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Topic: PCA9685 + N-channel MOSFETS = Gate resistors? (Read 1 time) previous topic - next topic

vile

Hello everybody,

I hope this is the right place for my question.

I am using an Arduino DUE and two PCA9685 to drive multiple n-channel mosfets (IRF7478 logic level mosfets).

The datasheet says on page 29, that setting INVRT to 0 and OUTDRV to 1 allows connecting N-type drivers directly to the outputs. Figure 13 shows this exact case.

But: Is there really no gate resistor necessary?
I am not sure about it, because the PCA9685 can only deliver 10mA as current source (and sink 25mA).
I do not want to destroy the chip if the mosfet gate capacitances draw high gate charging currents. I could not find any information about current limiting or overcurrent protection circuits.

If I use 330R resistors, at 3,3V the current will not exceed 10mA, but that causes bad rising and falling times.

I don't know what to do. I hope there are some experts out there!


Thanks in advance!

Vile

Grumpy_Mike


vile

Thanks for your reply.

Quote
Use A resistor.
But why? Because it is necessary or just to be sure?

With 330R the gate voltage rise time grows from about 200ns to about 3.6ms.

Is there any information about it in the datasheet?

Grumpy_Mike

Quote
Because it is necessary
Yes it is necessary.

A FET gate when off looks like a discharged capacitor. A discharged capacitor looks like a short circuit to ground. Therefore as you apply a step voltage there is a theoretical infinite current spike. You need the resistor to protect the Arduino's output pin.

 
Quote
Is there any information about it in the datasheet?
Yes it says the ABSOLUTE maximum a pin should be asked to supply is 40mA. Most people consider 20mA to be the maximum you should use. The FET does not care about this but your Arduino does. You can use a 130R resistor and have a better rise time or you can use a FET driver of some sort, say a transistor.

vile

I see, but the mosfets are not connected to the Arduino but to the PCA9685 outputs.

I know how the gate capacitance is charged and that it may draw huge currents.
But I don't know if the PCA9685 can drive mosfets directly as shown in the PCA9685 datasheet.

Grumpy_Mike

Quote
But I don't know if the PCA9685 can drive mosfets directly as shown in the PCA9685 datasheet.
Circuits in data sheets are often simplified versions. I would still use a resistor to keep the peak current below the 10mA it specifies.

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