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RT9271PE Datasheet(PDF) 11 Page - Richtek Technology Corporation

Part # RT9271PE
Description  White LED Step-Up Converter in Tiny Package
Download  15 Pages
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Manufacturer  RICHTEK [Richtek Technology Corporation]
Direct Link  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT9271PE Datasheet(HTML) 11 Page - Richtek Technology Corporation

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RT9271
11
DS9271-13 March 2007
www.richtek.com
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%.
Figure 7. Filtered PWM Signal for LED Dimming Control
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.
DS(ON)
DS
DS
OUT
DS
OUT
R
Ib
V
If
V
3VFf
V
R4
Ib
V
3VFb
V
R3
×
=
=
=
(1)
(2)
Figure 8
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.
VCC
EN
GND
LX
FB
RT9271
C1
1uF
C3
1uF
LX
10uH
R2
12
OVP
D2
D4
D3
R4
23.7k
R3
10k
D1
SS0520
V
IN
Rdc
100k
Cdc
0.1uF
PWM signal
0V
2.8V
V
FB
V
LED
VCC
EN
GND
LX
FB
RT9271
PWM >22kHz
C1
2.2uF
C3
1uF
D1
SS0520
LX
10uH
R2
5.1k
OVP
V
IN
2.4 to 6V
R1
239k
GPIO1
PWM
GPIO2
+
VFb
-
Ib
+
VFf
-
If
R3
R4
backlight
flashlight
V
OUT =12V
I
OUT MAX=80mA
PWM Duty Cycle vs. IOUT
0
5
10
15
20
25
10
20
30
40
50
60
70
80
90
PWM Duty Cycle (%)
(3)


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