I want to have a 5V trigger output signal that has rising and falling edges of up to 200ns max. into a load of 1-5 nF.

I thought about using MOSFET driver circuits (e.g. MC33152, many suppliers have similar). They are able to actively drive to GND and VCC (in my case 5V) with fast edges (range of 10-100ns) into some nF of load.

My problem: Because this 5V trigger will be an external pin, I need to make the driver output pins robust against, accidentially connecting external voltages (20-35V) to it and I have no idea how to do it.

I thought about using a schottky diode to block voltages from outside to go into the driver output. This however would also mean, that I could no longer drive the falling adge actively (because the diode will prevent that aswell). So I would need a pulldown resistor behind the diode, but this would not be enough to get steep falling edges.

How could I protect this trigger output against externally connecting 35V to the driver output pin without losing the possibility to drive the falling edge?

  • \$\begingroup\$ How about a Zener diode in parallel? \$\endgroup\$ – Claudio Avi Chami May 26 '16 at 10:47
  • \$\begingroup\$ I am not sure if I understand. How would the Zener allow the lowside switch to pull to GND? Do you mean anti parallel to the schottky? Could you try to explain? \$\endgroup\$ – Villotron May 29 '16 at 13:29
  • \$\begingroup\$ No schottky needed, only the Zener in parallel with the driver and a small series resistance to the output \$\endgroup\$ – Claudio Avi Chami May 29 '16 at 15:12
  • \$\begingroup\$ some MOSFET drivers have enable inputs with 40ns delay. Could I use the zener and a transistor to pull that enable low, so that the driver will be off? I cant judge if this would be fast enough to protect the device from destruction. \$\endgroup\$ – Villotron May 31 '16 at 8:43
  • \$\begingroup\$ @ClaudioAviChami : If I follow your suggestion with a parralel zener and a series resitor. If I make it large, the protection will be good but also my signal level from the output would suffer more, because a voltage drop would appear on the resistor. If I make it small, I guess the protection will be lower. How do I know what size I should use? \$\endgroup\$ – Villotron Jun 6 '16 at 13:13

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