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Window Comparator Output

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Prem Arjun

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Hello,
I have designed a Window comparator using LTC1040 for undervoltage lock out(LTSPICE). The battery supply should be cut of to hardware if the battery voltage falls out of range.
My operational criteria is
Battery supply is 39.2 V.
Upper limit Voltage 39.1V
Lower Limit Voltage 23.8V.
The comparator should give be High only in between the Limits.
Comparator is working good and the output at Nmos is as expected(39,2V).Although I have checked with the resistor value the output at the Vout is touching only to 36V only while it has to be 39.2
I could not understand what the problem is.
Please help me out.
Thank you.
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Well, first of all, you're exceeding the Vgs of M1. Absolute maximum is 20V; you've got 39.2 (assuming M2 pulls to ground). Not sure how Spice handles that. You might want to stick a zener or another resistor (voltage divider) in there and see what happens. Another thing to consider is that enormous gate resistance on M1. That's going to affect your switching time, if that's important to you.
 
@barry Thanks for the information. May I know where exactly to apply voltage divider and how the values has to be choosen?
 

Comparator is working good and the output at Nmos is as expected(39,2V).Although I have checked with the resistor value the output at the Vout is touching only to 36V only while it has to be 39.2
I could not understand what the problem is.
The "problem" is you upper comparator limit is set to 36V, not 39.2V.
But you are not going to get exactly 23.8V and 39.1V with standard 1% resistor values.
You will need to add a couple potentiometers to tweak them to that value.
And how accurate will that 5V source be?

Also you need to protect the gate of M1, as barry stated.
 

I recommend you think some more about your statement
that "the battery supply is". It seems you want to
use the supply that you are monitoring to also run the UVLO.
This then requires comparators which can operate well
outside the rails - and where, by the way, will you get the
39.1V reference if the battery has sunk to 23.8V?

I recommend you put the whole assembly under a
12V regulator, using the same 12V to derive ratioed
low and high references and dividing the VIN similarly
so that across the entire span of interest (and a bit
beyond) you are respecting the comparators' input
common mode range. The 12V rail would at least make
the FET gate happy (even if grossly under-driven, gate
current -wise).

You would want to "backstop" the window comparator
with some other crude scheme that prevents a false
high output while the VIN is between 0 and whatever it
takes for the local supply and references to get right.
 

@dick freebird. It looks to me like he's using 5v to power the comparator and his reference divider. I don't see a problem there.
 
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    Prem Arjun

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If you want to adjust the trip points to an accurate voltage, I suggest you use a voltage reference IC, such as a TL431, to generate the voltage for the divider, and use two separate dividers, each with its own potentiometer, to avoid the interaction in setting the two voltages that a single divider has.
 

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