SC624AEVB datasheet(13/23 Pages) SEMTECH

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SC624A

General Description

This design is optimized for handheld applications

supplied from a single Li-Ion cell and includes the

following key features:

A high efficiency fractional charge pump that

supplies power to all LEDs

Four matched current sinks that control LED

backlighting current, with 0.5mA to 25mA per

LED

Two adjustable LDOs with outputs ranging from

2.5V to 3.3V for LDO1 and 1.5V to 1.8V for LDO2,

adjustable in 100mV increments

An I2C compatible interface that provides control

of all device functions

High Current Fractional Charge Pump

The backlight outputs are supported by a high efficiency,

high current fractional charge pump output at the VOUT

pin. The charge pump multiplies the input voltage by

1, 1.5, or 2 times. The charge pump switches at a fixed

frequency of 250kHz in 1.5x and 2x modes and is disabled

in 1x mode to save power and improve efficiency.

The mode selection circuit automatically selects the

1x, 1.5x or 2x mode based on circuit conditions. 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 may be needed

momentarily to maintain regulation at the VOUT pin

during intervals of high demand, such as the droop at

the VIN pin during a supply voltage transient. The charge

pump responds to these momentary high demands,

setting the charge pump to the optimum mode (1x, 1.5x

or 2x), as needed to deliver the output voltage and load

current while optimizing efficiency. Hysteresis is provided

to prevent mode toggling.

The charge pump requires two bucket capacitors for

low ripple operation. One capacitor must be connected

between the C1+ and C1- pins and the other must be

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

•

the typical application circuit diagram. These capacitors

should be equal in value, with a minimum capacitance of

1μF to support the charge pump current requirements.

The device also requires a 1μF capacitor on the VIN pin

and a 2.2μF capacitor on the VOUT pin to minimize

noise and support the output drive requirements.

Capacitors with X7R or X5R ceramic dielectric are

strongly recommended for their low ESR and superior

temperature and voltage characteristics. Y5V capacitors

should not be used as their temperature coefficients

make them unsuitable for this application.

LED Backlight Current Sinks

The backlight current is set via the I2C compatible

interface. The current is regulated to one of 32 values

between 0.5mA and 25mA.The step size varies depending

upon the current setting. Between 0.5mA and 12mA, the

step size is 0.5mA. The step size increases to 1mA for

settings between 12mA and 15mA and 2mA for settings

greater than 15mA. This feature allows finer adjustment

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.

All backlight current sinks have matched currents, even

when there is variation in the forward voltages (ΔV

)

of the LEDs. A ΔV

of 1.2V is supported when the input

voltage is at 3.0V. Higher ΔV

LED mis-match is supported

when V

is higher than 3.0V. All current sink outputs are

compared and the lowest output is used for setting the

voltage regulation at the VOUT 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 less than four LEDs,

any unused output must be left open and the unused

LED driver must remain disabled. When writing to the

Backlight Enable Control register, a zero (0) must be

written to the corresponding bit of any unused output.

Applications Information

SC624AEVB 数据表(PDF) 13 Page - Semtech Corporation

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