Copyright © 2016, Texas Instruments Incorporated

Gain Control

1x, 1.5x

Charge Pump

Current

Control

Flash, Torch

Mode

Control

VREF

OSC

Ind.

Torch

Flash

EN1

EN2

LED

COUT = 2.2 µF

CIN = 2.2 µF

VIN

1 µF

C1+

C1-

C2+

C2-

RSET

ISET

GND

1 µF

LED

-

CPOUT

7

LM2758

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SNVS551E – APRIL 2008 – REVISED MAY 2016

Product Folder Links: LM2758

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Copyright © 2008–2016, Texas Instruments Incorporated

7 Detailed Description

7.1 Overview

The LM2758 is an adaptive 1× and 1.5× CMOS charge pump, optimized for driving flash LEDs in camera phones

and other portable applications. It provides a constant current of 500 mA (typical) for flash mode and 107 mA

(typical) for torch mode with RSET = 20 kΩ. These current can change (see Setting LED Currents).

There are four modes of operation for LM2758: the flash mode, torch mode, indicator mode, and shutdown mode

(see Table 1). Torch and flash modes sink a constant DC current while indicator mode operates in pulsating DC

at 1/32 positive duty cycle with same current magnitude as torch mode. The LED is driven from CPOUT and

connected to the current sink. LED drive current mode is programmed by connecting a resistor, RSET, to the

current set pin, ISET. The LM2758 device also controls CPOUT with variable gain (1× or 1.5×) and adjustable

impedance (ROUT) to provide an output voltage that would account for LED forward voltage drop and headroom

for the current sink to drive desired current through LED.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Charge Pump and Gain Transitions

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

pump is connected to the CPOUT pin. In 1× mode, as long as the input voltage is less than 4.7 V, the output

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

regulated to 4.7 V. In 1.5× mode, the output voltage is always less than or equal to 4.7 V over entire input

voltage range.

The gain of the charge pump is selected depending on the headroom voltage across the current sink of LM2758.

When headroom voltage VLED– (at the LED pin) drops below 300 mV (typical) the charge-pump gain transition

happens from 1× to 1.5× to maintain current regulation across the LED. Once the charge pump transition to a

higher gain, it remains 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.