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UCC28950: How to extend cycle-by-cycle current limiting

Chuck Sampson
Genius 3295 points
Part Number: UCC28950

Hello,

I need to extend the cycle-by-cycle current limiting beyond the time set by the soft start capacitor. Specifically I need to stay in cycle-by-cycle current limit until the output voltage drops to 30% of nominal. I have an idea to clamp the ss pin to 4.3V until the output voltage drops to the required value. Once it reaches the 30% of the nominal voltage I remove the voltage clamp from the ss pin and allow it to go into hiccup mode. Does this sound feasible?

I am asking because the TI data sheet is somewhat vague about its description on how the UCC28950 current limiting works. For example, the data sheet says that the time allotted for the cycle by cycle current limiting is set by the soft start capacitor and the discharge current, Ids. But the soft start capacitor also sets the soft start time. This makes it difficult to meet the standard specification I mentioned above. 

Has anyone tried extending the cycle by cycle current limiting?

  • 0
    TI__Mastermind 31580 points
    Hi Chuck,

    I have asked one of our applications engineers to respond to your post. You should see a reply later today.

    Regards

    Peter
  • 0 in reply to Peter Meaney
    TI__Mastermind 25570 points

    Hi Chuck

    Yes - I think your idea would work nicely. If you look at Fig 42 in the DS (below) It is of course possible to defeat the entry into Hiccup mode by making sure that the SS pin does not discharge below the 'Pull  up threshold - so for example a resistor tied to Vref that pulls a current of Ids before the pin falls below the Pull Up Threshold and you can also use a lower value resistor to extend the time taken before reaching this threshold but that's not what you want to do.

    Clamping the SS pin at 4.3V will allow normal operation including CbC current limiting. Releasing the clamp at Vo=30% will allow entry into Hiccup mode (providing of course that the system is still in current limit). Current limiting is done on the primary current of course and if Vo falls then unless Io increases by an equivalent amount the input current will also fall. If you need to do current limiting on the Output current then you need a different approach - second picture below.

    Anyhow, once you release the clamp the SS pin will fall at a rate determined by the SS capacitor and the discharge current IDS (15uA min, 25uA max) and Hiccup mode will start when the voltage falls to the pull up threshold (falling, VHCC) which is between 3.2 and 4.2V. Yuor starting or clamp level of 4.3V means you have somewhere between 100mV (4.3-4.2) and 1100mV (4.3-3.2) between the clamp level and the pull up threshold level. You should calculate the min and max times for this process in your application and consider if it's acceptable. - it might be better to simply pull the SS pin actively down to less than 3.2V and allow the IHCC to set the OFF time before Restart. Or else actively pull SS down to less than 500mV and release it after whatever HICCUP off time you want.

    Please let me know if you need any further clarification.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Genius 3295 points

    Thanks Colin, you've very helpful. Thanks for pointing out that the UCC28950 only provides cycle-by-cycle current limiting on the primary. I was hoping to avoid all the extra parts. Oh well.