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UCC28782: I have a question about the function of UCC28782CD

Part Number: UCC28782

Thank you for your response. Let me ask you one more question.

Please check the datasheet below.



It says that the waiting time for SCP(Output short-circuit protection)
operation is less than topp, does less than topp mean immediate?
  • Hello Masashi-san, 

    Thank you for your interest in the UCC28782 ACF controller. 

    In the case of a short-circuit on the output, the waiting time can be almost immediate, but not exactly immediate.

    A short-circuit condition is a special case of severe overload.  A normal overload is allowed to persist for tOPP (~160ms) before OCP shutdown.  During tOPP interval VDD is sustained above UVLO threshold (Vvdd(off)) by the output voltage.     

    In the case of a short-circuit, Vout is pulled too low to sustain VDD, so VDD falls toward UVLO.  While VDD is still > Vvdd(off), Vcst is driven above Vcst(opp) threshold by the feedback loop and the tOPP timer is started.  It takes some finite amount of time for VDD capacitor(s) to discharge to UVLO.  Once UVLO is reached, both conditions for SCP are satisfied and SCP shutdown happens at that time.  Not quite immediate, but much much sooner than tOPP.  

    Note, If the short circuit impedance is low enough such that the reflected voltage at VS is lower than the initial start-up threshold voltage ( Vvs < Vvs(sm2)), then this is an alternative criterion for triggering SCP shutdown.  But the time delay to shutdown still involves discharge of VDD to Vvdd(off).

    So SCP response is not quite immediate, but much much sooner than tOPP. 

    Regards,
    Ulrich