SC657 datasheet(10/22 Pages) SEMTECH

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Applications Information

SC657

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

Five matched current sinks that control LED

backlighting current, providing 0mA to 25mA

per LED

LEDs can be grouped in up to three indepen-

dently controlled banks

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 x, .5x, or 2x. 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 minimum capacitance of

µF to support the charge pump current requirements.

The device also requires at least µF of capacitance on the

IN pin and at least µF of capacitance on the OUT pin to

•

minimize noise and support the output current require-

ments of up to 90mA. For output currents higher than

90mA, a nominal value of 4.7µF is recommended for C

OUT

and C

IN. 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.

It is important to ensure the minimum capacitance value

of each capacitor does not drop below µF. This may

require the use of 2.2µF capacitors to be sure that the

degradation of capacitance due to DC voltage does not

cause the capacitance to go below µF.

LED Backlight Current Sinks

The backlight current is set via the SemPulse interface. The

current is regulated to a value 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 varia-

tion in step size allows finer adjustment for dimming func-

tions 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 a variation in the forward voltages (∆V

F ) of

the LEDs. A ∆V

F difference of .0V is supported when the

input voltage is at 2.9V. Higher ∆V

F LED mis-match is sup-

ported when V

IN is higher than 2.9V. 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 five 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.

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