© 2009 Fairchild Semiconductor Corporation

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FAN5701 • Rev. 1.0.3

9

Circuit Description

The FAN5701 is a white LED driver system based upon an

adaptive 1.5x/1x charge pump capable of supplying up to

180mA of total output current. The tightly matched current

sinks ensure uniform brightness across the LEDs. Each

LED is configured in a common anode configuration with

its peak drive current set during manufacturing (see

Charge Pump

The charge pump operates in either 1x mode, where

VOUT is connected to VIN through a bypass switch, or in

1.5x mode. The circuit operates in 1x mode until the LED

maintain current regulation. At that point, 1.5x mode

begins. If the lowest active cathode voltage is greater than

1.8V, the charge pump switches back to 1x mode.

In addition to hysteresis, a 1ms transition delay is provided

for the device to ignore short-duration input voltage drops

in deciding mode transitions.

IC Enable

When the EN pin is LOW, all circuit functions are disabled.

When the EN pin HIGH, the entire chip is enabled. Both

PWM inputs are now functional.

PWM Dimming

External PWM inputs (group A and group B) directly modulate

output currents in their corresponding LED channels to vary the

perceived LED brightness. Two PWM inputs are provided to

control two independent groups of LEDs, such as that of a

main display panel and a secondary panel. They can also be

connected to a single input to simultaneously dim all six LED

outputs.

The FAN5701 has integrated protection circuitry to prevent the

device from being short circuited when the output voltage falls

below 2V. If this occurs FAN5701 turns off the charge pump

and the LED driver outputs, but a small bypass switch is left

on. The device monitors the output voltage to determine if it is

still in short circuit condition and, once it has passed, FAN5701

soft-starts and returns to normal operation.

If the output voltage goes above 6V, the FAN5701 shuts down

until this condition has passed. The charge pump and LED

driver outputs are turned off. Once this condition has passed,

FAN5701 soft-starts into normal operation.