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UCC28780: The way to increase burst rate frequency on light load condition

Satoshi
Guru 19645 points
Part Number: UCC28780
Other Parts Discussed in Thread: UCC28782

UCC28780 datasheet is described burst rate frequency (fBR(UP) and fBR(LR)): 25kHz~34kHz for mitigate audible noise.

My question is; Is there any way to increase fBR(UP) frequency by external components?

Or, if UCC28780 can disable burst mode frequency (= set fixed PWM frequency) by any setting, it is also OK.  

My target is fBR(UP) set higher than 50kHz, because lower 50kHz frequency occur noise problem by one customer's application condition. 

Best regards,

Satoshi

  • 0
    TI__Genius 9790 points

    Hi, Satoshi

    it is better to use UCC28782 rather than UCC28780. 

    Jaden will give you more comments, please expect a response by next Monday.

    Yunsheng

  • 0 in reply to Yunsheng Qu
    TI__Expert 7080 points

    Hi Satoshi,

    The burst frequency range 25khz ~34kHz can't be changed , it's fixed by controller.Basically, it should not hear the noise if the burst frequency >20kHz , since the audible noise range for human is from 20hz to 20kHz. Could you please share some waveforms with a little long time scale when you can hear the noise ,I afraid the ABM mode on your board is unstable , it mixed with <20kHz and >25kHz burst frequency  that you just ignored the <20kHz portion.

    Thanks.

  • 0 in reply to Jaden Ning
    Guru 19645 points

    Hi Jaden

    Thank you for reply,

    I attached FFT waveform below.

    These noise is not hear by human, but inaudible noise occur on lower 50kHz.

    One customer is afraid to failed the measurement test by noise.

    If there way to suppress these noise, please let me know.  

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Expert 7080 points

    Hi Saoshi,

    May I ask you some questions:

    1. Since human can't hear the noise of 50khz , why your customer still concern about this frequency ?

    2. What test standard requested to test noise at 50khz and can you tell us what is the frequency test range of the standard.

    3. Think about that :  even if we can push the burst frequency higher than 34khz, such as 100khz , we still don't know if the customer has some concern on 100khz.

    Anyway,  when we define the burst frequency range 25khz ~34khz,  it has already higher than human can hear 20-20khz with some margin. it is fixed by controller, we can not help you to change it . the way to suppress the noise is another topic . firstly you have to find out the source if noise , is from transformer or is from MLCC ? before that , we should to know if it's a real issue or it just a concern .

    Thanks.

  • 0 in reply to Jaden Ning
    Guru 19645 points

    Hi Jaden

    Thank you for reply,

    I will confirm about the reason of noise, transformer or MLCC.

    Specifically, customer's condition is High resolution audio, these application is expanded frequency range of 50kHz.

    Actually human feel to hearing clearly and no trouble, but possibility to mention the concern point by customer internal measurement test, because its noise sensitivity application. 

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Expert 7080 points

    Hi Satoshi，

    Thanks for your check , let's discuss the specific question rather than a concern.

    Thanks.