Op amp power supply sequencing question

[Harry Bissell]

The latch up could be destructive or nuisance...

A lot of op amps will do this if you exceed the common mode range. If you run a voltage follower, it is easy to
exceed the common mode range (both inputs can swing wherever they are driven...) but in the inverting config... one
input is fixed at ground and the other "might as well be..." so it is much harder to get the overload you need.

In DC apps you still have to watch out...

The worst problem is phase reversal... an positive overdrive results in a negative output (instead of clipping at
the positive rail...) This can stay there until power is removed... or it may just be very sticky (wants to remain
at the neg. rail after the overdrive is removed... sometimes for many mS....) in recovery. The trick here is to
limit the circuit from ever reaching that condition... or use a "phase reverse free" op amp.

The 741 and 1458 do not phase reverse...
I think the TL07x and 08x can phase reverse if you let them... but they usually are OK in most apps...
The OP-07 does not... but its improved replacement the OP-77 does. I got screwed on that one in a very expensive
heater circuit.... (platinum wire heater). When it phucked up it cost $1000 (per event... not good)

Linear Tech and Maxim have some op-amps that boast of "no phase reversal" check the data sheets...

The MAD-1108 diode array from motorola may be a sub for the CA3019 (don't know about the matching... probably not
as good as the 3019) Ask TomG. The 3046 could be diode connected also... (havent tried it...)

I need a diode ring for a slew rate limiter I'm designing, so I will be trying this out soon...

:^) Harry

"Mike I." wrote:

> Hi Harry,
> Thanks for the info.(all this power supply protection stuff condensed into one place). I thought (maybe dreamt)
> I read somewhere that some op amp systems had the power supply electrolytics sized so that one side would
> always come up before the other to help avoid some potential destructive problem. I do not know where I put
> that reference (if it actually exists..). The "latch up to the negative rail" problem you mention is probably
> what they were referring to. Is this destructive or just a nuisance effect?  I would like to know which common
> op amps you avoid because of this. I  use the TL072, 4558 and TLC272 for the majority of applications. I once
> hooked my homemade synth (about 60 TL072's, some OTA's and some CMOS logic) BACKWARDS to the +/- 12 V supply.
> Wondered why the LED's didn't light... the fuse didn't blow. Everything survived except for a CMOS shift
> register which shorted out the -12 V side (backwards). The VCO's didn't need retuning, amazingly enough. The
> power supply was on the verge of losing regulation in normal operation. Maybe it just didn't have enough ooomph
> to to a good job of burning everything out. Has a beefier supply now and is permanently mounted in the cabinet
> to hopefully keep this from happening again. I had the phase reversal problem in a state-variable filter which
> would latch-up at high Q settings when it began to oscillate. It used TL074's as unity gain followers buffering
> the integration caps. JH mentioned on this list that  he had a similar problem and recommends using the TL072's
> as inverting integrators  to avoid the latch up. By the way, I cloned a KORG MS-50 diode ring VCF over the
> holidays and it sounds great. Uses a CA3019 diode array. It is a 2-pole Sallen + Key type filter.
> Unfortunately, the CA3019 is very difficult to find now. I had 2 kicking around in my parts box from twenty
> years ago. Regards, Mike