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PAM2801 Datasheet(PDF) 7 Page - Power Analog Micoelectronics |
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PAM2801 Datasheet(HTML) 7 Page - Power Analog Micoelectronics |
7 / 9 page 7 Dimming Control Using a PWM Signal to FB Pin Dimming Control Using a PWM Signal to SHDN Pin By using the PWM signal to FB pin as shown in the Figure 2, the LED turns on or off and its current operates at either 0mA or the set maximum current. The PWM signal can be considered as an adjustable DC voltage. As the PWM duty cycle increases, the LED current decreases. Typically, the PWM frequency ranges from 5KHz to 40KHz. Figure 2. Dimming Control Using a PWM Signal to FB Pin With the PWM signal applied to the SHDN pin of the PAM2801, the LED turns on or off. When the SHDN pin is high, the LED turn on and its current is at the set maximum current; when the SHDN pin is low, the LED turn off and its current turns to 0mA. The a verage LED c urrent increase proportionally with the PWM duty cycle. A 0% duty cycle produces 0mA of LED current; a 100% duty cycle corresponds to the set maximum current. The magnitude of the PWM signal should be higher than the SHDN input high (V ). The typical frequency of the PWM signal ranges from 100Hz to 1KHz. Figure 3 shows the LED current with the PWM duty cycle set to 50% and the PWM frequency set respectively to 100Hz and 1KHz. Figure 4 shows the LED current versus the PWM duty cycle, setting the PWM frequency to 100Hz. 3a 100Hz 3b 1 KHz Figure 3. Dimming Control Using a PWM Signal to SHDN Pin Figure 4. LED Current vs Duty Cycle SH () () SHDN FB I LED LED Current vs Duty Cycle 0 50 100 150 200 250 300 350 400 0 20406080 100 Duty Cycle(%) 08/2007 PAM2801 FB R1 R2 R 0.27 S Ω 1W WLED PWM 10KΩ 22kΩ 1KΩ 0.1uF Application Information(continued) SHDN FB I LED PAM2801 1W High Power White LED Driver , Power Analog Microelectronics Inc www.poweranalog.com |
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