Preliminary
Application Information
Inductor Selection
The recommended value of inductor for RT9272 applications
is 4.7μH. Small size and better efficiency are the major
concerns for portable device, such as RT9272 used for
dual panel mobile phone. The inductor should have low
DCR for better efficency. To avoid inductor saturation,
current rating should be at least 600mA for 3LEDs
application in the input range 2.7V to 4.5V.
Capacitor Selection
2.2μF input and 1μF output ceramic capacitors are
recommended for RT9272 applications. 2.2uF input
capacitor can reduce input ripple. For better voltage stability,
to increase the input capacitor value or using LC filter is
feasible, especially in the Li-ion battery application. 1μF
output capacitor is sufficient to reduce output voltage ripple.
For better voltage filtering, ceramic capacitors with low
ESR are recommended. X5R and X7R types are suitable
because of their wider voltage and temperature ranges.
Diode Selection
Schottky diode is a good choice for RT9272 because of
its lower forward voltage drop and faster reverse recovery.
Using schottky diode can get better efficiency. The high-
speed rectification is also a good characteristic of
schottky diode for high switching frequency. Current rating
of the diode must meet the root mean square of the peak
current and output average current multiplication as
following:
I
Diode (RMS)
≅
( I
OUT
×
I
PEAK
)
RT9272
Dimming Control
Dimming control is the current programming method when
output resistor fixes the brightness of LEDs. Using a PWM
signal on EN pin can achieve the dimming control. RT9272
is turned on or off by the PWM signal applied to EN pin.
The brightness of LEDs for CH1 or CH2 is depended on
PWM duty cycle. By selecting the different channel, it can
achieve the dimming control function for main panel or
sub panel. The average LED current increases
proportionally as the duty cycle added. The operation
frequency of PWM signal is 100Hz to 1kHz. The average
LED current is linear in different duty cycle. It should be
notice that the magnitude of PWM signal should be higher
than the minimum EN voltage.
Channel Selection
There is one set of LC filter for RT9272 to share two
channels power requirement. Each time turning on only
one channel can save the power dissipation and easily
extend the standby time. Selection pin is internal logic
controller. SP pin is pulled low to turn on the internal channel
1 and pulling high to turn on channel 2. As the input signal
change, internal MOSFET of LX pin will turn off 1ms to
prevent the over voltage damage by the output voltage
instant change.
Layout Guide
A full GND plane without gap break.
The input capacitor should be placed as close as possible
to the input for good filtering.
The switching path should be shorter to prevent EMI
problems.
The inductor and diode must be placed closely to the
switch pin.
The feedback path should be away from the inductor.
The feedback resistor should be connected directly to
the GND.
The diode’sreverse breakdown voltage should be larger
than the output voltage. SS0520 is recommended schottky
diode for rectifier.
LED Current Control
Current control is to change different brightness of LEDs.
To regulate the output resistor can get different currents of
LEDs as following formula.
FB_resister (
Ω
)
=
0.3 (V)
I
LED
(mA)
DS9272-09 August 2007
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
