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TPS54340B: TPS54340B Boot pin cap

Qinghong Yu
Prodigy 30 points
Part Number: TPS54340B

Dear TI

What is the minimum capacitance needed on the Boot pin? We have an application stepping down from 12V to 5V. One of our engineers is concerned about that 100nF is not enough if the tolerance of the cap considered and suggested two caps in parallel.

Thanks,

Qinghong

  • 0
    TI__Mastermind 23815 points

    Hi Qinhong, 

    I believe 0.1uF should be ok. With 0603 size and depend on your operation temperature either X5R and X7R so the tolerance value is smaller. 

    Most of designs i see is using 0.1uF. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Prodigy 30 points

    Hi Arief,

    Will 80nF be enough? Or 70nF? We try to use 0402 100nF and I want to use one only. Need have a reason to use any extra part. This cap should be charged every switching cycle to transfer its charge to the gate capacitance of the internal FET. I would think 47nF will be enough. 

    Thanks,

    Qinghong

  • 0 in reply to Qinghong Yu
    TI__Mastermind 23815 points

    Hi Qinghong,

    I think when we validate the silicon for the IC, we use the value of 100nF as a reference. 

    Since we have not use a smaller value, therefore i could not comment on 47nF. This value looks relatively small. Normally it is recommended to use around 0.1uF through 0.47uF for boot capacitor. 

    The condition where the boot capacitor might have an issue is during dropout condition where VIN is close to VOUT. Therefore if you choose a smaller value of capacitor, then you would have to test it to include this condition. 

    Therefore it is still recommended to use the 0.1uF capacitor for bootstrap capacitor. 

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    TI__Guru 57555 points

    Hello

    I will close this thread due to inactivity.

    Thanks

  • 0 in reply to Arief Hernadi
    Prodigy 30 points

    We are suing 100nF cap. However MLCC caps drop their capacitance when there are DC voltage applied across them. How much drop is allowed? Is 15% OK? 20%? In your app note X7R and X5R are used. These are not 5% cap - can be -20% easily. The bottom line is that nominal 100nF cap is not 100nF under certain conditions. Then what is the minimum it allows?

    Thanks,

    Qinghong

  • 0 in reply to Qinghong Yu
    TI__Mastermind 23815 points

    Hi Qinghong,

    I checked a couple of our reference design and the value that we use for the capacitor is 0.1uF with X7R, therefore it is +/- 10% tolerance.

    You can go with 0.1uF, 16V, X7R capacitor as mentioned on this reference design.

    http://www.ti.com/tool/PMP8851

    Thanks

    -Arief