FAN5609 Fairchild Semiconductor, FAN5609 Datasheet - Page 7

• Manufacturer Part Number: FAN5609
• Description: LED Driver with Adaptive Charge Pump DC/DC Converter
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Digital Control with PWM
A and B are PWM.
Application Information
Selecting Capacitors
It is important to select the appropriate capacitor types and
the values for use with the FAN5609. These capacitors
determine parameters such as power efficiency, maximum
sustainable load current by the charge pump, input and
output ripple and start-up time.
In order to reduce ripple, both C
ESR capacitor. Increasing the C
put ripple voltage. However this will increase the power-on
time. The C
ripple can be further reduced by powering the FAN5609
through a very small series inductor filter, as shown in Figure 3.
REV. 1.0.2 5/30/03
Notes:
5. Proportionally select the duty cycle to achieve a typical LED current between 1mA to 19mA.
A Input (PWM)
B Input (PWM)
Power
Supply
Input
I LED
IN
value controls input ripple. If necessary, this
0mA
0.1µF
I
LED
Figure 3.
100nH
(Average) =
OUT
IN
4.7µF
and C
Duty Cycle
Duty Cycle
capacitor reduces the out-
1KHz
30%
30%
OUT
(Continued)
δ
should be low
I
LED
x 20mA, where
V
GND
FAN5609
IN
(Average) = 0.3 x 20mA = 6mA
δ
is Duty Cycle. (Note 3, 5)
CAP1 and CAP2 control the current capability of the charge
pump and affect the overall efficiency of the system. A lower
value will improve efficiency, but it may limit the LED’s
currents at low input voltage. A capacitor of 100nF is opti-
mal for 4 × 20 mA load over the entire input voltage range of
2.7V to 4.2V. To save space and cost, and to increase effi-
ciency, this value may be reduced to 10nF for loads less than
4 x 7mA.
Pulse-Width-Modulated (PWM) Mode
Conversion errors are minimized and the best LED to LED
matching is achieved over the entire range of average current
settings, when PWM brightness control is used to modulate
the LED current between zero and the maximum value
(A=1, B=1).
PC Board Layout
For best performance, a solid ground plane is recommended
on the back side of the PCB. The ground tails of C
C
IC.
OUT
should be connected together close to the GND pin of
I
Duty Cycle
Duty Cycle
LED
1KHz
70%
70%
(Average) = 0.7 x 20mA = 14mA
OFF
IN
FAN5609
and
7