FAN5608

PRODUCT SPECIFICATION

REV. 1.0.3 6/28/04

5

Block Diagram

COIL DRIVER

DBB

START-UP

POWER GOOD

LINEAR REGULATOR

LINEAR REGULATOR

OSC

W_OR

REF

REF CH1

REF CH2

DAC1

DAC2

BG

V

OUT

V

IN

GND

B1

A1

CH1

IND

CH2

B2

A2

IND

Note: In the 8-pin version (analog version only), pins B1 and B2 are omitted.

Circuit Description

turned on, the bandgap reference acquires its nominal volt-

age and the soft-start cycle begins. Once “power good” is

achieved (0.5mA in the diodes), the soft-start cycle stops and

the boost voltage increases to generate the desired current

selected by the input control pins. If the second channel is

diodes are turned off.

The FAN5608 DC/DC converter automatically adjusts its

internal duty cycle to achieve high efficiency. It provides

tightly regulated output currents for the LEDs. An internal

circuit determines which LED string requires the highest

voltage in order to sustain the pre-set current levels, and

adjusts the boost regulator accordingly.

To maintain the regulated current at the selected value, the

difference in the number of LEDs between branches should

not exceed one. If only one branch is used, another branch

should be disabled, connecting the corresponding DAC

inputs to low. If the output external capacitor is shorted, the

Schottky diode can be damaged, therefore such a condition

should be avoided.

LED Brightness Control

The control inputs are A1, B1 for CH1 and A2, B2 for CH2.

B1 and B2 are digital inputs, thus they require LOW (GND)

are connected to an external stable voltage source via an

external resistor, and B1 and B2 inputs are connected to

ground. The current flowing through the resistor is scaled by

a factor of approximately 1000.