FAN5609

PRODUCT SPECIFICATION

4

REV. 1.0.2 5/30/03

Block Diagram

Linear

Regulator

Oscillator

Analog

Detector

Power

Good

Bandgap

Reference

Reference

D

R

I

V

E

R

S

P

U

M

P

Voltage

Selector

1

µF

1

µF

GND

Ref1

Ref3

Ref4 (BG)

Regulator

Low Battery Ref.

Range Selection

Mode Change

Ref2

5

µF

On Off

And

Current Range

DAC

A

B

Circuit Description

The FAN5609’s switched capacitor DC/DC converter auto-

matically configures its internal switches to achieve high effi-

ciency and to provide tightly regulated output currents for

the LEDs. An analog detector determines which diode

requires the highest voltage in order to sustain the pre-set

current levels, and adjusts the pump regulator accordingly.

Every diode has its own linear current regulator. In addition,

a voltage regulator controls the output voltage when the bat-

tery voltage is within a range where linear regulation can

provide maximum possible efficiency. If the battery voltage

is too low to sustain the diode current in the linear mode,

a fractional 3:2 charge pump is enabled. When the battery

voltage drops further and this mode is no longer sufficient to

sustain proper operation, the pump is automatically reconfig-

ured to operate in 2:1 mode. As the battery discharges and

the voltage decays, the FAN5609 switches between modes to

maintain a constant current through LED throughout the bat-

tery life. The transition has hysteresis to prevent toggling.

Supply Voltage

The internal supply voltage for the device is automatically

Soft Start

The soft-start circuit limits inrush current when the device is

initially powered up and enabled. The reference voltage con-

trols the rate of the output voltage ramp-up to its final value.

Typical start-up time is 1ms. The rate of the output voltage

ramp-up is controlled by an internally generated slow ramp,

and an internal variable resistor limits the input current.