© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5608 Rev. 1.0.5 • 8/8/06

5

Electrical Specifications

Recommended operating conditions and component values are per the Figure 18 test circuit. Typical values are at

Parameter

Conditions

Min.

Typ.

Max.

Units

Supply current in OFF mode

A input LOW, B input LOW

0.1

1

µA

LED current accuracy

A input HIGH, B input HIGH

18

20

22

mA

Current matching

(3)

A input HIGH, B input HIGH

3

%

Switching frequency

500

KHz

Analog Control Mode

1.2

V

Current multiplication ratio

850

1000

1150

Power efficiency (AVG)

(4)

80

%

HIGH

Digital Control Mode

LOW

0

0.6

Input A1, A2 threshold

Analog Control Mode

1.2

V

HIGH

Input B1, B2 threshold

Digital Control Mode

LOW

0

V

50

80

Input A1, A2 current

Digital Control Mode

0.1

µA

Input B1, B2 current

Digital Control Mode

0.1

µA

Notes:

3.

Current matching is the absolute value of the difference in current of the two LEDs channels, divided by the

average current in the two channels.

4.

Power efficiency is the ratio between the electrical power to the LEDs and the total power consumed from the

input power supply. Although this definition leads to a lower value than the boost converter efficiency, it more

accurately reflects the system performance in an application.

Circuit Description

enabled, the bandgap reference acquires its nominal

voltage (1.2V), and the soft-start cycle begins. Once

“power good” is achieved (when 0.5mA voltage flows

through the LEDs), the soft-start cycle stops, and the

boost voltage increases to generate the current selected

by the input control pins (A1, A2, B1, B2).

If CH1 is not selected, the CH1 output pin is HIGH, its

and its LEDs are OFF. The FAN5608 provides tightly

regulated output currents for the selected LED’s

channel. An internal feedback loop determines which

LED string requires the highest output voltage to sustain

the pre-set current. It also adjusts the boost regulator

based on CH1 and CH2 feedback voltage and A1, A2,

B1, B2 input settings. System efficiency decreases if the

channels’ strings of LEDs have different forward voltage.

If one channel is used, the other channel should be

disabled by connecting the corresponding DAC inputs to

shorted to GND, the internal Schottky diode can be

damaged, a condition which should be avoided.

LED Brightness Control

The control inputs are A1 and B1 for CH1 and A2 and

B2 for CH2. B1 and B2 are digital inputs that require

mode, A1 and A2 are connected to an external stable

voltage source via an external resistor, and B1 and B2

inputs are connected to ground. The current flowing

through the resistor is scaled by a factor of

approximately 1000.

LED dimming can be controlled with static or dynamic

inputs from CH1 and CH2. CH1 and CH2 outputs are

driven directly by A1, B1 and A2, B2 inputs, respectively.

Either channel can be configured as analog or digital.

Inputs A1 and A2 are analog inputs that can be

connected to an external regulated voltage source,

which can be calculated by:

EQ 1

where current multiplication ratio is according to the

Electrical Specifications table.

Inputs B1 and B2 are digital inputs and can only be set

by external logic of “0” (LOW) or “1” (HIGH).