SC624
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 all LEDs
Four matched current sinks that control LED
backlighting current, with 0.5mA to 25mA per
LED
Two adjustable LDOs with outputs ranging from
2.5V to 3.3V for LDO1 and 1.5V to 1.8V for LDO2,
adjustable in 100mV increments
A SemWire single wire interface that provides
control of all device functions
High Current Fractional Charge Pump
The backlight outputs are supported by a high e ciency,
high current fractional charge pump output at the VOUT
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 VOUT pin
during intervals of high demand, such as the droop at
the VIN 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
1μF to support the charge pump current requirements.
The device also requires a 1μF capacitor on the VIN pin and
a 2.2μF capacitor on the VOUT 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 coe cients make them unsuitable for
this application.
LED Backlight Current Sinks
The backlight current is set via the SemWire interface.
The current is regulated to one of 32 values between
0.5mA and 25mA. The step size varies depending upon
the current setting. Between 0.5mA and 12mA, the
step size is 0.5mA. The step size increases to 1mA for
settings between 12mA and 15mA and 2mA for settings
greater than 15mA. This feature allows ner 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.
All backlight current sinks have matched currents, even
when there is variation in the forward voltages (ΔV
F
)
of the LEDs. A ΔV
F
of 1.2V is supported when the input
voltage is at 3.0V. Higher ΔV
F
LED mis-match is supported
when V
IN
is higher than 3.0V. All current sink outputs are
compared and the lowest output is used for setting the
voltage regulation at the VOUT pin. This is done to ensure
that su cient bias exists for all LEDs.
The backlight LEDs default to the o 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 Control register, a zero
(0) must be
written to the corresponding bit of any unused output.
Applications Information
