stabilizing a comparator

[kell]

I was looking at this guy's voltage regulator

http://homepage.sunrise.ch/mysunrise/joerg.hau/mot/voltreg.htm

T1 acts as a comparator with positive feedback through R4 for
hysteresis. To eliminate oscillation, he adds the capacitor C1.

"Capacitor C1 stabilises the circuit at higher load. I have observed
that the switching behaviour of the circuit
without this capacitor is pretty "sharp" and stable if the load draws
less than ca. 0.5 A (typically); however
at higher current I observed a tendency to oscillate. This can be
observed best with an oscilloscope,
or a 12 V lamp as load: it will start to glow, instead of being
switched completely on or completely off.
C1 eliminates this instability"

I built a voltage regulator using that circuit and it works. It does
oscillate without the cap, and installing the cap
in the circuit prevents the oscillation.

I was wondering if I could use the same strategy with a comparator.
I have a project where a comparator will have high impedance,
slow-changing inputs, so I'm
concerned about oscillation.

I was skimming through the National Semiconductor datasheet for LM*111
comparators and they showed a couple of strategies
for preventing oscillation, but no cap from the output to the negative
input. Is there some reason it wouldn't work on a comparator?
I'm actually going to be using LM2903.

 

[John Popelish]

I was looking at this guy's voltage regulator

http://homepage.sunrise.ch/mysunrise/joerg.hau/mot/voltreg.htm

T1 acts as a comparator with positive feedback through R4 for
hysteresis. To eliminate oscillation, he adds the capacitor C1.

"Capacitor C1 stabilises the circuit at higher load. I have observed
that the switching behaviour of the circuit
without this capacitor is pretty "sharp" and stable if the load draws
less than ca. 0.5 A (typically); however
at higher current I observed a tendency to oscillate. This can be
observed best with an oscilloscope,
or a 12 V lamp as load: it will start to glow, instead of being
switched completely on or completely off.
C1 eliminates this instability"

I built a voltage regulator using that circuit and it works. It does
oscillate without the cap, and installing the cap
in the circuit prevents the oscillation.

I was wondering if I could use the same strategy with a comparator.
I have a project where a comparator will have high impedance,
slow-changing inputs, so I'm
concerned about oscillation.

I was skimming through the National Semiconductor datasheet for LM*111
comparators and they showed a couple of strategies
for preventing oscillation, but no cap from the output to the negative
input. Is there some reason it wouldn't work on a comparator?
I'm actually going to be using LM2903.

It is not so important to stop oscillations, (since this
regulator always oscillates between on and off) but to
control the frequency of oscillation. You want to keep the
switching down in the frequency range where the transistors
have lots of gain and the switching time of the darlington
is short, compared to the on or off time. In other words,
you want to stay away from high frequencies where the
switching becomes linear, instead of digital, and the
switching losses become very large.

You also don't want the regulator switching frequency to
lock on to the rectifier ripple.

All that said, I am not sure the schematic shows the best
way to accomplish those goals, but at least it rolls of the
gain faster than without the cap.

I see no reason (except, possibly ruggedness) that you
couldn't use an integrated comparator in place of the first
transistor. Keep in mind that the zener reference tempco is
compensating for the tempco if the transistors, so a
different reference (more temperature stable) may give a
more temperature stable result with a comparator.

 

[Chris]

I was looking at this guy's voltage regulator

http://homepage.sunrise.ch/mysunrise/joerg.hau/mot/voltreg.htm

T1 acts as a comparator with positive feedback through R4 for
hysteresis. To eliminate oscillation, he adds the capacitor C1.

"Capacitor C1 stabilises the circuit at higher load. I have observed
that the switching behaviour of the circuit
without this capacitor is pretty "sharp" and stable if the load draws
less than ca. 0.5 A (typically); however
at higher current I observed a tendency to oscillate. This can be
observed best with an oscilloscope,
or a 12 V lamp as load: it will start to glow, instead of being
switched completely on or completely off.
C1 eliminates this instability"

I built a voltage regulator using that circuit and it works. It does
oscillate without the cap, and installing the cap
in the circuit prevents the oscillation.

I was wondering if I could use the same strategy with a comparator.
I have a project where a comparator will have high impedance,
slow-changing inputs, so I'm
concerned about oscillation.

I was skimming through the National Semiconductor datasheet for LM*111
comparators and they showed a couple of strategies
for preventing oscillation, but no cap from the output to the negative
input. Is there some reason it wouldn't work on a comparator?
I'm actually going to be using LM2903.

If I could add to Mr. Popelish's observations, you need to be careful
if you're using a cap as feedback to avoid oscillations. If you're
using a cap for feedback, you might end up with the input voltage at
the inverting input going below the voltage at pin 4. If that happens,
you'll likely destroy the IC.

You might want to use a smaller value resistor in series with your cap,
and have that in parallel with the higher value feedback resistor
you're using to prevent oscillation.

But it really shouldn't be necessary to have very much hysteresis to
prevent oscillation with a slowly changing input voltage. Other causes
are usually the culprit of the bane of comparators (e.g. locallized
power supply bypassing, glitches in Vcc due to driving either large
loads or inductive loads). This oscillation *is* a problem, but the
solutions usually are elsewhere. Try setting switching voltages with a
voltage reference or something other than the Vcc, being very careful
with driving inductive loads like relays, and providing good local
bypassing of the comparator power (possibly including using a small
resistor in series with the power supply, like this:

|
| VCC
| +
| |
| 22 ohm .-.
| | |
| | |
| '-' 0.1uF
| | ||
| o---||---.
| | || |
| | ===
| |\| GND
| -|-\
| | >-
| -|+/
| |/|
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Another thing to remember is that some single supply op amps like the
LM358 can also be used like a slow comparator, if they're driving a
ground-referenced load like an NPN transistor base. It's not the most
elegant solution, but in some cases where switching oscillation is
driving you nuts and you can keep your input voltages more than a
couple of volts below Vcc, they can provide something that works.

Good luck
Chris