© 2003 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5611/FAN5612/FAN5613/FAN5614 Rev. 1.0.9

8

Application Information

Operational Descripton

The regulated current through each LED is a multiplica-

to the formula:

ulation is 36k

Ω.

The LED intensity can be adjusted by varying the duty

cycle of a square wave applied to the enable pin. Fre-

quency greater than 100Hz is best to avoid a "flickering"

effect. The maximum operation frequency is 10MHz.

Efficiency Considerations

The FAN561X driver’s low-dropout architecture can sig-

nificantly improve the efficiency compared to using sim-

ple ballast resistors. The system efficiency, defined as

the ratio between the LEDs’ power and the input sup-

plied power, can be calculated as:

ciency. Efficiency can be further improved by using a

Application Notes

The ultra-low voltage drop across the FAN561X series of

LED drivers allows the devices to drive white, blue, and

other color LEDs in a wide range of input voltages. The

driver can be used in many applications. Although only

the FAN5613 is shown in all three examples, any of the

FAN561X-series LED drivers can be used in the applica-

tions presented, due to their similar operation.

from single cell Li-ion

the full 20mA LED current. At 3.1V, there is 5mA typical

current available for the LEDs. The single cell Li-ion is

utilized in applications like cell phones or digital still cam-

eras. In most cases, the Li-ion battery voltage level only

goes down to 3.0V voltage level, not down to the full dis-

charge level (2.7V) before requesting the charger.

Key advantages:

• No boost circuit is needed for the LCD or keyboard

backlight.

• Drivers are directly connected to a Li-ion battery.

• No EMI, no switching noise, no boost efficiency lost,

no capacitor, and no inductor.

existing bus from 4.0V to 5.5V

the range of 3.2V to 4.0V. Driving these LEDs with the

maximum current of 20mA for maximum brightness,

usually requires a boost circuit for a single cell Li-ion

voltage range. In some cases, there is already a voltage

bus in the system, which can be utilized. Due to the ultra-

low voltage drop of the FAN561X series of LED drivers to

Key advantages:

• No boost circuit is needed for LCD or keyboard

backlight.

• Driver utilizes the existing bus.

• Ultra-low voltage drop provides the full 20mA LED

current at the lowest possible voltage level.

Efficiency

V

IN

V

CATHODE

–

() V

IN

⁄

=

I1

I2

I3

I4

GND

FAN5613

CTRL

V

CONTROL

ENABLE

ON/OFF