RT9273 데이터 시트보기 (PDF) - Richtek Technology
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RT9273 Datasheet PDF : 14 Pages
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Preliminary
RT9273
Application Information
Inductor Selection
The recommended value of inductor for RT9273
applications is 4.7uH. Small size and better efficiency
are the major concerns for portable device, such as RT9273
used for dual panel mobile phone. The inductor should
have low DCR for better efficiency. To avoid inductor
saturation, current rating should be at least 600mA for
3LEDs application in the input range 2.7V to 4.5V.
ILED
(mA)
5
10
12
15
20
LEDs feedback
resistor (Ω)
59
30.1
24.9
20
15
Capacitor Selection
2.2μF input output ceramic capacitors are recommended
for RT9273 applications. 2.2uF input capacitor can reduce
input ripple effectively. 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 RT9273 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:
IDiode(RMS) ≅ ( IOUT ×IPEAK )
The diode's reverse 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. The table for the selection
of feedback resistor is shown below.
LEDs Feedback_resistor = 0.3V
ILED
OLED output voltage control
The output voltage can be set using a resistor-divider as
Figure 2. To regulate the output resistor-divider (R2/R3)
can get different output voltage of OLED as following
formula.
VOUT = VFB (1+ R2 )
R3
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. RT9273
is turned on or off by the PWM signal applied to EN pin.
The brightness of LEDs is depended on PWM duty cycle.
The average LED current increases proportionally as the
duty cycle added. 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. The recommended dimming frequency is 200Hz
to 1kHz.
Dimming Control for 3LEDs
VIN = 3.7V
VEN
(5V/Div)
LX
(5V/Div)
VOUT
(5V/Div)
IOUT
(20mA/Div)
Time (2ms/Div)
Figure 4
DS9273-09 August 2007
www.richtek.com
9
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