500mA Buck/Boost

Charge Pump LED Driver

8

SGM3140B

SG Micro Corp

www.sg-micro.com

OPERATION

The SGM3140B is a charge pump regulator designed for

converting a Li-ion battery voltage of 2.7V to 4.2V to drive

a white LED used in digital still camera flash and torch

applications. The SGM3140B has two modes of

operation which are pin selectable for either Flash or

Torch. Both modes can be used continuously unless

thermal shutdown happens. Torch is often used for

several seconds in a digital still camera “movie” mode.

The SGM3140B also has two modes of operation to

control the output current: the 1× mode and 2× mode.

Operation begins after the enable pin EN receives a logic

high, the band-gap reference wakes up after 50μs, and

then SGM3140B goes through a soft-start mode

designed to reduce inrush current. The SGM3140B starts

in the 1× mode, which acts like a linear regulator to

control the output current by continuously monitoring the

feedback pin FB. In 1× mode, if the SGM3140B auto

detects a dropout condition, which is when the FB pin is

below the regulation point for more than 15μs, the

SGM3140B automatically switches to the 2× mode. The

SGM3140B remains in the 2× mode until one of four

things happens:

1) The enable pin EN has been toggled.

2) The FLASH pin has changed from high to low.

4) A thermal fault occurs.

The 2× mode is the charge pump mode where the output

can be pumped as high as two times the input voltage,

provided the output does not exceed the maximum

voltage for the SGM3140B, which is internally limited to

about 5.5V. In the 2× mode, as in the 1× mode, the

output current is regulated by the voltage at the FB pin.

In the Torch mode (FLASH = “GND”), the FLASH pin is

set to logic low and the SGM3140B FB pin is regulated to

47mV output:

When in Flash mode (FLASH = “High”), the FB regulation

RSET pin and SGND pin with the equation:

Where 1.26V is the internal band-gap reference voltage

and 10.2kΩ is an internal resistance used to scale the

The output current is then set in either Flash or Torch

mode by the equation: