LM2796

SNVS273 – MAY 2004

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CIRCUIT DESCRIPTION

OVERVIEW

The LM2796 is primarily intended for Lithium-Ion battery driven white-LED drive applications, and is well suited to

drive white LEDs that are used for backlighting small-format displays. The part has seven matched constant-

current outputs, each capable of driving up to 20mA (or more) through white LEDs. The well-matched current

sources ensure the current through all the LEDs is virtually identical. This keeps brightness of all LEDs matched

to near perfection so that they can provide a consistent backlight over the entire display.

The core of the LM2796 is a 1.5x/1x dual-mode charge pump. The input of the charge pump is connected to the

voltage and load current.

The charge pump operates in the 1.5x mode when the input voltage is below 4.75V (typ.). In this mode, the

input-to-output voltage gain of the charge pump is 1.5, and the voltage at the output of the charge pump will be

the voltage boost that is required to drive white LEDs from a Li-Ion battery. (White LEDs typically have a forward

voltage in the range of 3.3V to 4.0V. A Li-Ion battery typically is not considered to be fully discharged until the

battery voltage falls to 3.0V (approx.) )

The charge pump operates in the 1x mode when the input voltage is above 4.75V (typ.). In these conditions,

high.

The matched current outputs are generated with a precision current mirror that is biased off the charge pump

output. Matched currents are ensured with the use of tightly matched internal devices and internal mismatch

cancellation circuitry. Top-side current drive allows LEDs to be connected between each current output and

GND, simplifying PWB routing and connectivity.

There are seven regulated current outputs. These seven outputs are split into two groups, a group of 4 outputs

and a group of 3 outputs. There is an ON/OFF control pin for each group.

The DC current through the LEDs is programmed with an external resistor. Changing currents on-the-fly can be

achieved with the use of digital pulse (PWM) signals.

ENABLE PINS: EN, ENA, ENB

The LM2796 has 3 enable pins. All three are active-high logic (HIGH = ON). There are internal pull-down

resistors (300k

Ω typ.) that are connected internally between each of the enable pins and GND.

The EN pin is the master enable pin for the part. When voltage on this pin is low (<0.5V), the part is in shutdown

mode. All internal circuitry is OFF and the part consumes very little supply current when the LM2796 is

shutdown. When the voltage on the EN pin is high (>1.1V), the part is active. The charge pump is ON, and it is

possible to turn on the output currents to drive the LEDs.

ENA and ENB are used to turn the output currents ON and OFF. ENA activates/deactivates the four group-A

outputs (D1A-D4A). ENB activates/deactivates the three group-B outputs (D1B-D3B).

SETTING LED CURRENTS

The output currents of the LM2796 can be set to a desired value simply by connecting an appropriately sized

simplified in the equations below:

(1)

(2)

6

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