8 REV. 1.0.1 12/2/04
FAN5607 PRODUCT SPECIFICATION
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
R
SET
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
EN
On Off
And
Current Range
Figure 2. Block Diagram
Circuit Description
The FAN5607’s switched capacitor DC/DC converter auto-
matically configures its internal switches to achieve high
efficiency 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 FAN5607 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 0.4ms. 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.
