Teensy 3.1 open drain voltage tolerance?

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jwatte

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I have a project where I want to pull the gate of a power P-MOSFET down to ground using a Teensy output pin.
This gate is by default pulled up to +15V through a 47 kOhm resistor.
Can the open drain switch on the Teensy GPIO pins take that?

(And what happens before my program has managed to configure the pins to open drain?)

Or do I need to use the pins to drive a buffer (a signal MOSFET like the BS170) which in turns pulls down the P-MOSFET gate?
 
The Teensy 3.1 runs on 3.3V and I'm pretty sure there is a gate protection diode that will clamp any GPIO pin to one diode drop above 3.3V regardless of pin configuration (otherwise the chip would be too sensitive to ESD damage). That means that the gate of your P-Channel FET will be clamped to +4V by the Teensy, and the FET will never turn off, assuming the drain and source are supposed to be switching a +15V rail.
 
there is a gate protection diode that will clamp any GPIO pin to one diode drop above 3.3V regardless of pin configuration
Click to expand...

That's a very good observation! I'm going to have to go with the additional buffer transistor then.
 
But board space! I actually have four rails to control, so there are another four discrete transistors to pack in.
 
jwatte said:
But board space! I actually have four rails to control, so there are another four discrete transistors to pack in.
Click to expand...

Pre-biased transistors may be what the doctor ordered. One SOT23 per channel shouldn't be that bad re: board space.
 
If I had the luxury of surface mount and my choice of parts, I'd go with a driver array of some sort!
But as it is, my parts come from the existing bin of through-hole, and I need a pull-down resistor on the gate for the high-impedance state before the microcontroller comes on, and this is easily a square centimeter, each.
Which is where the idea of driving the power P-channel gate directly came from, but it is unlikely to work, so I know what to do....
 
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