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hello all, I just need one PWM chip. I hope its PWM frequency can be adjusted through outer signal such as analog voltage or others. Because I want to change its PWM frequency according to its work condition. Changing by adjustable resistor is not a good choice as we can not access it for its installation. I hope I can change it through a MCU's port or interface.
I've attached the 2013 User's Guide for power products. Starting on page 53 you will find a selection of PWM controllers. many of these teh frequency can be varied according to external criteria.
As an example if you use a UC3842 it is a simple process to change the operating frequency of the IC. One you understand how the IC works you will see (go the the to of page 6 of the spec.) that by connecting a second resistor (say 10 times Rt) to pin 4 and adjusting the voltage on the resistor you can effectively change the frequency (approximately by +/-10%) by change the charge time of the capacitor Ct.
Make certain the other components in the converter and the converter design can handle the entire frequency range you have selected. You wouldn't want to saturate the transformers or inductors.
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You can change the frequency of LM5006 easily by modulating its RON pin (within reason). For example, this app note varies the frequency for low dropout but a a uC DAC can also achieve the same result. You will have to verify all the boundary conditions for the IC rating and the power stage.
In reply to Vijay86929:
thanks, your answer is very good, resolving my problems.
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