FAN5606 Datasheet, PDF (4/9 Pages) Fairchild Semiconductor – Serial LED Driver with Current-Regulated, Step-Up DC/DC Converter

FAN5606
Block Diagram
V
IN
PRODUCT SPECIFICATION
V
OUT
OSC
DBB
COIL DRIVER
IND
B
DAC
A
START-UP
AMPLIFIER
LINEAR REGULA TOR
CAT
BG
REF
POWER GOOD
GND
Circuit Description
When the input voltage is connected to V_IN pin, the system is
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.
The FAN5606 DC/DC converter automatically adjusts its
internal duty cycle to achieve high efficiency. If the output
external capacitor is shorted, the Schottky diode can be dam-
aged, therefore such a condition should be avoided.
LED Brightness Control
The inputs A and B can be digitally controlled LOW (GND)
and HIGH (V_CC) signals. In analog mode, A input is con-
nected to an external stable voltage source via an external
resistor and B input is connected to ground. The current
flowing through the resistor is scaled by a factor of approxi-
mately 1000.
Digital Control
The FAN5606’s digital decoder allows selection of the fol-
lowing modes of operation: OFF, 5mA, 10mA, 20mA per
branch.
A
0
1
0
1
B
0
0
1
1
I_LED OFF 5mA 10mA 20mA
Analog Control with PWM
In analog control mode, the LED current can go up to 25mA.
Input A is used to control the LED currents. Input B should
be connected to GND (logic level "0"). An external resistor
(R) is connected from A to a stable voltage source (V_EXTER-
NAL^{) to control the LED current, I}LED^{. The I}LED ^{is given by}
the formula or the graph below:
I_LED
=


-V----^E---^x---^t--^e--^r--ⁿ--R-^a---^l--–-----V-----^R---^e---^f_
× Multiplication Ratio
4
REV. 1.0.3 1/7/05