Overvoltage protection on CMOS inputs

Adam-V

I have a mixture of Doepfer and Roland 100m which can supply gate and trigger levels ranging from 8-12V on the Doepfer (despite their specs saying 5V!) and 15V on the Roland and was thinking that it would be mightily handy to be able to just plug modular synth signals straight in to the Lunetta blocks without having to run them through a level converter block.

My initial thoughts were to use a 9V supply so that the outputs from the Lunetta blocks could be plugged directly into both modular systems (except for any of the audio inputs obviously) but then the Lunetta block inputs will be damaged by the signals coming from the modular so I was wondering if there is a relatively simple way to protect the inputs of CMOS devices from overvoltage?

Any ideas or suggestions would be greatly appreciated.

Cheers,
Adam-V

bugbrand · Posted: Wed Apr 02, 2008 12:38 am Post subject:

Well, actually, many CMOS chips have input protection diodes already on them (I was looking at the 4069 and 40106 datasheets last night).

Otherwise, I'd have thought a suitable zener (9v?) on the input? - could actually be a bit lower as triggering would usually be 1/2 of the voltage supply..

May be babbling!

blue hell · Posted: Wed Apr 02, 2008 5:02 am Post subject:

Do not use CMOS internal protection diodes for over voltage protection, unless you did the calculations to make sure that no more than around 0.3 uA will run through those diodes.

These diodes are not designed for over voltage protection and if you carelessly rely on them you will blow them. The device will still seem to work with blown protection diodes usually but the input impedance of the pin will be reduced to a few kOhm, they are not up to speed specs anymore and will draw much more current from the power supply.

Adam-V · Posted: Wed Apr 02, 2008 11:00 pm Post subject:

OK, I've had a bit of a dig around the interweb and although most of my searching resulted in pages about the internal protection diodes I did find some useful information.

I was thinking that (as suggested by Tom) just sticking a zener diode on each input as shown in diagram 2 below might be OK but it would appear that a circuit mimicking the internal protection circuit using schottky diodes is the safest way to go especially when coupled with a zener diode to keep the supply rails at a reasonable level. Something like the one in diagram 1 below.

Cheers,
Adam-V

InputProtection2.JPG

Description:

Diagram 1

Filesize:

12.74 KB

Viewed:

8792 Time(s)

InputProtection1.JPG

Description:

Diagram 2

Filesize:

7.57 KB

Viewed:

8792 Time(s)