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UCC25230: Zener diode COMP pin

Prodigy 30 points

Replies: 6

Views: 125

Part Number: UCC25230


Figure 9 in UCC25230 datasheet which depicts the typical application have D2 Zener diode connected to COMP pin.


What is the function of the D2 diode ? Is it mandatory ? UCC25230 Bias Power-Supply Design Review uses 3.9V Zener, what is the main reason to use this diode?

UCC25230 datasheet does not describe and explain D2 function.


  • Hi,

    We will reply you next Monday.

    Thanks & Best Regards

    Neal Zhang

  • Hi Lucasz,

    The COMP pin is the output of the error amplifier and the input to the PWM comparator.

    Page 7 of the datasheet states that a higher COMP voltage will result in a larger duty cycle.

    The Zener clamp on COMP limits the COMP Pin voltage, so functionally it prevents large duty cycles.

    This can be useful to control startup or short recovery, where the COMP voltage can hit the rail of the error amp.

    The diode will prevent the part from trying to run at 100% duty cycle and also reduce peak currents.

    I don't believe it is absolutely necessary, I'll update you otherwise.


  • In reply to Orlando Murray:

    Hi Orlando,

    Thanks for explanations.

    Is it safe for UCC25230 to run at 100, or 95% duty cycle?

    Thank you and Best Regards,


  • In reply to Lukasz Pulka:


    Are you running with a fly-buck output?

    The fly-buck output only conducts during the off-time of the switching converter, hence the Zener clamp keeps the duty cycle at 50% so that there is enough off-time to conduct the isolated output.

    If you are just running a buck output then it is safe to ignore the diode and run at maximum duty cycle.

    Hope this helps,


  • In reply to Orlando Murray:


    Yes I am running with fly-buck.

    Without zener diode and limiting the duty cycle, light load (far below 80mA)  on isolated output - we have bad experience - few chips died (I believe they died during disconnection of Vin). I added zener and so far so good. We do not have any load on primary output - now I know that some light load may help and even reduce voltage drop on isolated.

    I just found that TPS54308 may be better to my application as I do not need so high VIN. However I am looking for something that can handle 36Vin - does TI have some forced ccm bucks with up to 36V Vin and 2-3A max current? I can use it instead of UCC25230. Do you have any advice?




  • In reply to Lukasz Pulka:


    The part was blowing up because with short off-time the peak currents were getting really large.

    Other ICs for flybuck off of the top of my head are the LM5160 / LM5161 and the LM5017.

    However I recommend you use WEBENCH with your design to compare choices.