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Original question:

UC3825: About the soft-start

UC3825: About the soft start of UC3825

Alvin Lin
Prodigy 110 points
Part Number: UC3825

Hi TI's team:

In the application of UC3825, there is a question need your help to reply.

Does the capacitance of the SS pin participate in the loop response?

Would different capacitance values affect the loop stability？

Thank you very much.

  • 0
    TI__Genius 13660 points

    Hi, 

    SS pin doesn't affect the loop response. This pin provides a soft start time for start up, or a hold off time for PWM latch when the output shuts down by the fault.

    The output of internal error amplifier is compared with the soft-start capacitor voltage (SS) and the smaller of the two passes will being applied to the PWM comparator to determine the PWM duty cycle. There is a 1.25 volt offset on the ramp pin to facilitate zero duty cycle. Therefore, the soft start capacitor must charge to 1.25 volts before the PWM comparator is active.

    Regards,

    Teng

  • 0 in reply to Teng Feng
    Prodigy 110 points

    Hi Teng,

    hace a nice day.

    Thank you for your kindly reply.

    By the way,coud u help to reply another question below:

    Does pin 9（Ilimi） participate in the adjustment of pulse width? 

    I found that the power supply would  be damaged, when PIN9 is not connected.、

    Thanks so much.

  • 0 in reply to Alvin Lin
    TI__Genius 13660 points

    Alvin,

    The ILIM can't be floating. Please ground this pin if not used. Pulse-by-pulse current limiting is performed by the UC3825 by an input of the primary current waveform to the IC at pin 9. The small RC network are needed to suppress the leading edge glitch caused by turn-on of the mosfet and transformer parasitics. The input must be below the 1 volt threshold or current limiting will occur. Once reached, an input above the threshold will narrow the pulse width accordingly. When this reaches a 1.4 volts amplitude, shutdown of the outputs will occur, and the UC3825 will initiate a soft start routine.

    Teng

  • 0 in reply to Teng Feng
    Prodigy 110 points

    Whether the power supply can work normally, when I ground the ILIM pin,
    For the "Pulse-by-pulse current limiting" ,does it means the ILIM PIN involved in PWM modulation?

  • +1 in reply to Alvin Lin
    TI__Genius 13660 points

    Hi Alvin,

    Yes, the power supply can still work normally when ground the ILIM pin, but there is no current limit protection.

    I doesn't involve normal PWM modulation, with it is below 1V.    It will cycle-by-cycle turn off PWM when it is between 1V to 1.4V. It will shut down the PWM when it is above 1.4V.

    Regards,

    Teng

  • +1 in reply to Teng Feng
    Prodigy 110 points

    Thanks a lot