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
1µF
1µF
V
IN
V
OUT
GND
Ref1
Ref3
Ref4 (BG)
Regulator
Low Battery Ref.
Range Selection
Mode Change
Ref2
5µF
I. LIM.
I. LIM.
I. LIM.
I. LIM.
V
IN
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
selected from V
IN
or V
OUT
pins, whichever is higher.
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.
