LM25149-Q1: Question about MOSFET drive capability

As additional information, Customer usecase is as below.

・Vin=4V~36V(Load dump)

・Vout=3.3V

・Iout=23.5A

・Fsw=500kHz

Thanks,

Yuta Kurimoto

Hello Tim,

I got additional question from customer.

Q. How much output voltage of HO pin during output Hi?

The datasheet says the VCC to CBOOT voltage will be 0.8V at ICBOOT=20mA, But customer can't understand how much output voltage the HO pin become.

Thanks,

Yuta Kurimoto

Hello Tim,

Sorry for asking again about previous question.

Q. When customer use Qg=46nC MOSFET, how much time of "turn-on" and "turn-off"?

Is it difficult to estimate the rough value of turn-on and turn-off time at such low Vin? Customer would like to get the value(Roughly estimation is ok)

Thanks,

Yuta Kurimoto

Additional information.

The P.N. of MOSFET which Customer plan to use is SQJ858AEP.

Thanks,

Yuta Kurimoto

Hello Tim,

Let me confirm my understanding.

> More important for switching times is the Qgd of 6nC.

Q. Does the 6nC means below?

> Igate = Cgd*dv/dt = (5V - 3V) / 3.5Ohms = ~0.6A

>  (allowing 0.5Ohms for the gate driver and including the internal Rgate of this FET = 3 Ohms).

Q. Does the 5V means below?

Q. Does the "Rgate of this FET = 3Ohms" means below?

Q. Does the "0.5Ohms for the gate driver" means below? In my calculation, should it be 0.08V/100mA=0.8Ohms?

Q. For the turn on time calculation, My understanding is Q=it, t=Q/i=6nC/{(5V-3V)/3.5Ω} = 10.5ns. I think 120ns is too slow. Is my calculation correct?

>0.6A into 6nC gives a dv/dt of 0.1V/ns, so a typical voltage rise time for a 12V input is approximately 120ns.

Q. It would be helpful if you can explain why 12V conversion is needed

Thanks,

Yuta Kurimoto

Hello Tim,

No problem. Customer said appreciation to you that you showed detail calculation procedure. Then they finally got correct value.

So Let me close this question.

Thanks,

Yuta Kurimoto