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CAT4237TD Datasheet(PDF) 10 Page - Catalyst Semiconductor |
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CAT4237TD Datasheet(HTML) 10 Page - Catalyst Semiconductor |
10 / 13 page CAT4237 © 2005 Catalyst Semiconductor, Inc. 10 of 13 Doc. No. 5013, Rev. A Characteristics subject to change without notice 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 1 shows a 1kHz signal with a 50% duty cycle applied to the SHDN* pin. The recommended PWM frequency range is from 100Hz to 2kHz. Figure 1. Switching waveform with 1kHz PWM on SHDN* § Filtered PWM signal A filtered PWM signal used as a variable DC voltage can control the LED current. Figure 2 shows the PWM control circuitry connected to the CAT4237 FB pin. The PWM signal has a voltage swing of 0V to 2.5V. The LED current can be dimmed within a range from 0 to 20mA. The PWM signal frequency can vary from very low frequency up to 100kHz. Figure 2. Circuit for filtered PWM signal A PWM signal at 0V DC, or a 0% duty cycle, results in a max LED current of about 22mA. A PWM signal with a 93% duty cycle or more, results in an LED current of 0mA. |
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