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RT9271PE Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT9271PE Datasheet(HTML) 11 Page - Richtek Technology Corporation |
11 / 15 page 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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