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UC3842: About the application of UC3842

Alvin Lin
Prodigy 110 points
Part Number: UC3842

Hi  TI's taem

We want to use UC3842 as a Voltage Mode PWM.

Do RT/CT circuits need to be connected like the below picture from datasheet?

Can it be connected like the typical circuit (just using a resistor and capacitor)?

Will the voltage mode be better for avoiding entering pulse skipping when the output voltage is very small?

Thank you very much.

  • 0
    TI__Mastermind 25895 points

    Hi Alvin,

    I would leave the circuit as you see it in the data sheet. The idea is to completely buffer the ISENSE pin so that only the divided down, buffered RT/CT RAMP is fed into ISENSE.

    Steve M

  • 0 in reply to Steven Mappus1
    Prodigy 110 points

    Maybe my expression is a bit unclear.

    My question is that why should the circuit in the red box be connected like this ?

    How to determine the value of resistance and capacitance？

    Thank you so much.

  • +1 in reply to Alvin Lin
    TI__Mastermind 25895 points

    Hi,

    The circuit you've shown in the red box is a current source. The 1N4148 compensates the Vbe change with temperature, so the current is “constant” over temperature. A current source charging the CT capacitor provides a more linear RAMP at RT/CT instead of the RC time constant type waveform when you connect RT, CT directly from VREF.

    Since the RT/CT RAMP is linear, the voltage mode control ramp at ISENSE is also linear. The importance of this is that your small signal gain depends on the dV/dt of the ramp. With the linear ramp, you ensure that your gain remains constant.

    You can use the RC waveform if you can tolerate the changing loop gain. In practice, if your loop is slow and you have plenty of margin use the RC. If you want a little bit better performance and you push the gain bandwidth, use the current generator and the linear ramp. Your choice. Thanks for connecting through E2E.

    Regards,

    Steve M