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Original question:

UCC28740: The major difference between UCC28740 and UCC28742?

UCC28742: UCC28742 design document

Shivani Singhal
Prodigy 20 points
Part Number: UCC28742

How to decide the power on delay time so to calculate the start-up resistance to charge VDD capacitance?

  • 0
    TI__Expert 3431 points

    Hi, Shivani:

    Thanks for asking about UCC28742. May I know what's your target application which used UCC28742?

    The power on delay should be defined by system spec. May I know what's your system spec. ?

    Thanks. 

    Regards, 

    Wesley

  • 0 in reply to Wesley Hsu
    Prodigy 20 points

    It is  a 85-265 V rms to 12 V dc flyback converter for DCM operation.

  • 0 in reply to Shivani Singhal
    TI__Expert 3431 points

    Hi, Shivani:

    Thanks for your reply.  And I think the turn on delay is related to your end system. i. e, A 12VDC/65W adapter for a NB adapter, monitor, standby power for UPS/Solar....etc. This specification should be defined by 

    If there is no special requested, normally it can be set 1s~3s of turn on delay to be a start. 

    Once the turn on delay time is chosen, RSTR could be calculated by the questions as below.  ( You may refer the page 24 of UCC28742's datasheet. it has more description for RSTR and CDD chosen)

    Please note there is no HV pin of UCC28742. The power loss of RSTR would be (1.414*Vin- VDD)^2/RSTR. 

    Thus, higher RSTR gives lower power consumption. I would suggest you to use higher RSTR to reduce power loss if the tSTR is acceptable. 

    Please let me know if you need any assistance. 

    Regards, 

    Wesley.