Circuit Description

The LM2792 provides two matched current sources for driv-

ing high forward voltage drop LEDs from Li-Ion battery

sources. The part has on-chip current regulators which are

composed of current mirrors with a 25 to 1 ratio. The mirrors

control the LED current without using current limiting resis-

tors in the LED current path. The device can drive up to

34mA through the Leds.

The LED brightness can be controlled by both analog and/or

digital methods. The digital technique uses a PWM (Pulse

Width Modulation) signal applied to the shutdown input. The

analog technique applies an analog voltage to the brightness

(BRGT) pin (see Application Information section).

Application Information

SOFT START

LM2792 includes a soft start function to reduce the inrush

currents and high peak current during power up of the de-

vice. Soft start is implemented internally by ramping the

bandgap more slowly than the applied voltage. This is done

by holding the bandgap in shutdown for a short time. During

soft start, the switch resistances limit the inrush current used

to charge the flying and hold capacitors.

SHUTDOWN MODE

A shutdown pin (SD or SD) is available to disable the

LM2792 and reduce the quiescent current to 1µA maximum.

The LM2792 is available with both senses of shutdown

polarity.

During normal operation mode of the ’-L’ options, an active

high logic signal to the SD pin or tying the SD pin to V

enable the device. Pulling SD low or connecting SD to

ground will disable the device.

During normal operation mode of the ’-H’ options, an active

low logic signal to the SD pin or tying the SD pin to GND, will

enable the device. Pulling SD high or connecting SD to V

IN

will disable the device.

CAPACITOR SELECTION

Low equivalent series resistance (ESR) capacitors such as

X5R or X7R are recommended to be used for C

C

than or equal to 0.3 ohms ESR value are recommended for

this application. Table 1 below lists suggested capacitor

suppliers for the typical application circuit.

TABLE 1. Low ESR Capacitor Manufactures

Manufacturer

Contact

website

TDK

(847) 803 6100

www.component.tdk.com

MuRata

(800) 831 9172

www.murata.com

Taiyo Yuden

(800) 348 2496

www.t-yuden.com

SCHOTTKY DIODE SELECTION

A schottky diode (SD1) must be used between V

for proper operation. During start-up, the low voltage drop

across this diode is used to charge C

oscillator. It is necessary to protect the device from

turning-on its own parasitic diode and potentially latching-up.

As a result, it is important to select a schottky diode that will

carry at least 200mA or higher current to charge the output

capacitor during start-up. A schottky diode like 1N5817 can

20024204

FIGURE 1.

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