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CAT4238TD-GT3 Datasheet(PDF) 10 Page - ON Semiconductor |
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CAT4238TD-GT3 Datasheet(HTML) 10 Page - ON Semiconductor |
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10 / 14 page  CAT4238 Doc. No. MD-5019 Rev. E 10 © 2008 SCILLC. All rights reserved. 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 CAT4238 FB pin. The PWM signal has a voltage swing of 0V to 2.5V. The LED current can be dimmed within a range from 0mA 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. FILTERED PWM DIMMING [0V to 2.5V] 25 20 15 10 5 0 0 1020 30 4050 60 70 80 90 100 PWM DUTY CYCLE [%] VFB = 300mV 15Ω 1kΩ 3.1kΩ 3.73kΩ 0V 2.5V 0.22µF C1 RA RB R2 i R1 VIN CAT4238 FB SHDN SW GND PWM Signal LED Current |
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Similar Description - CAT4238TD-GT3 |
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