RT9271 데이터 시트보기 (PDF) - Richtek Technology
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RT9271 Datasheet PDF : 15 Pages
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RT9271
c. Using a Filtered PWM signal:
Another common application is using a filtered PWM signal
as an adjustable DC voltage for LED dimming control. A
filtered PWM signal acts as the DC voltage to regulate the
output current. The recommended application circuit is
shown in the Figure 7. In this circuit, the output ripple
depends on the frequency of PWM signal. For smaller
output voltage ripple (<100mV), the recommended
frequency of 2.8V PWM signal should be above 2kHz. To
fix the frequency of PWM signal and change the duty cycle
of PWM signal can get different output current as Figure
8. According to the application circuit of Figure 7, output
current is from 20.5mA to 5.5mA by adjusting the PWM
duty cycle from 10% to 90%.
C1
VIN 1uF
LX
10uH
D1
SS0520
VCC
LX
C3
1uF
EN
OVP
R3
VFB
10k
GND FB
RT9271
R4
23.7k
Rdc
100k
Cdc
0.1uF
2.8V
0V
PWM signal
D2
D3
D4
VLED
R2
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Constant Output Voltage for Backlight of Main
Panel and Flashlight:
Figure 9 is an application of RT9271 for backlight of main
panel and flashlight. Setting the divider-resistors (R1 & R2)
is to get a constant output voltage that depends on the
forward voltage and the numbers of series-LEDs. There
are three kinds of mode controlled by the switches -
backlight mode /flashlight mode /backlight + flashlight
mode. It can turn on backlight or flashlight at one time or
both at the same time. Applying different duty cycle of
PWM signal above 22kHz to backlight's switch can also
control the brightness. The following formula (1)(2) can
determine R3 and R4.
R3
=
VOUT
− 3VFb −
Ib
VDS
(1)
R4 = VOUT − 3VFf − VDS
(2)
If
VDS = Ib ×RDS(ON)
(3)
VIN
2.4 to 6V
C1
2.2uF
LX
10uH
D1
SS0520
C3
1uF
VCC
EN
LX
OVP
GND FB
RT9271
VOUT =12V
IOUT MAX=80mA
backlight flashlight
+
+
VFb
VFf
-
-
R1
239k
Ib
If
R3
GPIO1
PWM
R2
5.1k
R4
GPIO2
Figure 7. Filtered PWM Signal for LED Dimming Control
PWM Duty Cycle vs. IOUT
25
20
15
10
5
0
10 20 30 40 50 60 70 80 90
PWM Duty Cycle (%)
Figure 8
DS9271-13 March 2007
PWM >22kHz
Figure 9. Constant output voltage for backlight and
flashlight
Constant output voltage for backlight of main
panel and keypad:
Figure 10 is another application of RT9271 for backlight
and keypad. Setting the divider-resistors (R1 & R2) is to
get a constant output voltage that depends on the forward
voltage and the numbers of series-LEDs. It can turn on
backlight of main panel and keypad at the same time.
Applying different duty cycle of PWM signal above 22kHz
to the backlight's switch can also control the brightness of
main panel's backlight. The keypad's backlight will keep
the same brightness during the dimming control of main
panel. Otherwise the brightness of keypad's s backlight
can also change during the dimming control of main panel
by using the application circuit as figure 5. The following
formula (4)(5) can determine the resistors of Figure 10.
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