sc643 Semtech Corporation, sc643 Datasheet - Page 12

• Manufacturer Part Number: sc643
• Description: Light Management Unit With 4 Ldos And Sempulse Interface
• Manufacturer: 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 effi ciency,
high current fractional charge pump output. The charge
pump multiplies the input voltage by 1, 1.5 or 2 times. The
charge pump switches at a fi xed frequency of 250kHz in
1.5x and 2x modes and is disabled in 1x mode to save
power and improve effi ciency.
The mode selection circuit automatically selects the mode
as 1x, 1.5x, or 2x based on circuit conditions such as LED
voltage, input voltage, and load current. The 1x mode is
the most effi cient of the three modes, followed 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 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 effi ciency. Hysteresis is provided to prevent mode
toggling.
The charge pump requires two bucket capacitors for
proper operation. One capacitor must be connected
between the C1+ and C1- pins and the other must be con-
nected between the C2+ and C2- pins as shown in the
typical application circuit diagram. These capacitors
•
•
•
•
A high effi ciency fractional charge pump that
supplies power to all LEDs
Five matched current sinks that control LED
backlighting current, with 0mA to 25mA per
LED.
Four adjustable LDOs. LDO1 , LDO3, and LDO4
are adjustable with 15 settings from 1.5V to 3.3V.
LDO2 is adjustable with 7 settings from 1.2V to
1.8V.
An external enable pin for LDO4 allows it to be
used as a motor driver with hard-wired control.
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
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 coeffi cients 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 1mA for settings
between 5mA and 21mA. Steps are 2mA between 21mA
and 25mA. The variation in step size allows fi ner 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
fl exibility.
All backlight current sinks have matched currents, even
when there is variation in the forward voltages (ΔV
the LEDs. A minimum ΔV
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 suffi cient bias exists for all LEDs.
The backlight LEDs default to the off state upon power-
up. For backlight applications using fewer than fi ve 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.
IN
) is at 3.0V. Higher ΔV
IN
is higher than 3.0V. All current sink
(0)
F
of 1.2V is supported when the
must be written to the corre-
F
LED mis-match is
SC643
F
) of
12
12