LM2759SDEV datasheet(9/22 Pages) TI1

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LM2759

www.ti.com

SNVS577D – JUNE 2008 – REVISED MAY 2013

APPLICATION INFORMATION

CIRCUIT DESCRIPTION

The LM2759 is an adaptive CMOS charge pump with gains of 1x, 1.5x, and 2x, optimized for driving Flash LEDs

in camera phones and other portable applications. It provides a constant current of up to 1A (typ.) for Flash mode

and 180 mA (typ.) for Torch mode.

The LM2759 has selectable modes including Flash, Torch, Indicator and Shutdown. Flash mode for the LM2759

can also be enabled via the Strobe input pin. The LED is driven from VOUT and connected to the current sink.

The LED drive current and operating modes are programmed via an I2C compatible interface. The LM2759

adaptively selects the next highest gain mode when needed to maintain the programmed LED current level.

To prevent a high battery load condition during a simultaneous RF PA transmission and Flash event, LM2759

has a Flash interrupt pin (TX) to reduce the LED current to the programmed Torch current level for the duration

of the RF PA transmission pulse.

CHARGE PUMP AND GAIN TRANSITIONS

The input to the 1x, 1.5x, 2x charge pump is connected to the VIN pin, and the loosely regulated output of the

charge pump is connected to the VOUT pin. In 1x mode, as long as the input voltage is less than 4.7V (typ.), the

output voltage is approximately equal to the input voltage. When the input voltage is over 4.7V (typ.) the output

voltage is regulated to 4.7V (typ.). In 1.5x mode, the output voltage is regulated to 4.7V (typ.) over entire input

voltage range. For the gain of 2x, the output voltage is regulated to 5.1V (typ.). When under load, the voltage at

VOUT can be less than the target regulation voltage while the charge pump is still in closed loop operation. This is

due to the load regulation topology of the LM2759.

The charge pump’s gain is selected according to the headroom voltage across the current sink of LM2759. When

the headroom voltage VGDX (at the LED cathode) drops below 350 mV (typ.) the charge pump gain transitions to

the next available higher gain mode. Once the charge pump transitions to a higher gain, it will remain at that gain

for as long as the device remains enabled. Shutting down and then re-enabling the device resets the gain mode

to the minimum gain required to maintain the load.

SOFT START

The LM2759 contains internal soft-start circuitry to limit inrush currents when the part is enabled. Soft start is

implemented internally with a controlled turn-on of the internal voltage reference.

CURRENT LIMIT PROTECTION

The LM2759 charge pump contains current limit protection circuitry that protects the device during VOUT fault

conditions where excessive current is drawn. Output current is limited to 1.4A typically.

LOGIC CONTROL PINS

LM2759 has two asynchronous logic pins, Strobe and TX. These logic inputs function according to the table

below:

STROBE

FUNCTION

Current I2C programmed state (Off, Torch,

Flash, Indicator)

Current I2C programmed state (Off, Torch,

Flash, Indicator). If Flash is enabled via I2C

and TX is logic High, the LED current will be

at the programmed Torch level.

Flash Mode (Total LED "ON" Duration limited

by Flash Timeout)

Torch Mode (Total LED "ON" Duration limited

by Flash Timeout)

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