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TL494 PWM Controller IC external frequency control

David Darge
Prodigy 60 points

Other Parts Discussed in Thread: TL494

TL494 PWM Controller IC external frequency control. Typically the frequency is set by selecting the appropriate capcitor and resistor. In my current project the TL494 is operating properly as designed. For testing it is required to externally control the frequency with in the limits of the selected capacitor. Is there a way to implement a method to control the frequency with an external signal? The control signal can be a voltage, PWM or other. Since the capacitor/resistor determine the charge rate could a opto be installed, controlled with a PWM to either series or parallel the resistor to obtain the desired dynamic frequency control? Other suggestions are welcomed.

  • 0
    TI__Guru**** 171066 points
    David,

    The RT pin voltage is about 4.4V or 3.8V (one or two diode drops below VREF, I forget which). Total RT pin current sets the charge current for the CT pin capacitor. Therefore changing the current on RT pin will change the frequency of the oscillator.

    So connecting an adjustable power source via a large series resistor to the RT pin can be used to modulate the frequency. The power source return lead goes to the TL494 ground pin.

    If the application is line powered then the TL494 may have a hot ground so appropriate safety measures should be taken to prevent injury, death, or property damage.