LM25148: MOS Vth

Fred Cheng

Expert 4620 points

Part Number: LM25148
Other Parts Discussed in Thread: LM5145

Hi team,

I'm evaluating the MOS loss with our excel tool, but encounter a question.

as you can see the threshold voltage (1.8V) and miller plateau voltage (3.8V) are different

there's no input column for miller plateau, which one should I input to the calculator?

over 1 year ago

0 Timothy Hegarty over 1 year ago

TI__Mastermind 49387 points

Hi Fred,

You can use the Miller plateau voltage as that is used for switching time calculations in the file. The comments in that cell in the file should also indicate this.

Note 3.8V is somewhat high based on the LM25148 5V VCC. Most logic-level 40/60/80V FETs are in the 3V to 3.5V range, thus allowing adequate overdrive with a 5V gate drive amplitude, especially at cold where the gate threshold and Miller plateau levels increase.

Regards,

Tim

0 Fred Cheng over 1 year ago in reply to Timothy Hegarty

TI__Expert 4620 points

thanks for your instant reply,

but I'm confused why should I input miller plateau voltage if it's named gate threshold voltage in the excel ?

I mean they are different thing originally.

0 Timothy Hegarty over 1 year ago in reply to Fred Cheng

TI__Mastermind 49387 points

Hi Fred,

Apologies for the ambiguity - we will update that in the next revision cycle.

Regards,

Tim

0 Fred Cheng over 1 year ago in reply to Timothy Hegarty

TI__Expert 4620 points

Hi Timothy,

so it's supposed to be miller plateau voltage, got it.

BTW, if I want to use parallel MOS to decrease the Rds_on LS,

does this mean I have to double Qg, Qgd, Qgs, Coss and GFS right?

is there any thing else I have to double?

0 Timothy Hegarty over 1 year ago in reply to Fred Cheng

TI__Mastermind 49387 points

Yes, correct, double the capacitances, gate charges. reverse recovery charge and transconductance. Half the Rdson and gate resistance. The effective thermal resistance will be lower as well (but maybe not half).

Note we typically do not need paralleled FETs unless it's high output current (>15-20A), depending on max ambient temperature and airflow as well.

0 Fred Cheng over 1 year ago in reply to Timothy Hegarty

TI__Expert 4620 points

Hi Timothy,

yes my customer spec is 18A,

and actually I found paralleling only helps LS. HS is dominated by Cgd

+1 Timothy Hegarty over 1 year ago in reply to Fred Cheng

TI__Mastermind 49387 points

Thanks, Fred. You can also reduce the Rdson of the FET. Depending on the figure of merit, it might be better. Take a look at the LM5145 EVM, 5V/20A/225kHz using 80V FETs.

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LM25148: MOS Vth