How can I speed up slow-to-rise LM393 comparator open collector outputs?

Thread Starter

Norman Greenbaum

Joined Oct 12, 2023
22
Hi guys, first post here so hoping I don't inadvertently break any rules.

I'm driving the Set and Reset inputs of a latch (a CMOS 555 chip in bistable mode, operating at 8V) from the outputs of the the two comparators in the LM393 dual comparator chip.

The comparators give sharp falling edges, but because of their open collector outputs I get a slow rising edges. In fact the rising output seems to enter a narrow linear domain before reaching saturation.

I don't want to use a low value pullup resistor because it will be powered from a very small solar panel so I have to conserve energy.

Any suggestions on how I might interface these two devices?

Thanks.
 

Papabravo

Joined Feb 24, 2006
21,008
Choose a comparator with push pull output.
A comparator with a push pull output may not be available for a reasonable price. The output of an open collector circuit looks like an RC network. This can be a problem on the rising edge of a signal. The falling edge is not a problem since there is an active device pulling the output low. To make the rising edge faster you can reduce the capacitance, or you can reduce the resistance of the pullup. You cannot reduce it to far or you will force the output to sink more current than it is capable of. If you can look at the rising edge on a scope you should be able to estimate the total capacitance that goes along with the value of the pullup resistor.
 

eetech00

Joined Jun 8, 2013
3,823
Hi guys, first post here so hoping I don't inadvertently break any rules.

I'm driving the Set and Reset inputs of a latch (a CMOS 555 chip in bistable mode, operating at 8V) from the outputs of the the two comparators in the LM393 dual comparator chip.

The comparators give sharp falling edges, but because of their open collector outputs I get a slow rising edges. In fact the rising output seems to enter a narrow linear domain before reaching saturation.

I don't want to use a low value pullup resistor because it will be powered from a very small solar panel so I have to conserve energy.

Any suggestions on how I might interface these two devices?

Thanks.
Just add some hysteresis to each opamp.

Can you post your comparator circuit?
 

dl324

Joined Mar 30, 2015
16,691
I have no idea how to breadboard with SMD devices.
SOIC8 to DIP adapters are readily available.
1697157514450.png
This is another option (I use it for programming 93C46R in SOIC8):
1697157575258.pngBe careful with these. There are two widths and the sellers don't seem to comprehend that.

Neither of these sellers offered free shipping.

When I only had SOIC16 adapters, I cut one in half:
1697157898804.png
 

WBahn

Joined Mar 31, 2012
29,867
Hi guys, first post here so hoping I don't inadvertently break any rules.

I'm driving the Set and Reset inputs of a latch (a CMOS 555 chip in bistable mode, operating at 8V) from the outputs of the the two comparators in the LM393 dual comparator chip.

The comparators give sharp falling edges, but because of their open collector outputs I get a slow rising edges. In fact the rising output seems to enter a narrow linear domain before reaching saturation.

I don't want to use a low value pullup resistor because it will be powered from a very small solar panel so I have to conserve energy.

Any suggestions on how I might interface these two devices?

Thanks.
What size pullup are you using currently?

How slow are they currently rising and how fast do you need them to rise?
 
Top