Comparator with driver output

Without knowing the output drive requirements, the TLV360x may be a place to start:

You can add hysteresis to most comparator circuits; something like a NE555 has builtin hysteresis and good pullup and pulldown capability.

What input and output range do you need? What's the load? How fast do you need?

I don't understand that. I use opamps as comparators now and then.

Mouser and Digikey are good for searching for stuff like this, from multiple vendors.

This is when I use the Digikey search. If you type in comparator it will take you to a search page with many parameters to adjust. You can select the output type and many other features.

Yeah, but... that's in-the-box searching, and it'll miss possibilities like using an op amp (or an unused section of a quad) or a discrete two-transistor circuit, or NE555, or TLV431, or power sequencer, all of which have comparator character. The "comparator" label isn't always the useful first-search-box criterion.

LTC8741 appears to have a symmetric output.

Clifford Heath

Not sure what your point is. If you want a comparator, Digikey is a good way to search for one. If you want an alternative circuit, then search for that. What do the two have to do with one another?

Interesting you mention transistor circuits. I needed to design a FF to remember power state when everything is shut down, including the MCU, which would only wake periodically. Because of the circuit, it worked better to power it from 12V, which could be as high as 15V, iirc. Digital logic was a bit too fast, making it subject to noise, etc and other concerns I don't recall at the moment (I think it had to do with undefined behavior with both set and reset asserted). So I opted for a transistor circuit, which included several more FETs for input and output drive, some in other than common source configurations. I checked it nine ways from Sunday, so I had confidence it would work fine. Since a handful of transistors would be easier to source than a couple of 4000 series CMOS, it wanted to use the transistors. Being a group project, some guy who never saw a Linear Tech device he didn't like (meaning it wasn't his money he was spending, why use something cheap when several things expensive would do?) kept criticizing it, not in engineering terms, but negging terms like kludge, etc. What he really meant, was, he didn't understand it.

So how do you use two transistors to form a reasonable comparator? Is this something that only works in some special set of conditions? Some sort of positive feedback, perhaps?

I can't think of any time when an opamp would be preferable to a comparator, unless the requirements were very lax and you simply wanted to reduce the BoM.

(I interpret this to mean he wants to exclude midrange parking of the output... which generally just requires some positive feedback, creating hysteresis)

The application could be comparison to midrange of power supply, and benefit from hysteresis, and require a high current output... so, an NE555 is a good off-the-shelf solution. Or it could just need a repeatable ON threshold, and a CMOS power monitor would do that. A two-transistor Schmitt trigger also has bistable output and an input threshold. True comparators with input common-mode insensitivity are not the only choices in reality, but are the only ones that will show up in a search.

I often use resistors to add positive feedback to a comparator, and of course the Schmitt trigger is a useful two-transistor circuit, might be preferable. Once you use positive feedback, an op amp is interchangeable with a comparator (no need for stability as with negative feedback, just gain).

One time I used an op amp that way was an old design, LM301 (no compensation required) when I wanted a near-positive-rail input; the old comparators didn't match that requirement. On other occasions, I've simply had good stock of the op amps but not of comparators... I was mainly doing one-off jobs, didn't want to delay for a trip to/shipping from a supplier. Yeah, lax requirements is a major 'reason'.

Microchip has some nice little comparators with push-pull outputs: MCP6541 (low power) and MCP6561 (fast)

piglet

Am 17.11.22 um 21:20 schrieb snipped-for-privacy@gmail.com:

LMV7219 M7

cheers, Gerhard

Not so, but far otherwise. Internally-compensated op amps trade off a lot, a lot of slew rate for feedback stability.

Plus external positive feedback is fairly slow, on account of input capacitance and internal delays, potentially leading to artifacts for wideband inputs. Internal hysteresis can be nice and fast.

Oh, and of course it causes kickout from the noninverting input.

I used to really like the MAX900 quad 10-ns comparator, circa 1996--it had a latch output that could be driven from its own output (via an RC highpass) without misbehaving. That made it easy to get a repeatable pulse without kickout.

Cheers

Phil Hobbs

Some of the rrio opamps are pretty good comparators. OPA197 is a nice ca 1 us comparator. It comes off rails clean and fast.

Nice trick.

MAX9690 was a wonderful 1 ns ECL comparator... until it wasn't.

ADI makes the only fast comparators that I know of. ADCMP562 has programmable hysteresis. ADCMP582 is 37 picoseconds and has well under

No spice models.

and, FWIW, there is a 3 (?)-or-so BJT comparator circuit in Tom Fredericksen's Intuitive IC Op Amps book. (probably there to explain the core of a LM311) So, Rick, you're in good company.

Yeah, you've told me how to build a watch, but not answered the question, why would I care? If I'm looking for a comparator, the Digikey search gets me what I want. If I want something else, why would I search for a comparator?

Ok, so special conditions. Got it.

Ok

John Larkin likes using LVDS receivers as fast comparators. they will have driver outputs.

If the input signal levels are compatible and you can wear the typically larger offset votlage you may find that could be a solution.

The trouble with using a digital differential input as an analog comparator, is they are not specified for the same sort of performance. For example, the input offset can typically be rather large. Maybe in experiments, they prove to work well with low offsets, but there's no good reason to use such a device in an application it is not specified for... or I should say, unspecified for.

Also, LVDS receivers seldom have wide common mode ranges.

No part is cheap if it takes up space on a crowded board.

Many Greenpak devices have comparators in them, but few have opamps.

What are you talking about???

I like that you picked a part that comes in tiny packages, like DIPs. Even the SOIC is a large package these days.

Figure 1. SOIC and PDIP Packages

WTF is going on with you? An opamp and a comparator is always going be a larger package than a simple comparator that does the job.

Beware note 5!