Open collector comparator maximum pull-up voltage ?

[Flyback]

Hello,

I've got the MCP65R46 open collector comparator pulling up to 12V with a 330K resistor.
Is this OK...?
The datasheet appears to say that 10V is the maximum?
Vdd pin is 3V3

MCP65R46 datasheet:
https://www.electro-tech-online.com/custompdfs/2013/06/22269A.pdf

 

[MikeMl]

The footnote seems to imply that it is ok to connect the pull-up to +10V, but the data sheet is ambiguous. I would call their Application Engineering dept, ask the question, and give them hell for writing such a poor data sheet.

You are violating the Output pin no more positive than Vdd+0.3V Absolute Maximum Rating, but if there is an exception for the open drain output, it should be plainly stated there.

 

[Flyback]

Thanks, but i rarely get much luck phoning microchip (or any other semicon)

The datasheet says "do not short the output of the MCP65R46 comparator above VSS+10V" (bottom pg 4)

What on earth this means i have no idea.........why anyone would want to short the output of a comparator to any voltage is totally beyond my comprehension?
Nowhere in the datasheet does it say that the pullup voltage is 'limited' to 10V?

 

[MikeMl]

...
Nowhere in the datasheet does it say that the pullup voltage is 'limited' to 10V?

Nowhere in the datasheet does it say that it is ok to connect the pullup resistor to >Vdd+0.3V, either. It says that the open-drain version can be used as a level shifter from 3.3V logic to 10V logic swing, so that implies that Vpu can be 10V. Where is it stated or implied that it can be higher than 10V?

 

[Flyback]

...old time engineers usually know that if you have enough limiting resistance then you can connect pins to high voltages and always get away with it........surely you agree with me that an extra 2V isnt going to cause a lightning storm in the chip?
12V/330K = 36uA...................surely you agree that 36uA isnt going to weld the chip to the pcb traces?

Sorry I'm not being funny, its just that im trying to see this from the rational sense by elucidating

 

[crutschow]

You asked if it's OK to go above 10V on the output pullup. The data sheet says you can go up to 10V. Anything more than that is likely exceeding the output transistor rating. Higher than that may not cause problem. But it's not something to do if you want to design a reliable circuit. Just add a transistor buffer if you want to go above 10V.

 

[MikeMl]

...old time engineers usually know that if you have enough limiting resistance then you can connect pins to high voltages and always get away with it........surely you agree with me that an extra 2V isnt going to cause a lightning storm in the chip?
12V/330K = 36uA...................surely you agree that 36uA isnt going to weld the chip to the pcb traces?

Sorry I'm not being funny, its just that im trying to see this from the rational sense by elucidating

I see three possibilities:

You get an email from Microchip saying that Vpu=12V is ok.

You wing it, and you luck out, and the issue never comes up again,

You wing it, and then have to explain to your boss why some of the units are being returned with a blown open-drain comparitor, and it was too much trouble for you to close the loop with Microchip.

 

[Flyback]

Thanks, i may use a 3v3 zener in there....in series with the 330K...to limit the voltage

 

[ChrisP58]

...old time engineers usually know that if you have enough limiting resistance then you can connect pins to high voltages and always get away with it

Sorry, I can't agree with that. At least not with the all inclusive statement that you are trying to make.

It might be true if the pin were internally clamped to Vdd, but an open drain output is just that, open. Connecting the drain of a mosfet to a voltage higher than it's rated source drain voltage, even with a limited current, is asking for eventual failure.

 

[Flyback]

i cant understand why on page 2 "absolute maximum" ratings, it doesnt say what is the maximum pull up voltage......its the first thing anyone would want to know for an open collector device. I am wondering if i put a 10V zener on the open collector pin it will be ok......theres a tolerance so maybe it could go to 10.5V but is 0.5v going to kill me.

 

[blueroomelectronics]

Never ever design a commercial product that goes beyond the maximum ratings of a device, that's just asking for trouble. Always give yourself some headroom instead.

I hope this isn't something you plan to sell commercially. Is it part of your emergency lighting system?

 

[ericgibbs]

...old time engineers usually know that if you have enough limiting resistance then you can connect pins to high voltages and always get away with it

hi flyback.

I am a 'very' old time professional electronics engineer and I would advise any wannabe budding engineer to operate his devices BELOW the ABSOLUTE ratings specified by the manufacturer of the device.
Otherwise one day the equipment you have built and works OK in the comfort of your workshop bench will fail in the outside world environment.

E.

 

[ronv]

Take a look at the LMP7300. (Mouser) Don't know it will work for you but it's good at 12 volts. Or if your going to add parts why not just a 36 volt comparator and a voltage reference? On the second page of DC characteristics your part specs 10 volts max. l

 

[alec_t]

Can't you just add a resistor from collector to ground to form a voltage divider with the collector pull-up? That way the collector voltage can be kept below 10V.

 

[Flyback]

sorry i cant because it would turn on the bjt.

Anyway, thanks for your advice , i have gotten rid of the >10V.

i redesigned the hysteretic converter to now give 9V4 average....it peaks at 10V (instead of 12V) now but that gets filtered out by the cap that i put below the 330K which was previously connected to the open collector.
Theres also a 1K feeding into the output of the comparator now, to stop a surge current going out of the filter cap and through the open collector when it turns on.

no more 10V... thanks for telling me i def couldnt get away with it

 

[MrAl]

Hello,


From the data sheet, it states that the device can be used as a level shifter from 1.6v to 10v.

What it does *NOT* say is that it can be used as a level shifter from 1.6v to 12v.

What this tells me is that it can probably work up to 12v as a true max but for reliability they quote 10v. They cant say 12v because that is the exact point where the leakage current starts to become too great with various other spec limits also in place, and although it still works with the majority of the packages tested, the number of failures at 12v exceeded some lower limit set as a threshold for determining what the max voltage should be. Thus they rate it slightly under their threshold point and know that this is a good practice from experience.

So going over 10v means that you risk that some units will fail (even though others may not), so it becomes more of a risk that there may be a failure much sooner than expected.

We see these kinds of ratings more on data sheets that actually do show the maximum ratings too. For example, i would not be surprised to see something like "12v absolute maximum" in a revised data sheet. We see this for 5v parts too a lot like "6.5v absolute maximum" but we dont want to run it at 6.5v as a normal operating point. 5.5v maybe, but not 6.5v because that is the point where probably many parts failed.

Now we also know that when this kind of test is done that the other specs are held as a max too, so there is also a chance that the unit would survive for a long time if that was the only spec that was violated. So 12v at 25 degree C might work forever, while 12v at 70 degree C may see failures of 50 out of 100 (far too great).

So there's a chance that it could work if no other specs are violated, but it's a risk other wise and it's still a little bit of a risk anyway even if there are no other spec out of whack.

So stick with 10v and you should see long product life. If that's not possible, then modify the circuit a little. Adding two diode in series with the output resistor subtracts about 1.4v from the supply, bringing the max down to 10.6v, and one more diode (three in total) brings it down to about 9.9v which is under the max.