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dc2176af

DEMO MANUAL DC2176A

Description

LT3909

2-String × 50mA, 2MHz Step-Up LED

Driver with ±1.8% Current Matching

Demonstration circuit 2176A features the LT®3909, a

2-string × 50mA, 2MHz step-up LED driver with ±1.8%

current matching.

The LT3909 precisely regulates two LED strings for equal

current, supports LED dimming that reduces brightness

down to a 1/40,000 ratio, protects against faults and also

reports faults. The LT3909 consists of a step-up converter

withafixed2MHzswitchingfrequencyandcurrentsources

for the LED strings.

The operating voltage range for DC2176A is from 7V to

36V. The maximum current in each LED string is 40mA

resistor can be changed to 20.0kΩ to program the string

current to 50mA – see the data sheet for an application

example with 50mA string current.

The LT3909 utilizes voltage feedback during PWM dim-

ming and when there are faults on both LED strings. Volt-

age feedback resistors R2 and R3 on DC2176A optimize

operation for LEDs that are 35V maximum. Efficiency is

86% at 12V input, 35V for the LED string forward voltages

and 40mA per string.

LED current can be reduced by an external voltage applied

to the CTRL terminal. Resistor R8 pulls up CTRL to the

40mA maximum when there is no external CTRL voltage

applied. There are placeholders to add voltage dividers

LED current at low input voltages. A resistor divider from

high temperatures. There are placeholders for an RC filter

between the CTRL terminal and the CTRL pin. The RC filter

limits the ramp time of the CTRL pin and prevent false

faults when CTRL ramps too quickly. The FAULT terminal

L

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

Technology Corporation. All other trademarks are the property of their respective owners.

Resistor divider R4 and R5 from the input to the EN/UVLO

below 5V with 0.9V of hysteresis. The circuit can also be

turned off by connecting the EN/UVLO terminal to ground.

An external voltage must be applied to the PWM terminal

in order for DC2176A to operate. The external voltage can

be a DC level or an appropriate PWM dimming signal. A

common frequency for PWM dimming is 100Hz. There is

an external voltage on PWM for operation is unnecessary.

There is a placeholder that allows an external diode to

be placed in parallel with the internal LT3909 rectifier for

higher current capability. DC2176A uses an inductor that

is optimized for high efficiency, and the input supply for

power to the inductor can be separated from the LT3909

50V tantalum polymer capacitor protects the input from

ringingthatmayoccurwhentheinputishot-pluggedusing

long leads. There are placeholders occupied by 0805 size

0Ω jumpers on the back of the PCB for two LED strings

and also placeholders for SOD323 diodes to clamp the

LED1 and LED2 voltages from ringing below ground.

2-string operation, close current matching, an extremely

wide dimming range and fault protection make the LT3909

ideal for functionally advanced back lighting and illumi-

nation circuits. DC2176A features the LT3909EDD in a

thermally enhanced 12-lead 3mm × 3mm DFN package.

The LT3909 data sheet must be read in conjunction with

this demo manual to properly use or modify DC2176A.

Design files for this circuit board are available at

http://www.linear.com/demo/DC2176A