SC653 Datasheet, PDF (12/22 Pages) Semtech Corporation – Light Management Unit with 2 LDOs and SemPulse®Interface

SC653
Applications Information
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
• Four matched current sinks that control LED
backlighting current, with 0mA to 25mA per
LED
• Two adjustable LDOs with outputs ranging from
1.5V to 3.3V for LDO1 and 1.2V to 1.8V for LDO2,
adjustable in 100mV increments
High Current Fractional Charge Pump
The backlight outputs are 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 fre-
quency 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 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 efficient 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 efficiency. Hysteresis is provided to prevent mode
toggling.
Note that small package capacitors can decrease in value
by up to 50% under DC loading, so it is strongly recom-
mended that 2.2uF capacitors be selected when using
0402 size ceramic capacitors. The device also requires a
2.2μF capacitor on the IN pin and a 2.2μF capacitor on
the OUT pin to minimize noise and support the output
drive requirements. 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 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. The step sizes are 0.5mA for current set-
tings between 0mA and 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 finer adjustment 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 reg-
ister for maximum flexibility.
All backlight current sinks have matched currents, even
when there is variation in the forward voltages (ΔV ) of
F
the LEDs. A ΔV_F of 1.2V is supported when the input
voltage is at 3.0V. Higher ΔV LED mis-match is supported
F
when V is higher than 3.0V. All current sink outputs are
IN
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 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
should be equal in value, with a nominal capacitance of
1.0μF to support the charge pump current requirements.
The backlight LEDs default to the off state upon power-
up. For backlight applications using less than four LEDs,
any unused output must be left open and the unused
LED driver must remain disabled. When writing to the
Backlight Enable register, a zero (0) must be written to
the corresponding bit of any unused output.
12