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CAT4139 Datasheet(PDF) 10 Page - ON Semiconductor |
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CAT4139 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 13 page CAT4139 www.onsemi.com 10 Dimming Control There are several methods available to control the LED brightness. PWM Signal on the SHDN Pin LED brightness dimming can be done by applying a PWM signal to the SHDN input. The LED current is repetitively turned on and off, so that the average current is proportional to the duty cycle. A 100% duty cycle, with SHDN always high, corresponds to the LEDs at nominal current. Figure 23 shows a 1 kHz signal with a 50% duty cycle applied to the SHDN pin. The recommended PWM frequency range is from 100 Hz to 2 kHz. Figure 23. Switching Waveform with 1 kHz PWM on SHDN Filtered PWM Signal A filtered PWM signal used as a variable DC voltage can control the LED current. Figure 24 shows the PWM control circuitry connected to the CAT4139 FB pin. The PWM signal has a voltage swing of 0 V to 2.5 V. The LED current can be dimmed within a range from 0 mA to 20 mA. The PWM signal frequency can vary from very low frequency and up to 100 kHz. Figure 24. Circuit for Filtered PWM Signal VIN SW CAT4139 0 V 2.5 V R1 15 W R2 1 k W 3.1 k W 3.73 k W C3 0.22 mF LED Current PWM Signal FB GND RA SHDN VIN RB VFB = 300 mV A PWM signal at 0 V DC, or a 0% duty cycle, results in a max LED current of about 22 mA. A PWM signal with a 93% duty cycle or more, results in an LED current of 0 mA. Figure 25. Filtered PWM Dimming (0 V to 2.5 V) 25 20 15 10 5 0 0 102030 4050 60 70 8090 100 PWM DUTY CYCLE (%) |
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