PRODUCT SPECIFICATION FAN5609
REV. 1.0.2 5/30/03
7
Digital Control with PWM
(Continued)
A and B are PWM.
I
LED
(Average) =
δ
x 20mA, where
δ
is Duty Cycle. (Note 3, 5)
B Input (PWM)
A Input (PWM)
s
s
1KHz
s
s
30%
Duty Cycle
1KHz
70%
Duty Cycle
70%
Duty Cycle
30%
Duty Cycle
s
s
0mA
OFF
I
LED
I
LED
(Average) = 0.3 x 20mA = 6mA
I
LED
(Average) = 0.7 x 20mA = 14mA
Notes:
5. Proportionally select the duty cycle to achieve a typical LED current between 1mA to 19mA.
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
IN
and C
OUT
should be low
ESR capacitor. Increasing the C
OUT
capacitor reduces the out-
put ripple voltage. However this will increase the power-on
time. The C
IN
value controls input ripple. If necessary, this
ripple can be further reduced by powering the FAN5609
through a very small series inductor filter, as shown in Figure 3.
Figure 3.
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
IN
and
C
OUT
should be connected together close to the GND pin of
IC.
100nH
0.1µF
4.7µF
Input
Power
Supply
V
IN
GND
FAN5609
