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I am trying to use FDMQ8403 IC which has 4 nmos. It will be used for an h-bridge, so 2 of these will be for high side and 2 for low side.

I am trying to drive the hide side nmos with MIC5018.

On page 5 of the driver chip it shows the functional diagram shown below. I just started learning about gate drivers. My understanding is we use a gate resistor to prevent the MCU or whatever is driving the gate to have excess current drawn from it, thus saving the driving component.

The question is why don't they include a resistor between the gate of the mosfet and pin G of the driver?

Thanks

enter image description here

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  • \$\begingroup\$ Who is to say that it isn't inside the charge pump. \$\endgroup\$
    – Andy aka
    Oct 6, 2020 at 15:32

2 Answers 2

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The gate driver can only sink a few mA (maybe as much as 20mA with a 9V supply) and can only source at most about 100uA so it's unnecessary.

This kind of driver is only suitable for very slow switching. Milliseconds in the case of turn-on, because that charge pump is really wimpy. Say the internal capacitor is 30pF, a rather large capacitor for integration on a chip. It only operates at ~80kHz so it will only be able to supply a very small current. Q3 is another matter, but you can tell from the discharge times that the typical current is quite modest.

enter image description here

A driver that can sink or source amperes such as the TC4420 is much more likely to need the on/off times extended.

Of course there is nothing stopping you from adding an external resistor if you want to make it even more glacial in response time.

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So,

How fast the gate needs to be drive and how fast you want to turn ON /OFF the MOSFET both factor used in deciding the gate resistor (if the driver is able to provide that much inrush current) and bootstrap capacitor.

As fast you will turn MOSFET ON/OFF EMI in your product will increase. As slow you will turn MOSFET ON/OFF more heat will be dissipated.

so MOSFET driver manufacturer that's why don't put the resistor inside to let the user choose the resistor according to need.

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