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TPS53318: Cout_min at load transient of TPS53318

takahiko yokota
Mastermind 7135 points
Part Number: TPS53318

Hi

I have a question about Cout_min of TPS53318.
My customer use TPS53318 the following condition.
I found the document about Cout at load transient as below.
But I could not find the calculation formula at under shoot.
Could you tell me that calculation method of Cout_min?
Or are there the limitation about Cout_min?

 Vin:6.5~12V
 Vout:5A
 Iout:1A ⇔ 7A

 Document: http://www.ti.com/lit/an/slva874/slva874.pdf

Best regards,
Yokota
  • 0
    TI__Mastermind 35165 points

    Hi Yokota-san,

    I have the equation below to estimate the minimum capacitance for a target undershoot for DCAP type control. This includes the min off-time of the converter because will limit how quickly the inductor current can ramp up after a load step up. With DCAP type control the converter will always switch with a fixed on-time. So when there is a load step up, the converter will turn on the HS FET for the fixed on-time then wait for the min off-time to turn on again. It will do this until the output voltage is back in regulation.

    In a 5 Vout application like this the undershoot does start to become more important. This is because with a 5 Vout the overshoot will be less because the larger Vout will discharge the inductor current quicker. With a 1 Vout there is a smaller voltage across the inductor when the SW node is low so the current discharges slower. than a lower output voltage like 1 V.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Mastermind 7135 points

    Hi Anthony-san

    Thank you for the information.
    Please let me ask you further questions.
    Could you tell me adequate value of ΔVout calculating Cout_min?

    Best regards,
    Yokota
  • 0 in reply to takahiko yokota
    TI__Mastermind 35165 points
    Hi Yokota-san,

    ΔVout depends on the requirements for the output of the converter, 5V in this case. If there is not a strict requirement in this application, 4-5% of Vout (0.2-0.25V for 5 V) is reasonable.

    Best Regards,
    Anthony