SC653 datasheet(12/22 Pages) SEMTECH

12 / 22 page

SC653

General Description

This design is optimized for handheld applications sup-

plied 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 0mA to 25mA per

LED

Two adjustable LDOs with outputs ranging from

1.5V to 3.3V for LDO1 and 1.2V to 1.8V for LDO2,

adjustable in 100mV increments

High Current Fractional Charge Pump

The backlight outputs are supported by a high efficiency,

high current fractional charge pump output at the OUT

pin. The charge pump multiplies the input voltage by 1,

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

quency 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 mode

as 1x, 1.5x, or 2x based on circuit conditions such as LED

voltage, input voltage, and load current. The 1x mode is

the most efficient of the three modes, followed by 1.5x

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 (1.5x 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 C1+ and C1- 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

1.0μF to support the charge pump current requirements.

•

Note that small package capacitors can decrease in value

by up to 50% under DC loading, so it is strongly recom-

mended that 2.2uF capacitors be selected when using

0402 size ceramic capacitors. The device also requires a

2.2μF capacitor on the IN pin and a 2.2μF capacitor on

the OUT 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 SemPulse interface.

The current is regulated to one of 29 values between

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

current setting. The step sizes are 0.5mA for current set-

tings between 0mA and 5mA. The step size increases to

1mA for settings between 5mA and 21mA. Steps are 2mA

between 21mA and 25mA. The variation in step size

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. 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 reg-

ister for maximum flexibility.

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 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 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 register, a zero (0) must be written to

the corresponding bit of any unused output.

Applications Information

SC653 Datasheet(PDF) 12 Page - Semtech Corporation

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