SC644 datasheet(12/26 Pages) SEMTECH

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SC644

General Description

This design is optimized for handheld applications sup-

plied from a single cell Li-Ion and includes the following

key features:

A high efficiency fractional charge pump that

supplies power to all LEDs

Six matched current sinks that control LED back-

lighting current, with 0mA to 25mA per LED.

Four adjustable LDOs. LDO , LDO3, and LDO4

are adjustable with 5 settings from .5V to 3.3V.

LDO2 is adjustable with 7 settings from .2V to

.8V.

High Current Fractional Charge Pump

The backlight outputs are supported by a high efficiency,

high current fractional charge pump output. The charge

pump multiplies the input voltage by , .5 or 2 times. The

charge pump switches at a fixed frequency of 250kHz in

.5x and 2x modes and is disabled in x mode to save

power and improve efficiency.

The mode selection circuit automatically selects the mode

as x, .5x, or 2x based on circuit conditions such as LED

voltage, input voltage, and load current. The x mode is

the most efficient of the three modes, followed by .5x

and 2x modes. Circuit conditions such as low input voltage,

high output current, or high LED voltage place a higher

demand on the charge pump output. A higher numerical

mode (.5x or 2x) may be needed momentarily to main-

tain regulation at the OUT pin during intervals of high

demand. The charge pump responds to momentary high

demands, setting the charge pump to the optimum mode

to deliver the output voltage and load current while opti-

mizing efficiency. Hysteresis is provided to prevent mode

toggling.

The charge pump requires two bucket capacitors for

proper operation. One capacitor must be connected

between the C+ and C- pins and the other must be con-

nected between the C2+ and C2- pins as shown in the

typical application circuit diagram. These capacitors

should be equal in value, with a nominal capacitance of

2.2µF to support the charge pump current requirements.

The device also requires a 4.7µF capacitor on the IN pin

and a 4.7µF capacitor on the OUT pin to minimize noise

•

and support the output drive requirements. Capacitors

with X7R or X5R ceramic dielectric are strongly recom-

mended for their low ESR and superior temperature and

voltage characteristics. Y5V capacitors should not be

used as their temperature coefficients make them unsuit-

able for this application.

LED Backlight Current Sinks

The backlight current is set via the SemPulse interface.

The current is regulated to one of 29 values between

0mA and 25mA. The step size varies depending upon the

current setting. Between 0mA and 5mA, the step size is

0.5mA. The step size increases to mA for settings

between 5mA and 2mA. Steps are 2mA between 2mA

and 25mA. The variation in step size allows finer adjust-

ment for dimming functions in the low current setting

range and coarse adjustment at higher current settings

where small current changes are not visibly noticeable in

LED brightness. A zero setting is also included to allow

the current sink to be disabled by writing to either the

enable bit or the current setting register for maximum

flexibility.

All backlight current sinks have matched currents, even

when there is variation in the forward voltages (∆V

F ) of

the LEDs. A minimum ∆V

F of .2V is supported when the

input voltage (V

IN) is at 3.0V. Higher ∆VF LED mis-match is

supported when V

IN is higher than 3.0V. All current sink

outputs are compared and the lowest output is used for

setting the voltage regulation at the OUT pin. This is

done to ensure that sufficient bias exists for all LEDs.

The backlight LEDs default to the off state upon power-

up. For backlight applications using fewer than six LEDs,

any unused output must be left open and the unused

LED must remain disabled. When writing to the backlight

enable register, a zero (0) must be written to the corre-

sponding bit of any unused output.

Backlight Quiescent Current

The quiescent current required to operate all five back-

lights is reduced when backlight current is set to 4.0mA

or less. This feature results in higher efficiency under

light-load conditions. Further reduction in quiescent

current will result from using fewer than six LEDs.

Applications Information

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