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TPS51220A: Calculation of output capacitance

Part Number: TPS51220A

Hi there,

As stated in the TPS51220A datasheet, Rgv is needed for determining Co.

Now, Vdroop is needed for determining Rgv. However, I have no idea about how to obtain Vdroop.

Could you please advise?

Thanks!

Best regards,

Roy Hsu

  • Hello, Roy,

    Normally, Vdroop is set as 1% of Vout voltage. Vdroop must be less than 3% of Vout.

    best regards.

    edwin.

  • Hi Edwin,

    Thanks!

    Another question I have is, why is the output voltage of TPS51220A limited to 12V?

    My customer is now designing a 19Vin, 12Vout/10A application, would TPS51220A be a good fit?

    Thanks!

    Best regards,
    Roy Hsu
  • Hello Roy,

    The internal loop parameters can cover 1-12Vout. if Vout is larger than 12V, it's unstable.

    Yes, you can use it to design 19Vin, 12Vout/10A.

    best regards,

    edwin,

  • Hi Edwin,

    Thanks for the explanation. I got two other questions below.

    1. Driving capability

        My customer uses 2 MOSFETs in parallel at both the high and low side. How do I know if TPS51220A is capable of driving the FETs?

        Is there any systematic way to calculate this?

        The MOSFETs used is: 

    2. Overcurrent Limit

        The result I got using Eq. (20) in the datasheet is different from the one in the TPS5122x Calculation Tool attached below.

     

    Besides, the OCL value also varies with input voltage which doesn't match Eq. (20).

    Could you please let me know what's the correct way to calculate OCL value?

    Thanks!

    Best regards,

    Roy Hsu

  • Hello Roy,

    1, In MOSFET datasheet, you can find Qg total.

    In TPS51220A datasheet EC table, you can find DRVH/DRVL resister.

    so you can calculate switching rise time and fall time using below equation:

    5/DRVH resister *t= n* Qg.

    n is quantity of MOSFET.

    if rise time/fall time is about 10ns -20ns. it's OK.

    2, please follow datasheet to calculate the OC value.

    best regards

    edwin.

  • Hi Edwin,

    1. There are many Qg parameters in the datasheet. Which one should I use and why?

    2. So Vin should not affect the OCL value, right?

    Actually I couldn't find the TPS5122x Calculation Tool on our website, can we still use that as reference?

    Thanks!

    Best regards,

    Roy Hsu

  • Hello Roy,

    1, you should use Qg(tot). MOSFET is fully open until it is charged Qg(total).

    2, Actually, there is very little difference about the OCL value at different Vin. We can ignore it. if you want know, I can try to explain to you by email.

    yes, we still use that as reference.

    my email is edwin-zang@ti.com

    best regards.

    edwin.

  • Hi Edwin,

    No problem, Let's follow up through email.

    Thanks for your support!

    Best regards,

    Roy Hsu