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UC28025D delay time from pin8 to pin14

Ken Lam
Prodigy 40 points

Will it appear the randomly delay time, say 720usec to 1.6msec, from SS pin8 to PWM output pin14 in UC28025D?

Any adjustable timing between pin8 & pin14?

  • 0
    TI__Mastermind 25130 points

    Ken,

    I am not certain what youare asking. The voltage on the SS pin  (pin 8) limits the votlage on pin 3 EAOUT. If that votlage is below 1.25 volts there will be no pulses on pin 14. Pulses will only occur when the voltage on pin 3 is greater than 1.25 volts above the instantaneous voltage on pin 7  RAMP.

    Send me your schematic and a scope shot of pins 3, 8, 7 and 14 (same time scale and ground)

  • 0 in reply to John Bottrill
    Prodigy 40 points

    Hi John,

    Thanks for your comment. I will send you the schematic and waveforms by email.

     

  • 0 in reply to Ken Lam
    Prodigy 40 points

    Hi John,

    Please find attached the SS vs Output pin waveform.5078.UC28025 Soft Start vs Output (TI).XLS

  • 0 in reply to Ken Lam
    Prodigy 40 points

    Hi John,

    I also have some questions about the error amp (pin1,2 &3),

    1. What's happen in pin3 (or output pin14) if pin2 below 0.6V compare with pin1?

    2. If pin2 is greater than 7V (out of the max rating of error amp input) and pin1 is 4.5V, what is the effect at pin3 and pin14?

    Thanks  

  • 0 in reply to Ken Lam
    TI__Mastermind 25130 points

    Ken,

    The soft start should not be holding th e error amp output low inb that time frame. I need the complete schematic with values.

    From the above question you are driving one of the eror amplifier inputs above its common mode operational range of 1.5 to 5.5 and even going above the ABS MAX rating on the part.

    Exceeding the common mode range can cause the error amp to saturate and remain in that state for significant time. Going above the ABS MAX can cause not only saturation be it can destroy the part.

    If that is what the customer is doing itis probably the answer to his question.