Application Information

CIRCUIT DESCRIPTION

The LM2758 is an adaptive 1x and 1.5x CMOS charge pump,

optimized for driving Flash LEDs in camera phone and other

portable applications. It provides a constant current of 500 mA

(typ.) for Flash mode and 107 mA (typ.) for Torch mode with

R

CURRENTS.

There are four modes of operation for LM2758: the Flash

Mode, Torch Mode, Indicator Mode and Shutdown Mode (see

EN1 and EN2 truth table). Torch and Flash modes sink a

constant DC current while Indicator mode operates in pulsat-

ing DC at 1/32 positive duty cycle with same current magni-

tude as Torch mode. The LED is driven from CP

connected to the current sink. LED drive current mode is pro-

grammed by connecting a resistor, R

pin, I

or 1.5x) and adjustable impedance (R

put voltage that would account for LED forward voltage drop

and headroom for the current sink to drive desired current

through LED.

CHARGE PUMP AND GAIN TRANSITIONS

The input to the 1x/1.5x charge pump is connected to the

V

is connected to the CP

input voltage is less than 4.7V, the output voltage is approxi-

mately equal to the input voltage. When input voltage is over

4.7V the output voltage gets regulated to 4.7V. In 1.5x mode,

the output voltage is always less than or equal to 4.7V over

entire input voltage range.

The charge pump’s gain is selected depending on the head-

room voltage across the current sink of LM2758. When head-

room voltage V

the charge pump gain transition happens from 1x to 1.5x to

maintain current regulation across the LED. Once the charge

pump transition 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 will resets the gain mode to the

minimum gain required to maintain the load.

SOFT START

The LM2758 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 ref-

erence.

CURRENT LIMIT PROTECTION

The LM2758 charge pump contains current limit protection

circuitry that protects the device during V

where excessive current is drawn. Output current is limited to

1.2A typically.

LOGIC CONTROL PINS

LM2758 has two logic pins, EN1 and EN2. The operating

modes of the part function according to the tables below:

EN1

EN2

Mode

0

0

Shutdown

1

0

Indicator

0

1

Torch

1

1

Flash

FLASH TIME-OUT FEATURE

Flash Time-out Protection Circuitry disables the current sinks

when the signal on EN1 and EN2 is held high for more than

814 msec (typ.). This prevents the device from self-heating

due to the high power dissipation during Flash conditions.

During the time-out condition, voltage will still be present on

CP

current through the Flash LED. When the device goes into a

time-out condition, placing a logic Low signal on EN1 and EN2

will reset the time-out; a subsequent logic High signal on EN1

or EN2 will return the device to normal operation.

SETTING LED CURRENTS

The current through the LED can be set by connecting an

appropriately sized resistor R

LM2758 and GND.

The LED current in Torch mode is approximately 1639 times

greater than the current of I

mode is approximately 7650 times of the same I

The feedback loop of an internal amplifier sets the voltage of

the I

in the equations below:

I

The maximum recommended current through LED is 500 mA

in Torch mode / 700 mA in Flash mode. Note: If the I

Torch Mode setting at 500 mA, the Flash mode would be over

700 mA (max). See the graph LED Current vs R

part in conditions where the junction temperature might rise

above the rated maximum requires that the operating ranges

and/or conditions be de-rated. The printed circuit board also

must be carefully laid out to account for high thermal dissipa-

tion in the part.

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