTAGE (V) 0.4 –45 –20 55 30 80 5 TEMPERATURE (°C) 3476 G10 105 130 3476 G11 35.5 35.0 34.5 34.0 33.5 33.
PAGE 6
LT3476 Block Diagram PVIN 33V CBYP 2.2µF EXTERNAL COMPONENTS BUCK MODE RSNS (EXT) 0.1Ω CAP 3, 4, 9, 10 LED 2, 5, 8, 11 VADJ 14, 15, 36, 37 1.25V PWM 16, 17, 34, 35 + + – Q3 A4 ERROR AMPLIFIER THERMAL LIMIT 145°C VC 1, 12, 13, 38 VIN 3V 25k PWM – Q2 + DRIVER R A2 PWM COMPARATOR Q1 MAIN SWITCH Q S + ∑ RSW 0.02Ω A3 – VIN 33 ISRC 300µA REF 7 V1 + RAMP GENERATOR CURRENT SENSE AMPLIFIER 200kHz to 2MHz OSCILLATOR – 1.
PAGE 7
LT3476 Applications Information Layout Hints The high speed operation of the LT3476 demands careful attention to board layout. Several items are worthy of note. The exposed pad of the package is the only GND terminal of the IC and is also important to thermal management for the IC, so it is crucial to achieve a good electrical and thermal contact between the exposed pad and the ground plane of the board.
PAGE 8
LT3476 Applications Information switch, the state of the PWM pin is sampled only during a narrow time slot at the beginning of each switch period. Inductor Selection The inductors used with the LT3476 should have a saturation current rating of 2.5A or greater. For best loop stability results, the inductor value selected should provide a ripple current of 350mA or more. For buck (step-down) or boost (step-up) configurations, and using a 21kΩ resistor on RT (TSW ~ 1µs), inductor values from 4.
PAGE 9
LT3476 Applications Information Ceramic type capacitors using X7R dielectric are best for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor or larger case size to get the required capacitance at the operating voltage. Always check that the voltage rating of the capacitor is sufficient. Table 3 shows some recommended capacitor vendors. Table 3.
PAGE 10
LT3476 Applications Information through a resistor divider, or connected directly to REF to give the full-scale threshold of 105mV. A DAC may also be used to drive the VADJ pins. The VADJ pins should not be left open. If the VADJ input is connected to a voltage higher than 1.25V, the default regulation threshold across CAP and LED is 125mV (typ). The VADJ pin can also be used in conjunction with a PTC thermistor to provide overtemperature protection for the LED load as shown in Figure 2.
PAGE 11
LT3476 Typical Applications Buck Mode 100W Quad 1A × 8 LED Driver CAP2 CAP1 100mΩ 100mΩ 0.22µF L1 10µH D1 L2 10µH SW1 CAP1-4 LED1-4 VIN PWM1-4 SHDN PWM1-4 SHDN 2.2µF L3 10µH SW2 2.2µF ×4 1A 0.22µF D2 VIN 2.8V TO 16V LED4 1A 0.22µF 100 100mΩ LED3 1A 1A Efficiency vs LED Current for Buck Mode CAP4 100mΩ LED2 LED1 UP TO 8 LEDS CAP3 0.22µF D3 L4 10µH D4 LT3476 92 5 LEDS 88 80 4.99k VIN = 3.3V PVIN = 33V WHITE LEDS VF = 3.6V AT 1A 0 0.2 0.6 0.4 LED CURRENT (A) 0.8 1.
PAGE 12
LT3476 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHF Package 38-Lead Plastic QFN (5mm × 7mm) (Reference LTC DWG # 05-08-1701 Rev C) 0.70 ± 0.05 5.50 ± 0.05 5.15 ± 0.05 4.10 ± 0.05 3.00 REF 3.15 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 5.5 REF 6.10 ± 0.05 7.50 ± 0.05 RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 ± 0.10 0.75 ± 0.05 PIN 1 NOTCH R = 0.30 TYP OR 0.35 × 45° CHAMFER 3.
PAGE 13
LT3476 Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 11/11 Updated Features, Absolute Maximum Ratings, Pin Configuration, Order Information, Electrical Characteristics notes, Typical Performance Characteristics, and Pin Functions sections. Revised Table 4, moved drawings to Typical Applications section, and updated Related Parts list. Changed RT pin to RT pin and VC pin to VC pin throughout data sheet.
PAGE 14
LT3476 Typical Application Quad Boost 200mA × 8LED Driver PVIN 14V TO 18V L1 47µH L2 47µH D1 D2 2.2µF L3 47µH L4 47µH D3 D4 Boost Efficiency vs LED Current 2.2µF UP TO 8 LEDS CAP3 CAP2 0.25Ω LED1 LED2 200mA PWM1-4 SHDN 2.2µF 110k 100k 0.25Ω LED3 200mA SW1 VIN 3.3V 2.2µF 2.2µF CAP4 0.25Ω LED4 200mA SW2 200mA SW3 SW1 SW2 VIN PWM1-4 SHDN REF VADJ1-4 LT3476 SW4 CAP1-4 LED1-4 8 LEDS 92 88 80 10pF VC1-4 6 LEDS VIN = 3.3V PVIN = 14V WHITE LEDS VF = 3.