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UCC28780: ucc28780- WHEN does RUN signal assert?

robin gangopadhya18
Mastermind 6296 points
Part Number: UCC28780

Ulrich:

We reported REF required 1uF in our PCB /circuit to go to 5V.

But up to 50V, no RUN signal on pin4

Compared with Eval kit, up to 50V, Eval kit RUN is also low( kind of in hi Z state).

Eval kit RUN goes high around 100V DC input. And Pulses appear as well.

The d/s sheet indicates that RUN stays low during STARTUP.

But the Debug document lists REF &RUN to be high as exit criteria for getting PWML around 25V rms.

Should ignore that exit clause & ratchet up VBULK until we see Pulses?

-robin

  • 0
    TI__Mastermind 40580 points

    Hello Robin,

    I expect RUN to go high shortly after REF goes high, provided none of the Table 3 fault protections are active.
    The sequence is shown in Figure 30 of the datasheet, and is described in Section 7.4.8. 

    It should be dependent only on VDD achieving the start-up threshold of 17.5V, not on any specific input AC level or bulk DC level.
    The input does have to get high enough to get enough current through the depl-Fet to get VDD up to 17.5V in the first place, though.
    I'll have to have a look at our EVM next time I get into the lab to see how it behaves.

    RUN stays low during part of startup, until a) REF comes into spec (>4.8V) and b) HVG comes down to 11V.

    Then RUN goes high to allow PWML to switch, to drive the MOSFET to be able to sample Vbulk to determine if it is high enough to continue switching.

    I think you can go to ~100V.  RUN should have gone high by then, if there are no faults.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Mastermind 6296 points

    Ulrich:
    I have to  dig deeper into the circuit  for making sure this is valid:

    The input does have to get high enough to get enough current through the depl-Fet to get VDD up to 17.5V in the first place, though.

    This is loaded with meaning:  I will recheck 17.5v - I think it does but for how long? ( I ask this question to traffic police if they tell me I did not "stop"...for how long? 1ms?)

    I  have added 66 uF tantalum...VDD will stay high ( but  of course starts downward slope from 17.5V) & going down over long stretch maybe 300ms.

     But no RUN.

    So we will check if anything else is contrary to startup setup & then take it to 100V or so( AUX & sec will be appropriately terminated)

    - Will  report asap.

    robin