RT9271 datasheet(11/15 Pages) RICHTEK

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RT9271

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)

OUT

3VFf

3VFb

−

(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

GND

RT9271

1uF

10uH

OVP

23.7k

10k

SS0520

Rdc

100k

Cdc

0.1uF

PWM signal

2.8V

LED

VCC

GND

RT9271

PWM >22kHz

2.2uF

1uF

SS0520

10uH

5.1k

OVP

2.4 to 6V

239k

GPIO1

PWM

GPIO2

VFb

VFf

backlight

flashlight

OUT =12V

OUT MAX=80mA

PWM Duty Cycle vs. IOUT

PWM Duty Cycle (%)

(3)

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