June 21-07 revH

SP7680: Complete Backlight Solution



2007 Sipex Corporation

Page 7

THEORY OF OPERATION

The SP7680 is a charge pump regulator

designed for converting an input voltage of 2.7V

to 5.5V to drive backlight white LEDs used in

portable applications. It has a total of 7 LED

driver outputs consisting of 4 Main, 2 Sub and

one Auxiliary keypad output. The Main and Sub

outputs can drive up to 31.5mA for each LED

while the Aux output can drive up to 63mA. With

the I

on/off independently so any combination of 7

outputs is available. Three 6-bit DACs are

included to provide precise current level setting.

The drive current is set simultaneously for one

group. The brightness integrity and good current

matching among channels are assured by this

control method.

The SP7680 is a fractional charge pump and

can multiply the input voltage by 1 or 1.5 times.

The charge pump switches at a high fixed

frequency of 2MHz which allows for reduced

external component sizes. The internal mode

selection circuit automatically switches the mode

between 1x and 1.5x mode based on the input

voltage, output voltage and load current. This

mode

switching

maximizes

the

efficiency

throughout the entire load range. When the

battery voltage is high enough, the SP7680

operates in 1x mode to provide maximum

efficiency. Dropout detection is provided on all

four MAIN LED outputs and the two SUB LED

outputs, to determine when the SP7680 needs

to transition to 1.5X mode. If the battery voltage

is too low to sustain the LED current, the 1.5x

mode is automatically enabled. As the battery

discharges and the voltage decays, the SP7680

automatically

switches

between

modes

to

maintain a constant current to drive LEDs

throughout the battery life.

Overtemperature Protection

When the temperature of the SP7680 Rises

above 170°C, the overtemperature protection

circuitry turns off the output switches to prevent

damage to the device. If the temperature drops

back

down

below

145°C,

the

SP7680

automatically recovers and executes a soft start

cycle.

Overvoltage Protection

The SP7680 has overvoltage protection. In

normal operation if the current-sinks are all

open-circuited, the output voltage will rise only to

the regulation voltage of 4.2V. When the current-

sinks are no longer open-circuited, the device

resumes normal operation.

Shorted LED Protection

Shorted LED protection is provided.

If the

SP7680

detects

a

shorted

LED

the

corresponding LED output will turn off (no

current).

I²C interface

The I²C interface allows a simple way to adjust

each bank of channel currents in order to

provide ultimate flexibility in driving LEDs. The

SP7680 has five data registers which can be

programmed serially via the I²C interface.

The STATUS register is used to enable/disable

the part as well as for fault mode readback. The

CNTRL register contains information regarding

the state of each of the 7 individual LEDs. The

final

three

registers

contain

information

regarding the current level for the MAIN, SUB

and AUX channels.