PRODUCT SPECIFICATION

FAN5609

REV. 1.0.2 5/30/03

7

B Input (PWM)

A Input (PWM)

ss

1KHz

ss

30%

Duty Cycle

1KHz

70%

Duty Cycle

70%

Duty Cycle

30%

Duty Cycle

ss

0mA

OFF

ILED

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.

put ripple voltage. However this will increase the power-on

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

IC.

100nH

0.1

µF

4.7

µF

Input

Power

Supply

GND

FAN5609