SC628A
12
General Description
This design is optimized for handheld applications
supplied from a single Li-Ion cell and includes the
following key features:
A high e ciency fractional charge pump that
supplies power to the ash LED
An LED ash output that provides up to 1000mA
of momentary current or up to 275mA of
continuous spotlight current
An I
2
C compatible interface that provides control
of all device functions
High Current Fractional Charge Pump
The ash output is supported by a high e ciency, 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 xed frequency of
250kHz in 1.5x and 2x modes and is disabled in 1x mode
to save power and improve e ciency.
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 ash 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 e ciency. 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.
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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 coe cient of the 0402
size (1005 metric), 0603 size (1608 metric) capacitors are
strongly recommended to achieve the rated electrical
speci cations 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 coe cients
make them unsuitable for this application.
LED Flash and Spotlight Current Sink
A single output current sink is provided to drive both
ash and spotlight functions. In ash mode, this current
sink provides up to 1000mA for a ash 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 ash 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 ash 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
