SC662 datasheet(12/30 Pages) SEMTECH

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SC662

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.

Six matched current sinks that control LED back-

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

Up to three independently controlled LED

banks.

Selectable charge pump frequency — 250kHz or

1MHz options.

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 1x, 1.5x, or 2x. The

output of the charge pump is delivered to the LED anodes.

The charge pump switches only in 1.5x and 2x modes and

is disabled in 1x mode to save power and improve

efficiency.

The charge pump switches at a fixed frequency of either

250kHz or 1MHz. The charge pump switching frequency

is set via the SemPulse interface by the FSEL bit. The

250kHz setting is selected by setting FSEL = 0, while the

1MHz setting is selected when FSEL = 1.

The mode selection circuit automatically selects one of

the following modes; 1x, 1.5x, or 2x based on circuit condi-

tions such as LED voltage, input voltage, and load current.

The 1x mode is the most efficient of the three modes, fol-

lowed 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 maintain 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 optimizing efficiency. Hysteresis is

provided to prevent mode toggling.

•

The charge pump requires two bucket capacitors. 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. Bucket capacitors should be equal in value to

support current sharing between C

1 and C2.

OUT , CIN , C1 , and C2 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 coef-

ficients 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 32 values between 0mA and

25mA. The step size varies depending upon the current

setting. The lowest settings are 0, 50, 100, and 200µA.

From 0.5mA to 5mA, the step size is 0.5mA. 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. When

there is a variation in the forward voltages (∆VF ) of the

LEDs, mis-matched LED voltages do not degrade the accu-

racy of the backlight currents. The voltages of all BLn pins

are compared, and the lowest of these voltages is used as

feedback 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 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. Detailed information

about programming of the registers is provided in later

sections, beginning at SemPulse Interface on page 21.

Applications Information

SC662 Datasheet(PDF) 12 Page - Semtech Corporation

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