# [SOLVED] Can you explain this formula to get MOSFET total charge at given Vgs ?

#### kiweered

Joined Dec 13, 2021
28
Hey,
I'm working on a Buck convert (LM5116). I must calculate the gate drive current supplied by the internal VCC regulator to make sure that it's lower than the regulator limit current.
The datasheet says :
The gate driving current is supplied by the internal linear regulator. The gate drive current supplied by the VCC regulator is calculated as:

IGC =(Qgh + Qgl) x fSW
Qgh + Qgl represent the gate charge of the HO and LO MOSFETs at VGS = VCC

To ensure start-up, IGC must be less than the VCC current limit rating of 15 mA minimum when powered by the internal 7.4-V regulator. Failure to observe this rating can result in excessive MOSFET heating and potential damage.
The used mosfet has the following charactéristic :

On a spreadsheet offered by TI, I found the following equation :
$Qg_{(Vgs=VCC)} = Qg_{(Vgs=4.5)}+(VCC-4.5)*Qg_{(Vgs=10)}-Qg_{(Vgs=4.5)}/(10-4.5)$

I can't find the explaination of it, maybe it's just basic linear math and I can't see the it but I'm not sure.
Do you understand it ?

My bad, I spent 1h looking at the maths behind the formula but I didn't read it correctly on Excel... The formula was $Q4.5 + (VCC-4.5)(Q10-Q4.5)/(10-4.5)$ which makes a lot of sense...