Quote
Cefiar
Regarding the outpus being on when VCC has no voltage:
This is due to the way the pull-up resistors are hooked up and the fact we're using N channel FETs to drive more N-channel FETs.
Basically with no input on the gate of the DMN2075U's (the ones matching outputs 1-4), they are left in their open state. This means that the gate on the IRLB8743's gets all of +VGATE, switching it on.
This basically means that the existing Dual FET arrangement is broken by design if the 3.3v supply fails. :/
Yes, this is a serious design flaw :) Looking at it now, it is too risky to have heaters attached which may turn on, if the Due loses power (e.g. USB is disconnected). It really must be fail safe.
I think there are several options which have been suggested:
1) add a 5V regulator, run the buffers at 5V
2) run the buffers at 5V but remove the DMN2075 drivers and drive FETs at 5V
3) use some P channel FETs
I quite like 2), as it removes several components, including an easy to get wrong jumper. I think the IRLB8743 should be OK at 5V, especially if we use ACT buffers, they probably have more drive than a 5V AVR. The two main aspects are having a high enough gate voltage, which according to data sheet and testing by Traumflug, 5V should be fine. The other is being able to source enough current to have a fast switch on/off, to reduce time in the transition zone.
If the FETs are driven with positive logic, there is no need for a separate 5V, although if we now have room, it would be a good idea to add one. Ideally a switching type which can run 12-24V.
This looks like a really useful way to optimize the design, as well as fix a nasty gotcha.