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UCC25630-1EVM-291: Question about feedback loop design

Filippo Appignani
Prodigy 20 points
Part Number: UCC25630-1EVM-291
Other Parts Discussed in Thread: TLVH431, TL431, TLV431

Hi everyone,

I don't understand how feedback work in the evaluation board UCC25630-1EVM-291. I don't have the ev. board to measure voltages or currents so I post this question.

When the output voltage is regulated at 12V the current that pass through the LED of the optocoupler is about 1.45mA. With the assumption that CTR is 100%, the opto collector current must be 1.45mA. From the datasheet the FB current is 85uA and FB voltage must settle at 5.4V so the 1.45mA sink from the collector where it came from?

Is it possible that every time the output voltage goes a little bit up the set point, the FB voltage will drop and the system goes in burst mode?

Thanks for the support.

Filippo.

  • 0
    TI__Mastermind 47415 points
    When the output voltage goes up, U4 pin 1 voltage goes up, TLVH431 internal will control to sink more current which causes U2 VO618A more current thrrough its collector which pull more current out of U1 FB pin so cause FB drops. If FB reaches burst level, then the operation will be in burst.

    How did you determine 1.45mA going through the VO618A LED? If you try pull down more than FB can supply, FB will be in saturation, i.e., it won't be able to supply any more current. But the first thing is you show how you conclude 1.45mA through the LED?
  • 0 in reply to Hong Huang
    Prodigy 20 points

    I determined that value because I made the assumption that the cathode voltage on TLVH431 is 1.24V when the output voltage il regulated at 12V, and the forward voltage drop of the LED diode is about 1V.

    From the answer you have write I suppose that the cathode voltage it isn't fixed at 1.24V.  So, how much is this voltage? The value of R23 where it came from?

    Thanks.

  • 0 in reply to Filippo Appignani
    TI__Mastermind 47415 points
    TLV431 cathode voltage may not stay on 1.24V, which is its ref voltage, unless you short its cathode to its ref. its cathode voltage is controlled by its ref. How much is its cathode is determined by how much error on its ref. You may review TL431 datasheet to find out how TL431 (or TLV431) works.

    R23 is usually determined by trial-and -error but its value will setup opto-coupler gain.

    In any case, you would have to confirm your assumption by bench test before you conclude your assumption is true.