Hysteresis Comparator Methods

[Jim Thompson]

Conserning use of comparators and SR latchs to make a hysteresis
comparator..

I'm keeping an eye on that SR latch idea..
The SR latch has that illegal input state (race condition).
My spec is 100mV of hysteresis and this circuit is part of a
smps controller..
Could be like finding a quiet spot in a noisey dance club

I suspect as hysteresis approachs 0 there's more probability
that the SR latch gets simultaneous S and R signals due to noise
presented to the comparators. Just getting scared by this quirk,
but I won't rule it out yet..

I had to review my latch operation and found:
http://www.allaboutcircuits.com/vol_4/chpt_10/4.html
Which describes the D latch without illegal states.

For choice of latch in the circuit...
Perhaps SR is good for wide hystersis and clean signals and D
latch for small hysteresis and dirty signals??

D

Except that the circuit at the above URL is NOT a "D-latch" in the
conventional sense of the word... a "D-latch" is edge-triggered, and
the circuit at the URL is "re-settable" as long as "E" is high and
thus is called a "transparent latch".

...Jim Thompson

 

[D from BC]

Ooops..You're right..
I'm looking at the wrong latch to review...
I'll be reading...
http://www.allaboutcircuits.com/vol_4/chpt_10/5.html

Also...
I've been writing D latch all this time...
Saying positive edge triggered D type flip flop would be more
descriptive.. Like the SN74AHC74.

I hope I don't get bounced into the electronics.basic group for
that one...

D

 

[Jim Thompson]

Ooops..You're right..
I'm looking at the wrong latch to review...
I'll be reading...
http://www.allaboutcircuits.com/vol_4/chpt_10/5.html

Also...
I've been writing D latch all this time...
Saying positive edge triggered D type flip flop would be more
descriptive.. Like the SN74AHC74.

I hope I don't get bounced into the electronics.basic group for
that one...

D

No. Happens all the time... promulgated by lousy data sheet writers
;-)

...Jim Thompson

 

[John Larkin]

Conserning use of comparators and SR latchs to make a hysteresis
comparator..

I'm keeping an eye on that SR latch idea..
The SR latch has that illegal input state (race condition).

It's not illegal. And in this case, simultaneous set and reset
shouldn't happen. If it did happen, it would just extend the prop
delay of the whole thing a little.

For choice of latch in the circuit...
Perhaps SR is good for wide hystersis and clean signals and D
latch for small hysteresis and dirty signals??

Can't see why. Using edge-triggered flops just seems, to me, to add
asymmetry and complexity to a simple concept.


John

 

[Ken Smith]

In regards to using the 555 as a hysteresis comparator....

The 555 is a great suggestion.
I'm somewhat stuck using those internal resistors.
But that might turn out ok...Will be checking specs.
Nationals LinMos sounds like an improvement to the 555.
It's one of the few IC's I know of with comparators and a latch.

IIRC: There nothing stopping you from connecting a reference to pin 5.
This makes the THR voltage fairly well known.