[sdiy] Latch-up question

[harrybissell]

Essentially, you've got it. There are a few problems.

1) IF the circuit is capable of restoring the input condition. Often
this is not the
case... you are expecting a positive output that suddenly becomes
negative...

2) usually the recovery from phase reversal is slow. The circuit
basically gets
'stuck' to the rails.

I had phase reversal in a servo system... the OP-07 did NOT "feature"
phase
reversal, the OP-77 IMPROVED OP-07 (may it burn in hell) did.   This
made
my heater circuit run for the rails... at about 900C the platinum heater
(yessir, platinum
18 gauge wire !!!) would start to sputter off onto the Vicor tube
(quartz glass)...
It made VERY pretty Jack Frost patterns on the glass and cost about
$1500 every
time it happened in the field.  And it happened more than once before we
got to SEE
what the cause was...

I'd suggest NOT allowing the inputs to reach that condition, or choose
an opamp that
does not have that feature.

Usually... diffamps are immune to this (because the voltage divider
action reduces the
'common' mode).  Inverting amps are usually OK (one input is ground, and
the feedback
makes it tough to reach any voltage other than ground)... but voltage
followers can really
get hit... comparators are even worse.

In a comparator case... maybe using input resistors and back to back
clamp diodes
across the inputs might help... This makes the inputs have a lower
impedance (that of the
resistors) but it beats having the circuit latch up :^P

H^) harry

fmg wrote:

> Hi all
>
> I'm working in an adsr-r/env_gen and am facing the posibility
> of phase-reversal issue in one (or two) of TL08x I included
> in the design.
>
> What follows is an excerpt of National's AN447:
>
> >"In open-loop operation (i.e., no feedback from output to in-
> >verting input, or, during any time that the opamp is slew-rate
> >limited), taking the non-inverting input below the inverting
> >input causes the output to slew toward the negative supply
> >rail. If the voltage at the non-inverting input is more negative
> >than the negative common-mode limit, the input stage ceas-
> >es to function properly and the output swings to its positive
> >limit. This apparent ``phase-reversal'' is temporary; bringing
> >the non-inverting input back within the legal input common-
> >mode range restores the part's normal operation."
>
> Now, wich is the "negative common-mode limit" ?
>
> In the TL081 datasheet is spec'ed:
>
> >Differential Input Voltage +/-30V
> >
> >Parameter           Condition       Min        Typ
> >------------------------------------------------------------
> >Input Common-Mode       Vs +/-15V     +/-11V    +15V/-12V
> >Voltage Range
>
> Should I consider -11V the limit for the non-inverting input to
> be in a safe area from latch-up?
>
> ...even if the inverting input is at a diode below Vdd (+14.4V) ?
>
> Are these assumptions wrong?
>
> I'll appreciate any comments about.
>
> Thanks,
>
> Fabio Gonzalez