What is the largest Qg, P channel mosfet that the TPS40200 is rated to drive? I realize this is not a black/white answer because it depends on what efficiency you are willing to accept. From the date sheet is appears the drive capability is limited compared to discrete mosfet drivers. Obviously limited current drive and high Qg will have a negative impact on the drain's voltage rise and fall time which will effect efficiency and mosfet dissipation.
I noted that the FDC5614P is used quite often in the TI application notes, it has a max Qg of 24nc.
Can driving a mosfet with a high Qg, ex 150nc, damage the TPS40200's driver stage or is they internal protection?
For the TPS40200, driving higher Qg may cause more power consumption in the part, therefore, the maximum ambient temperature to run the part will decrease. The calculation steps are as follows.
Ploss = (Fsw x Qg + Iq) x Vin, where Iq is the quiescent current of TPS40200.
Ta,max = Tj,max - Ploss x theta_ja, where theta_ja is the thermal resistance from junction to ambient.
However, I am not concerned much about the loss in TPS40200, but the rise/fall time of gate voltage of the mosfet to cause more switching loss in the fet. Considering the limited driving ability, the 200mA driving current, it will take 0.75us to charge 150nC. The switching loss in the mosfet will roughly be
Psw = 0.75us x Fsw x Vin x Iout
I am not sure your application, but you are right about the impact of high Qg on the mosfet dissipation/efficiency and that is where the major problem is.
My question was general in nature as I often have to remind EEs of the tradeoff between Qg and Rdson.
Thanks for your response.
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