LT3909 Datasheet, PDF (1/8 Pages) Linear Technology – 2-String × 50mA, 2MHz Step-Up LED Driver

DEMO MANUAL DC2176A
LT3909
2-String × 50mA, 2MHz Step-Up LED
Driver with ±1.8% Current Matching
Description
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
with a fixed 2MHz switching frequency and current sources
for the LED strings.
The operating voltage range for DC2176A is from 7V to
36V. The maximum current in each LED string is 40mA
and is programmed by 24.9kΩ I_SET resistor R1. The I_SET
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
INTV_CC voltage of the LT3909 so the LED current is at the
40mA maximum when there is no external CTRL voltage
applied. There are placeholders to add voltage dividers
either from V_IN or INTV_CC to the CTRL pin. A voltage
divider from V_IN to the CTRL pin can be used to reduce
LED current at low input voltages. A resistor divider from
INTV_CC to CTRL allows an NTC to reduce LED current at
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
is pulled up by 100kΩ R6 to V_IN, but there is a placeholder
to pull up FAULT to INTV_CC instead.
Resistor divider R4 and R5 from the input to the EN/UVLO
pin programs the circuit to turn off when V_IN decreases
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
a placeholder for a resistor to pull up PWM to INTV_CC so
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
V_IN pin to allow operation from two input sources. Ceramic
capacitors are used at the input, output and for INTV_CC. A
50V tantalum polymer capacitor protects the input from
ringing that may occur when the input is hot-plugged using
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
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Technology Corporation. All other trademarks are the property of their respective owners.
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