1. Home
  2. Electronics Design
  3. Comparator with Wide Common Mode Range
  4. Page 2

Comparator with Wide Common Mode Range

r

I might be able to get around the need for 15 volts on the output. That on e comparator is pulling down the source of an nFET with the gate at 3.3 vol ts and the drain at 15V. In theory the 15 volts won't make it through to t he comparator. But the nFET has some leakage. With damage not happening u ntil exceeding 7 volts that would make Vgs -3.7 volts which should turn the nFET off pretty hard. Just to be sure, I added a 1 Meg resistor to ground to drain away any leakage and it should be good.

The higher voltage parts both cost more and use more power supply. At leas t two of these devices are used for alarms which need to be operative durin g the power "off" mode which is really just a minimum power level. The bat tery has a self discharge current of around 80 uA, so these parts at

Reply to
Ricketty C
Loading thread data ...

Well, the 393 is good for most things on higher supplies, and with 3.3V VDD Rick can drive a 2N7002W to higher current than the OD comparator could handle. It's only an occasional extra SC70, no biggie.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

Minimalist design is an art form like sonnet writing. It's worth spending a bit of extra time on it, both for the fun and because it helps keep one sharp. (My designs are prettier than my poetry by all accounts, including mine.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

RRIO,

or

I think you are missing the issue. The output of the 393 is not a problem. I also don't want to add transistors to the outputs of the comparators. The 393 has two problems, high power consumption, nearly a mA and poor comm on mode input range.

In the one circuit where the TLV7042 output might possibly be exposed to a higher voltage than the rated 7 volts, adding a single resistor should prot ect the output in that case.

Maybe I don't understand your comments.

--

  Rick C. 

  +-- Get 1,000 miles of free Supercharging 
  +-- Tesla referral code - https://ts.la/richard11209
Reply to
Ricketty C

I wouldn't use a 393 in a battery-powered application, but at ~10 mW dissipation it takes quite awhile to use enough line power to equal the cost of a $1 comparator. 15 cents/kWh = $4E-8 per joule, so at 10 mW it takes 76 years by my arithmetic. (Not counting compound interest on the money saved.) ;)

The CM range goes from a bit below ground to VCC-1.5V. That's inconvenient in a 3.3V system but really not much of a worry when VCC >

5V or so. Certainly to save a buck one can think of ways to avoid high-side sensing near the rail, or use an op amp.

You were worrying about the drain leakage of a 2N7002 being potentially large enough to damage the OD output of the TLV comparator, and added a resistor for safety. A 2N7002W costs 2 cents in reels, takes 4.5 square millimetres of board space, and solves Pimpom's issue entirely as well. So getting higher-voltage capability on the OD outputs isn't a worry at all.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

t

It is battery powered and even with a 2 AHr SLA a single LM393 blows away t he self discharge current. Run time is not the issue, as it's powering a 2 amp motor! Storage time is the issue. The thing has to survive storage f or as long as possible. Draining the battery in a few weeks from thirsty c omparators is not a good idea.

The device we are using is spec'd for Vcc-2V. Even in a 5 volt system that half a volt makes a difference. 2.75v instead of 3.25v. I could live wit h 3.25 volt considering the signal range is up to 3.3 volts.

Not sure why you are talking a buck. We are down to less than $0.40 delta in the total board cost now and I can live with that.

I have no idea why an op amp should be added. That makes no sense to me. Yes, I could simply add more than a dozen resistors to scale the various vo ltage values down into the remaining ~1V range of the comparator... or I ca n ditch this witch and use a rail to rail part which is what I am doing.

Not sure what you are complaining about. If I can do the job with a resist or for $0.002, why use a transistor for twenty times that price? I don't s ee the 2N7002W being sold for a tuppence at any quantity, btw. Digikey doe s get a bit below $0.04.

My initial thoughts were to be retain the option of running a comparator fr om 15 volts, but that is a less likely scenario now.

Anyway, thanks for the advice.

--

  Rick C. 

  +-+ Get 1,000 miles of free Supercharging 
  +-+ Tesla referral code - https://ts.la/richard11209
Reply to
Ricketty C

I'm not the one doing the complaining, dude.

Cheers

Phil Hobbs

Reply to
pcdhobbs

What's going on with you? We all discussed the issues here, I thanked you for the advice. What makes you think I am complaining?

Well, it was a fruitful thread and the mission is accomplished. Thanks to everyone who contributed.

--

  Rick C. 

  ++- Get 1,000 miles of free Supercharging 
  ++- Tesla referral code - https://ts.la/richard11209
Reply to
Ricketty C

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.