LTC7803 Bootstrap capacitor value

Hi team,

I am considering a circuit driven by a FET with Qg of 50nC.

In the datasheet of LTC7803, it says "The value of the boost capacitor, CB, needs to be 100 times that of the total input capacitance of the topside MOSFET.".

To drive a 50nC FET, it needs 50nC x 100 = 5000nC

I think that it means CB = Q / Vintvcc = 5000nC / 5.15V = 970nF.

Is it correct?

If the above is correct, the CB will be 1uF.

Can LTC7803 keep driving 1uF CB?

I'm worried if the boot voltage can be maintained if the switching frequency is set to 3MHz or if the Duty increases.

I would like to set the switching frequency to 2MHz or higher if possible.

I'm thinking of making 5V from a 12V battery, but I still don't know what the battery drop will be.

I would like to know the restrictions on whether the bootstrap can be retained even if it drops to 7V.

Best regards,

T-yoshi

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    •  Analog Employees 
    on Oct 15, 2021 8:15 AM in reply to T_yoshi

    Hi T-yoshi,

    Sorry for late reply. I hope you got answer to this already. I see Figure 12 in datasheet using two FET with only 0,1uF boost cap so in most applications 0.1uF may be sufficient. If your FET is really big, please try to check in LTspice first and then on a proto board. INVTVCC will also be tied to the boost pin that I believe will somehow help too.

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