opamp tri state output
DenisMark said:
The first of all you need some detection circuit which will detect vdd1<4V condition and force opamp output in Z state. There aren't any problems with output nmos.
About pmos there are several option possible:
1) Use diode in series with output pmos if you have enough room. It will work in forward region during normal operation and in reverse in Z state. Be care in classical CMOS process you can't use forward biasing w/o latch-up possibility.
2) Use other pmos in series. But exchange source and drain. So it's parasitic diode will be forward biased in normal condition. Use gate to eliminate parasitic diode action in normal condition (rule of thumb: drop on this pmos should be less 350mV).
This is feasible solution for CMOS. It's some kind of controling rectifier.
3) if your pmos has similar breakdowns for source and drain junction, you can switch bulk through pmos switches and connect it either source or drain in depence of operation mode.
All of this solution are frequently used in "high voltage tolerant" I/O buffers.
May be not all of this points are clear explained. So let me know if you will face with some doubts.
Thank you, DenisMark, for your great information.
Yes, My design has a detection circuit which generate indication signal, which is equal to Vdd1 whenever Vdd1 >4V, and 0V whenever Vdd1<4V. I have no probem with output NMOS.
I can not use the first method you gave, which uses a series diode, because my deisgn need rail to rail output.
For second method, I attached a brief Class AB Op Amp based on your suggestion, that is, M14 and M15 in series and M15's source and drain is swapped.The two diodes are parasitic ones related to M14 and M15 respectively. Normal opamp only have M14 here, for tri-state, your suggestion is to introduce M15, right? I still have some question about this method. When Vdd1 is from 0 to 2V, and output is pulled down, the gate of M14/M15 is not well defined, that means, M14 and M15 could not be totally turned off. Then there will be some current runs through M14 and M15. That means output is not 0V. So, how to bias the gate of M14 and M15 so that M14 and M15 are turned off totally? Thank you very much for your help. Also, how to make the drop on M15 less than 350mV. My design require rail-to-rail output. Thanks!