© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5608 Rev. 1.0.5 • 8/8/06

9

decreases due to the resistive loss in the switching

power FET. Using L = 3.3μΗ and increasing the load to

maintain a constant 20mA current through LEDs. An

inductance L = 4.7μH ensures proper operation for 2 x 4

increased to improve the system efficiency. Application

examples are given in Figures 16 - 25.

The peak current in the inductor depends on the

maximum battery voltage and the inductance, according

to the equation:

L

V

S

I

IN

pk

max

4

.

1

×

=

μ

EQ 7

which gives the maximum rated current for the inductor.

current should be at least 1A.

Capacitors Selection

Low ESR capacitors should be used to minimize the

layer ceramic capacitor is recommended.

A larger value input capacitor placed as close as

possible to FAN5608 may be needed to reduce the input

voltage ripple in noise-sensitive applications. An

additional LC filter between the battery and the

FAN5608 input can help reduce the battery ripple to the

level required by a particular application.

Driving Higher Current LEDs

To increase the LED current range to 50mA, the CH1

and CH2 outputs may be connected, as shown below:

INDB

INDA

GND

CH1

DAC Inputs

L = 4.7µH

4.7µF

2.7V to 5.5V

CH2

Figure 14. Digital Brightness Control

INDB

INDA

GND

CH1

L = 4.7µH

4.7µF

2.7V to 5.5V

CH2

Figure 15. Analog Brightness Control

The current feeding the string of LEDs is the sum of the

currents programmed for each channel in digital or

analog mode. Using all four inputs in digital mode, the

LED current can be programmed within the 0 to 40mA

range, according to the following table:

Driving Two Independent Strings of LEDs

For best efficiency, the two strings of LEDs driven by the

FAN5608 should have close forward voltages. This

recommendation is met when the same number and

type of LEDs are serially connected in each channel. If

the application requires the two channels be significantly

unbalanced, an external resistor may be added in series

with the shorter string of LEDs to reduce the voltage

difference between the two channels and enhance

performance. The maximum differential LEDs connected

in series to CH1 and CH2 is not recommended to

exceed three LEDs.

Specific Layout Recommendations

The FAN5608 is available in a dual Die Attach Pad

(DAP) package. One DAP is connected to GND and

external connection to the DAPs. Since the internal

power dissipation is low, the 4x4mm MLP package is

capable of dissipating maximum power without requiring

an extra copper field on the printed circuit board. When

viewing the bottom of the dual-DAP device package, two

electrically isolated exposed metal islands are visible.

The two IND pins should be connected externally.

Depending on the application noise sensitivity, an

external filter may be required between the battery and

the IC input.