SP7682 datasheet(6/19 Pages) EXAR

6 / 19 page

Sept 27-07 rev G

SP7682: Backlight and Flash Driver

Page 6

THEORY OF OPERATION

The SP7682 and SP7682A are charge pump based

backlight and flash LED drivers with I

2C and serial single

wire interface (SWI) control respectively. The SP7682

family provides very accurate current drive capability for

up-to four backlight LED channels and a flash LED. The

backlight LED current can be set through a serial

interface controlled 6-bit current DAC with 0.5mA steps,

up to 31.5mA. Similarly, FLASH LED current is set thru a

serial interface controlled 6-bit current DAC with 10mA

steps, up to 500mA.

Backlight and Flash current loops make sure that the

corresponding output currents at each individual LED

output are regulated to their DAC set current values. The

inherent decision and control logic decides the operation

mode of the "gear-box" charge pump to maximize overall

efficiency. In order to decide on the most efficient

operation mode for the charge pump, the decision and

control logic observes the battery voltage, output

voltage, load current, dropout voltage, over-voltage,

over-current conditions and in-regulation feedback

signals and forces the charge pump to operate in one of

the 1x, 1.5x and 2x modes and dynamically switches

between modes to maximize overall efficiency.

The charge pump used in the heart of the design is a

regulated charge pump and regulates the minimum of

the LEDx outputs and FLASH output to be 350mV

typical. Regulated output voltage depends on the

forward voltage drop of the external LEDs used. The

charge pump switches at a high frequency of 2.4MHz

which allows tiny 1uF external capacitors to be used as

flying capacitors.

When any of the LED outputs or the FLASH output

voltage drops below the "dropout" voltage (200mV

typical), the charge pump decides to switch up to a less

efficient mode, i.e. in 1x mode it switches to 1.5x mode,

in 1.5x mode it switches to 2x mode. This switching to a

less efficient mode guarantees the part to sustain LED

currents in regulation. For the decision to switch back to

a more efficient mode, the mode decision logic uses

battery voltage, output voltage and load current

information and relies on preset margins on the mode

switching comparator thresholds.

The FLASH channel can be used in torch mode as well

as in Flash time-out mode. When used for a momentary

Flash output, it is recommended to activate the I

control for timeout. Once the I

2C timeout is activated and

for as long as the Flash is active, the SP7682 charge

pump will operate in 2x mode in order to provide the

continuous voltage and high current needed for Flash.

For the SP7682A, which uses the single wire interface,

the Flash channel will be in 2x mode whenever the Flash

current is at least 230mA.

The SP7682 and SP7682A are furnished with under-

voltage lockout, current limit, thermal shutdown and over

voltage protection features.

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