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UCC28631: Load Transient

Hirotsugu Kobayashi
Mastermind 8725 points
Part Number: UCC28631
Other Parts Discussed in Thread: UCC24650

Hi, team.

Out customer is evaluating a load response test in UCC28631.

In a transient test from zero load, very big drop is observed.

But test from sub mA load, it is improved.

Is there difference in an operation mode by no load and several mA?

Best Regards,

  • 0
    TI__Expert 3260 points

    Hi Hirotsugu,

    Thanks for your interest in TI. During the no load condition, the device will enter the sleep mode to minimise the controller power consumption. The controller will not be able to react the transient load instantaneously and the output voltage could drop significantly as described on Page 44 in the datasheet.

    An external wake-up circuit with UCC24650 could be implemented to speed up the transient response during the sleep mode. Please refer to page 43 to 44 for the recommend Wake-up circuit and how it works.

    Another alternative is to add some pre-load resistor to keep controller always awake, but the standby power performance will be comprised if the pre-load resistor is added

    I hope this would clarify your question, please let us know if you have any further question.

    Thank you very much.

    Best Regards,

    Wei-Hao Chen

  • 0 in reply to Wei-Hao Chen (TW-FAE)
    Mastermind 8725 points

    Hi, Chen.

    How many pre-load is need?

    How much is lowest frequency of no-load? We observed about 200Hz at no-load, is it correct?

    Best Regards,

  • 0 in reply to Hirotsugu Kobayashi
    TI__Expert 4015 points

    Per the electrical table and figure 41 of the datasheet 200Hz is the minimum switching frequency. Adding preload will increase the switching frequency, how much can be observed by looking at the DRV pin of the IC. You can calculate the minimum switching frequency from the maximum voltage drop you can accept and the output cap and load current. Then you can see how much load is required to get you to that switching frequency. 

    Also check out this app  note explaining how to improve the transient response by using the UCC24650.  

    Thanks

    Billy 

  • 0 in reply to Billy Long
    Mastermind 8725 points
    Hi, Billy.
    Is there no method to increase the minimum frequency at no-load?
    Our customer thinks that minimum frequency is too low, so load transient response is slow.
    Our customer is requesting that minimum frequency is about 1.5kHz.

    Best Regards,
  • 0 in reply to Hirotsugu Kobayashi
    TI__Expert 4015 points

    The minimum switching frequency is fixed inside the IC. Increasing the pre-load will increase the minimum frequency which the converter switches at.