3.3V supply opamp design with 1.8V devices

[rampat]

Hi All

I need to design a opamp with 3.3V supply and only 1.8V devices are available from the process.How to take care of the reliability issues for the devices? If anyone knows solutions or any reference/papers to the above problem it will be a great help.


Thanks
rampat

 

[erikl]

If you need a 3.3V supply, in any case your process must provide this option, i.e. a thicker (3.3V capable) gate oxide. If this is possible, you could create your own 3.3V transistors both in schematic, symbol, ..., layout if you know how to take care of, but what about the simulation models? Even if you could get hold of such models from a very, very similar process technology, would you trust them enough for a tape-out? May be for a university project study, but I'd never take the risk for a real company project.

 

[rampat]

Erik l,Thanks for your reply ....
I missed out provided couple of information...The process has devices which can handle VDS=3.3( drain extended mos) or VGS=3.3( thick oxide).How do use these devices so that either of the junction VDS or VGS should not experience reliability issues especially during start up..I am looking for circuit techniques which can solve these issues..

Thanks

 

[jgk2004]

If you make sure you have lots of cascoding to make sure each transistor doesn't see more then 1.8V you can do it. If there is a place where you can't ensure that a voltage across the device is less then 1.8 then use the thick oxide devices. You can do what your saying but it can be tricky.

Jgk

 

[erikl]

... so that either of the junction VDS or VGS should not experience reliability issues especially during start up..I am looking for circuit techniques which can solve these issues..

Hot electron injection is the problem.

You should use an enable facility anyway. During start-up (and shut-down) have all these critical MOSFETs either fully disabled (Vgs=0) or fully enabled (Vgs=VDD_3.3V rising/falling). The latter category you'll need to disable the others.

Try to avoid full-scale Vgs changes (0 <--> 3.3V) during Vds > 1.8V .