SC644 SEMTECH [Semtech Corporation], SC644 Datasheet - Page 12

• Manufacturer Part Number: SC644
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information
General Description
This design is optimized for handheld applications sup-
plied from a single cell Li-Ion and includes the following
key features:
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 , .5 or 2 times. 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 nominal 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
•
•
•
A high efficiency fractional charge pump that
supplies power to all LEDs
Six matched current sinks that control LED back-
lighting current, with 0mA to 25mA per LED.
Four adjustable LDOs. LDO , LDO3, and LDO4
are adjustable with 5 settings from .5V to 3.3V.
LDO2 is adjustable with 7 settings from .2V to
.8V.
and support the output drive requirements. Capacitors
with X7R or X5R ceramic dielectric are strongly recom-
mended for their low ESR and superior temperature and
voltage characteristics. Y5V capacitors should not be
used as their temperature coefficients make them unsuit-
able 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. 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 variation in step size allows finer adjust-
ment 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 register for maximum
flexibility.
All backlight current sinks have matched currents, even
when there is variation in the forward voltages (∆V
the LEDs. A minimum ∆V
input voltage (V
supported when V
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 fewer 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
sponding bit of any unused output.
Backlight Quiescent Current
The quiescent current required to operate all five back-
lights is reduced when backlight current is set to 4.0mA
or less. This feature results in higher efficiency under
light-load conditions. Further reduction in quiescent
current will result from using fewer than six LEDs.
IN
) is at 3.0V. Higher ∆V
IN
is higher than 3.0V. All current sink
(0)
F
of .2V is supported when the
must be written to the corre-
F
LED mis-match is
F
) of
2