SC628AEVB datasheet(12/21 Pages) SEMTECH

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SC628A

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 the flash LED

An LED flash output that provides up to 1000mA

of momentary current or up to 275mA of

continuous spotlight current

An I2C compatible interface that provides control

of all device functions

High Current Fractional Charge Pump

The flash output is 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 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 OUT

pin during intervals of high demand, such as the high

current of an LED flash or the droop at the IN 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

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. Capacitor

case size is an important factor in the performance of

the SC628A. Due to the poor DC coefficient of the 0402

size (1005 metric), 0603 size (1608 metric) capacitors are

strongly recommended to achieve the rated electrical

specifications and performance shown in the typical

characteristics section.

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 Flash and Spotlight Current Sink

A single output current sink is provided to drive both

flash and spotlight functions. In flash mode, this current

sink provides up to 1000mA for a flash LED or array of

parallel LEDs. Flash current settings are available in 21

values from 300mA to 1000mA. Flash steps increments

are no larger than 75mA nominally. Some additional

settings that have smaller increments of 50mA, 25mA

and 12.5mA. Table 1 on page 12 contains detailed current

setting information for flash mode. The FLEN pin directly

triggers the FLASH function when pulled high, or it can

be wired to IN to enable software control via the serial

interface.

In spotlight mode, the output can be set for up to 275mA

of continuous current. Settings are available in 25mA

increments from 25mA to 275mA, with some additional

settings that have smaller increments of 12.5mA. Table 2

on page 13 contains detailed current setting information

for spotlight mode. Continuous operation above 275mA

is not recommended due to high power dissipation.

Flash and Spotlight Safety Timer

A safety timer disables the flash and spotlight output

current sink if the sink remains active for an extended

period. The timer protects the SC628A and the LED from

high power dissipation that can cause overheating. The

Applications Information

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