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UCC256404: Excel Design Spreadsheet

Kok Khuan P'ng
Intellectual 450 points
Part Number: UCC256404

Hi TI,

I have several questions on UCC25640x Design Calculator as below.

  1. For LL/SS, why actual Vssinit in cell - 'C208' is 0V? Although it is Option 5 where LL/SS pin is pulled down by internal 1.2kOhm, it should not get 0V. With 13V of Vrvcc, the actual Vssinit = 13 x 1.2k//316k / (1.2k//316k + 549k) = 0.0282V.
  2. Can you enable the "table and calculation" tab? I am in a 'black box' in LL/SS calculation.
  3. Issup value in cell - 'C197' is 37.5uA while it is 36uA in its datasheet. Which value should we follow?

4. Equation in cell - 'C193' involves actual OVP value, which is incorrect, right? The actual OVP value in cell - 'C192' should be the Upper resistor value in cell - 'C188', right?

5. Would it be OK if switching frequency at no load based on Mg_ꚙ on Mg vs fN curve is more than maximum switching frequency since it would enter burst mode at low load?

6. Is UCC256404 max switching frequency 350kHz?

Regards,

KK

  • 0
    TI__Mastermind 40350 points

    VHello KK,

    Thank you for your interest in the UCC256404 LLC controller.

    Here are responses to your questions:

    1. It was deemed that the Vssinit voltage was close enough to zero to simply initialize C208 to 0V. 

    2. The hidden tab contains background calculations and is not well organized because it was not expected to be shown. If you have a question about some specific LL/SS calculation we can help you with that.

    3. The nominal value of Issup was originally 37.5uA, but changed to 36uA before release.  The calculator tool missed this change.  36uA is the correct value.

    4. You are correct; there is an erroneous cell reference.  It should be to the Upper resistor in cell C188. 

    5. Yes, this is okay.

    6. The UCC25640x series is capable of switching up to 1Mhz, but usually is used lower than this.  A full-load steady-state frequency of 350kHz is comfortable upper design target limit to allow for higher frequencies during start-up, light loads, and transients.    

    Thank you for helping us find these errors.  We'll be upgrading the calculator tool shortly, though I cannot predict when the new version will become available.
    Hopefully very soon.

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 450 points

    Hi Ulrich,

    Thank you for your answers.

    I have further question on previous item-1 and other questions

    1. With Vssinit = 0V, VTH value cannot be acquired according to above equation captured from UCC25640x datasheet (Equation-76). Is Vssinit= 0V correct? Since VTH value cannot be acquired, LL/SS Upper & Lower Resistance values cannot be acquired according to Equation-78 and -79 in its datasheet. If Vssinit is not 0V, what should we do to get LL/SS Upper & Lower Resistance values for Option-5?
    2. The suggested range is 3V and 4.5V according to its datasheet instead of between 3V and 5.5V according to TI Design Calculator for VCR pin voltage peak to peak. Which is correct?
    3. The common mode voltage of VCP pin from datasheet is 3V while is 3.02V from Design Calculator. Which is correct?

    Regards,

    KK

  • 0 in reply to Kok Khuan P'ng
    TI__Mastermind 40350 points

    Hello KK,

    1.  I've been informed that the calculations for the Upper and Lower LL/SS resistors proceeds with the value that Vssinit would be as if Option 5 was not selected.

    2.  3V to 5.5V is okay for the suggested range.

    3. The Vcm is 3.00V.  The typical value in the datasheet specification table was adjusted slightly from a previous 3.02V after the calculator had been developed. This 20mV offset from typical makes less than 1% difference in the results. 

    Regards,
    Ulrich

  • 0 in reply to Ulrich Goerke
    Intellectual 450 points

    Hi Ulrich,

    Can TI share the calculation details on cells - 'C202, 'C204, 'C205', 'C208' and 'C209'?

    Regards,

    KK

  • 0 in reply to Kok Khuan P'ng
    TI__Mastermind 40350 points

    Hello KK,

    I apologize for my delayed reply.

    The cells that you are asking about are carry out the calculation procedure of Section 9.2.2.19 in the datasheet.

    Your cell references appear to be based on the previous version of the Excel calculator tool, SLUC675A.
    This has been updated recently to SLUC675B on the TI website, and I do not have access to the older version.

    Please download this new version, plus make sure you have the latest datasheet version, too.

    Regards,
    Ulrich