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TPS61099EVM-768: different switching frequency for the coil on the EVM

Expert 6280 points
Part Number: TPS61099EVM-768

Team,

we are evaluating 'TPS61099EVM-768' and experienced an unwanted feature. As per the calculation mentioned in the datasheet, the coil WE 74404024010 (1uH, Saturation current 2.6A) should be switched with around 280KHz (Uin = 2V; Uout = 3V), but it is getting a switching frequency of around 930KHz.

Hence, we need to know few things about the evaluation board:

1. Whether there are some undocumented changes in the EVM design or not.

2. We considered efficiency of 92% - is it reasonable?

3. We biased the Feedback with 1V. Does it need to be provided with some other bias?

Thanks for your help,
TI Customer

  • Hi Bart:
    The calculation is for CCM. May you share the waveform with IL, Vsw, VIN, Vout?
    I also did a calculation with typical value in CCM: f=Vin*(Vout-Vin*n)/(L*Ilh*Vout)=2*(3-2*0.9)/(1uh*0.3A*3V)=2.6Mhz. Please have a double check.

    1. Whether there are some undocumented changes in the EVM design or not.
    You could found the latest one in TI.com. We have two types EVM, for adjust part and fixed part.

    2. We considered efficiency of 92% - is it reasonable?
    What's the load conditions?

    3. We biased the Feedback with 1V. Does it need to be provided with some other bias?
    Why you need to bias it? And how?
  • Hi Bart:
    I haven’t heard back from you, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out) Thank you~
  • Hi Minqiu,

    We checked the formula again, it should be switching in fact around 364KHz. But we are getting much higher than that.

    Please see the attached waveforms.

    About point 3. - there is a feedback bias (1V) applied to the pin FB (DRV - 3 / YFF - C2) as suggested in datasheet, by a divider sub-circuit (Clause - 9.2.1.1.2).

    Looking forward for your feedback.

    Thanks & regards

    plots.zip

  • Hi Bart:
    May you share the schematic and calculation for me with E-Mail? Thank you.

    You needn't add bias for FB pin. Divider circuit is enough for almost case.
  • Hi Minqiu,

    I've sent to you the schematic via e-mail.

    Calculations:

    Input V [V] Output V [V] Coil Frequency (Hz) Real
    1.993 2.883 930232.5581
    2.193 2.896 784313.7255
    2.392 2.902 714285.7143
    2.587 2.92 625000
    2.791 2.944 588235.2941
    2.89 2.91 800000
    2.942 2.91 800000
    2.99 2.91 800000
    3.186 2.916 740740.7407
    3.25 2.91 0
    3.26 2.92 0
    3.27 2.918 0

    Load = 3.3K.

    The calculation for the frequency we have done as per the equation provided by you earlier. And for feedback we are using 500K as R2.

    Kind regards
  • Hi Bart:
    I'll close the post firstly. Let's contact by Email.