Comparators -- an evolving component Todays parts differ from the "ordinal" comparator, if there ever was such a beast, and tomorrows will likely differ again.
" The other way of looking at it, is it’s the 'same direction' of behaviour as an op amp, but without the upper drive-high output transistor." These days, Open Collector and a push pull outputs are both available. Beyond application considerations the main delta is that OC parts are much slower. I would guess that the OC only paradigm was suplanted in the last quarter of the 20th century. The use of external components / feedback to define / extend hysteresis is well known. However, the devil is in the details as the voltage fed back is the comparator output (sometimes pull up) voltage. How much shy of the rails varies with technology (generation), these days for a push pull output the answer can be the rails but back in the day ... The Op amp rule of thumb, that the input impedance at the inverting and non-inverting inputs should be equal - to systematically minimise input offset (due to bias current) - was carried over from (Bipolar) OpAmps. Some contemporary comparators, eg TLV3501, have pA input bias current and it is the last thing you should do - a few 10's of Ohms on the inverting input (as a current limiter) is the new vanilla. The bottom line is that the data sheet for THE part and its Spice model have to be inhaled and applied to attain understanding. That the output is the inverted difference of the inputs controlling an OC / PP output is at best a first order model. Somewhere along the Spice modeling / Bench experimentation continuum lies understanding. For information sources beyond the data sheet: TI, LT/AD and ST all have many apps notes; Books - eg Jung IC OpAmp Cookbook ISBN 0-13-889601-1 gives a good cooks tour of the old men's external circuitry tricks and covers more than the basics Horrowitz and Hill The Art of Electronics 3e ISBN 978-0-521-80626-9 present a useful cameo / survey in section 12.3 Martin -----Original Message----- From: Warner Losh via cctalk [mailto:[email protected]] Sent: 15 October 2025 21:30 TI is one of the worst for this.Especially if the chip implements an industry standard or is compatible with some other chip. In those cases, you barely get enough to understand. I've had to many times in the past hunt down an industry standard or get the datasheet for the part it's compatible with. << snippety>> Warner -----Original Message----- From: Rob Jarratt via cctalk [mailto:[email protected]] Sent: 15 October 2025 06:42 I have a Texas Instruments datasheet that does explain it, but much further down in the Application section. I have to say that many datasheets assume you already know an awful lot about the devices and how they work, which is definitely not the case for someone like me. -----Original Message----- From: Brent Hilpert via cctalk [mailto:[email protected]] Sent: 14 October 2025 22:14 To: General Discussion: On-Topic and Off-Topic Posts ([email protected]) <[email protected]> Cc: Brent Hilpert <[email protected]> Subject: [cctalk] Re: Rainbow H7842 PSU On 2025Oct 14,, at 1:42 PM, Rob Jarratt <[email protected]> wrote: >> -----Original Message----- >> From: Brent Hilpert via cctalk <[email protected]> >> >> There’s some confusion here somewhere. >> Those input V's would imply the comp. output should be loZ to >> Vsupply– pin, around –12V; not hiZ, +7.5V. > > Oh my! I have clearly got my understanding the wrong way around, not sure how > I did that because I read the datasheet carefully. Somehow, I got confused. I > re-measured and found 1IN+=5.5V, 1IN-=9.4V, Power OK=6.7V, but GND (on the > 393) is -13V. So as you say Power OK should be -13V and AC OK H would be > asserted. I guess this must mean that the comparator itself is faulty. I have > some 393s, so I will replace it and see what happens. I don’t think I’ve ever seen a comparator datasheet that explicitly laid out the input-to-output function - contrast with other device datasheets with detailed truth tables galore. The comp. datasheets always seem to assume “everybody knows that”. You can figure it out if you look at some of the example circuits or squint closely at just the right parameters in the specs and graphs or trace the operation through the internal schematic if present. In the absence of that, a lot of people seem to (wrongly) assume that “well, + > – would be 1, so transistor ON”. The other way of looking at it, is it’s the 'same direction' of behaviour as an op amp, but without the upper drive-high output transistor.
