SP7682 EXAR [Exar Corporation], SP7682 Datasheet - Page 6

• Manufacturer Part Number: SP7682
• Description: Backlight and Flash Driver in QFN 3mm x 3mm
• Manufacturer: EXAR [Exar Corporation]

Sept 27-07 rev G
The SP7682 and SP7682A are charge pump based
backlight and flash LED drivers with I
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
SP7682: Backlight and Flash Driver
2
C and serial single
Page 6
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
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.
© 2007 Exar Corporation
THEORY OF OPERATION
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C timeout is activated and
2
C