LM2794TLX datasheet(9/16 Pages) NSC

9 / 16 page

Functional Block Diagram

20028530

Application Information

CIRCUIT DESCRIPTION

The LM2794/5 is a 1.5x/1x CMOS charge pump with four

matched constant current outputs, each capable of driving

up to 20mA through White LEDs. This device operates over

the extended Li-Ion battery range from 2.7V to 5.5V. The

LM2794/5 has four regulated current sources connected to

the device’s 1.5x charge pump output (P

OUT). At input volt-

ages below 4.7V (typ.), the charge-pump provides the

needed voltage to drive high forward voltage drop White

LEDs. It does this by stepping up the P

OUT voltage 1.5 times

the input voltage. The charge pump operates in Pass Mode,

providing a voltage on P

OUT equal to the input voltage, when

the input voltage is at or above 4.7V (typ.). The device can

drive up to 80mA through any combination of LEDs con-

nected to the constant current outputs D

1-D4.

To set the LED drive current, the device uses a resistor

connected to the I

SET pin to set a reference current. This

reference current is then multiplied and mirrored to each

constant current output. The LED brightness can then be

controlled by analog and/or digital methods. Applying an

analog voltage in the range of 0V to 3.0V to the Brightness

pin (BRGT) adjusts the dimming profile of the LEDs. The

digital technique uses a PWM (Pulse Width Modulation)

signal applied to the Shutdown pin (SD). (see I

SET and

BRGT PINS section).

SOFT START

Soft start is implemented internally by ramping the reference

voltage more slowly than the applied voltage. During soft

start, the current through the LED outputs will ramp up in

proportion to the rate that the reference voltage is being

ramped up.

LM2794TLX Datasheet(PDF) 9 Page - National Semiconductor (TI)