Why do I observe ringing on gate driver logic input?

Question

I have a 3v3 signal that has a nice sharp edge when probed in isolation. However, when I connect this to the input of a gate driver IC (TC42X series) driving a MOSFET switching an 8A load, I see strong ringing when probing the input (yellow). The logic input is supposed to draw less than 1uA. The blue trace is the gate voltage. What is happening here?

Out of curiosity I added an RC filter to the input and observe similar ringing on the input pin when the Schmidt trigger hits threshold in the gate driver. What could be the origin of this strange transient on the data line?

With the load disconnected it’s still there but small

Circuit diagram below

Answer 1

As the FET turns on, two often-unexpected thing happen.

1. as the drain voltage falls, the capacitance between D and G slows down the rise of VGS (Miller effect) -- this because the gate drive has to supply charge to the G-D capacitance. Thus the FET spends a long time in the high gain linear state. This can trigger ringing and oscillations in the circuit (depending on your layout; 8A and some fly-wire components on a perf. board is likely to have issues such as this).
2. 8A switching suddenly through the FET and into the source will cause the source V to rise due to perhaps 10's of nH inductance in the GND line. This will then to turn off the FET and while usually the loop gain is below 1, the system will still ring.

To get cleaner waveforms, you should:

1. move the 22 Ω as close as possible (mm) to the gate.
2. try to get as close to an ideal and single GND as possible. Wire the GND of your driver to the S of the FET. If your gate driver IC doesn't have input hysteresis, then you'll also have to ensure your input control signal also refers to the same GND.