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maximum hysteresis from a LM311?

M

Mook Johnson

Jan 1, 1970
0
I'm looking to use a LM311 to square up a signal from a opamp on +/-15V
rails.


The configuration I'm using is to have positive feedback to the +pin via a
100k resistor and a 10K resistor from the plus pin to ground. Output is 0 -
3.3V (1k pullup) so that gives about 300mV of hysteresis. The signal is
applied to the - pin.

I'd like to increase this to at least twice this level. I've been warned
not to give too much positive feedback or it will oscillate. How much
hysteresis have you guys gotten out of this part with this type of
configuraion.

The designs in the app note have a high parts count and I'd like to keep it
simple if possible.
 
I'm looking to use a LM311 to square up a signal from a opamp on +/-15V
rails.

The configuration I'm using is to have positive feedback to the +pin via a
100k resistor and a 10K resistor from the plus pin to ground. Output is 0 -
3.3V (1k pullup) so that gives about 300mV of hysteresis. The signal is
applied to the - pin.

I'd like to increase this to at least twice this level.

So drop the 100k resistor to 47k.
I've been warned
not to give too much positive feedback or it will oscillate.

Try to remember who warned you, and pay less attention to their advice
in the future.

Hysteresis in comparators is usually used to kill oscillations, by
forcing the comparator to switch rapidly between one stable
configuration and another. Getting a comparator stable when you
haven't got much hysteresis to keep the inputs away from the sensitive
region requires you to be very careful with layout and power supply by-
passing, to block the sorts of feedback paths that can support
oscillation.
How much
hysteresis have you guys gotten out of this part with this type of
configuration.

The standard triangular wave and sawtooth wave generators typically
run with a hysteresis of about one third of the supply voltage. 600mV
should be fine.
The designs in the app note have a high parts count and I'd like to keep it
simple if possible.

Changing the value of one resistor won't increase the parts count.
 
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