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I will use this comparator with 12V DC single supply.

Some of the pulse inputs to this comparator will exceed 12V that is to say the inputs to its inverting input can be up to 24V. So input voltage will exceed the supply voltage.

So my question is:

Can I use this comparator safely when it is powered by 12V and the input to its inverting input is 24V?

EDIT:

I decided to use 8.2V Zener because the commentators said the common mode should not exceed Vcc-2V which is 10V in my case since I’m gonna use 12V single supply. But I don’t know which type suits good for this application where the inputs to the circuit(not the comparator input) will be between 12v and 24V pulses with max freq. 900Hz or so. Another thing is R5 is 1.5k. In average it dissipates 70mW but peak in simulation exceeds 250mW. Would that be an issue. The R5 is soldered so I don’t want to change it if not necessary.

Circuit basically:

enter image description here

Which zener type?:

enter image description here

Power dissipation of R5 for 24V input:

enter image description here

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  • \$\begingroup\$ What are the values of the Zener diodes? If the value is less than 24V how can it be that there is more than 24V (+0.6V) at the input of the comparator? (Were the Zener diodes included in the circuit of the original question?) When you say "inputs to this comparator " do you really mean inputs at the comparator or inputs at the node labeled "Input"? Please make your question more accurate and make sure any sentence still makes sense even after edits. \$\endgroup\$ – Curd Aug 23 '17 at 9:52
  • \$\begingroup\$ I agree please see my edit. I decided to use 8.2V Zener because the commentators said the common mode should not exceed Vcc-2V which is 10V in my case since I’m gonna use 12V single supply. But I don’t know which type suits good for this application where the inputs to the circuit(not the comparator input) will be between 12v and 24V pulses with max freq. 900Hz or so. Another thing is R5 is 1.5k. In average it dissipates 70mW but peak in simulation exceeds 250mW. Would that be an issue. The R5 is soldered so I don’t want to change it if not necessary. \$\endgroup\$ – HelpMee Aug 23 '17 at 10:05
  • \$\begingroup\$ If power dissipation in R5 is a problem of course depends on what power R5 can handle (mean and pulse; no information given). Also: I think you can (and should) remove Z2. \$\endgroup\$ – Curd Aug 23 '17 at 10:23
  • \$\begingroup\$ I thought Z1 and Z2 together serves as over voltage protection. Why Z2 is useless? If I keep it would it have a bad effect. \$\endgroup\$ – HelpMee Aug 23 '17 at 10:30
  • \$\begingroup\$ If you intend to use this on battery 12V, then use an automotive grade comparator with protection. Also Vcc has to be protected in your circuit, keep in mind that high currents dumps from battery can generate more than +/-100V spikes. \$\endgroup\$ – Marko Buršič Aug 23 '17 at 11:00
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Can I use this comparator safely when it is powered by 12V and the input to its inverting input is 24V?

Short answer:

Yes!!!
This comparator acts differently than many as regards allowable Vin voltages.
As long as one input is in the 0V to (Vcc-2V) range then the other input may be anywhere in the 0V to 36V range, without damage and with correct operation. This applies for any Vcc value from 2 to 36V.

Because:

There are limitations on what happens when one or both inputs exceed Vcommon-mode - see below.

In Table 2 on page 4/19 it states -
Vicm: Common mode input voltage range for Tmin ≤ Tamb ≤ Tmax 0 to VCC+ -1.5 (or Vcc -2 in another row).
A look at the data sheets from 6 manufacturers shows that:

At least one input must be within the Vicm range of 0V to Vcc - 2V * The other input may be above Vcc - as much as +36V in some versions.

  • *(For full temperature operation. Vcc-1.5V at 25C)

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The TI data sheet lists the conditions most clearly:

8.2.2.1 Input Voltage Range
When choosing the input voltage range, the input common mode voltage range (VICR) must be taken in to account. If temperature operation is above or below 25°C the VICR can range from 0 V to VCC– 2.0 V. This limits the input voltage range to as high as VCC– 2.0 V and as low as 0 V. Operation outside of this range can yield incorrect comparisons.

Below is a list of input voltage situation and their outcomes:

    1. When both IN- and IN+ are both within the common-mode range: (a) If IN- is higher than IN+ and the offset voltage, the output is low and the output transistor is sinking current
      (b) If IN- is lower than IN+ and the offset voltage, the output is high impedance and the output transistor is not conducting
    2. When IN- is higher than common-mode and IN+ is within common-mode, the output is low and the output transistor is sinking current
    3. When IN+ is higher than common-mode and IN- is within common-mode, the output is high impedance and the output transistor is not conducting
    4. When IN- and IN+ are both higher than common-mode, the output is low and the output transistor is sinking current

The Diodes Inc data sheet appears the most generally useful.
(MB: Diodes Inc often make better than most spec products so be wary if using their data sheets for other brands. This noted, they can be very useful).

On page 5 note 13 they say:
"13. The input common-mode voltage of either input signal voltage should not be allowed to go negative by more than 0.3V (@ +25°C). The upper end of the common-mode voltage range is VCC -1.5V (@ +25°C), but either or both inputs can go to +36V without damage, independent of the magnitude of VCC"

Note that if both inputs are above Vcmr the output will be low (according to the TI notes above.)

______________

ONSEMI datasheet
ROHM data sheet
ST data sheet
Fairchild data sheet is now shared with ONSEMI


Notes:

(1) This is based on a comment I made to to Andrew and is worth noting.

They seem to have a "different to usual" interpretation of what 'common mode voltage' means. After looking at data sheets from 6 manufacturers I think the TI summary in my answer above probably best states what happens.

ONE input must be in the 0 - (Vcc-2) range** BUT the other can be 0 - 36V without damage AND with correct operation!
Overall this is a very unusual interpretation of terms and mode of operation.

Note that Iin can be very large, unlike usual devices.

  • **
    Diodes inc say lower Vin limit is -0.3V
    Vcc-2 is for full temperature range.
    Vcc-1.5 at 25C.

(2) Based on a comment that I made to Curd

I looked at 6 manufacturer's data sheets in order to elucidate [tm] what they appear to be saying.
These indicate that this device has an unusually accommodating spec and, if ONE input is in the 0 - (Vcc-2V) range, then regardless of the value of Vcc*** it will allow the OTHER input to cover the 0-36V range without damage and with correct operation.

  • ***-
    Vcc allowed = 2-36V.
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  • \$\begingroup\$ Please see my edit. Vcc will be 12 single supply. So I decided to use a 8.2V zener to ease the input. But there are many types. In my case the Input will be 12 to 24V pulses with max freq. 900Hz. Which type of zener would be good for this application? I attached the options in my edit. \$\endgroup\$ – HelpMee Aug 23 '17 at 9:32
  • \$\begingroup\$ Thanks just one more thing. I will use 12V single ended power supply. With 24V input if I use 15V zener just before the comparator input which means if the max pulse voltage input to the inverting input will be 15V and the non-inverting input will not exceed 3V by R3/R4(see my edit circuit). So in this case maximum common mode voltage will be around 12V right? Does that mean I have a problem because the common mode is exceeding 10V ? Another question is some suggested to remove Z2, but I always see the zeners back to back in such applications. Do you also think Z2 is a bad idea? \$\endgroup\$ – HelpMee Aug 23 '17 at 12:03
  • \$\begingroup\$ @doncarlos See my updated answer. This is a very unusual IC. It seems that if ONE input is within the 0 to Vcc-2 range then the other input may be in the 0-36V range without damage and with correct operation. Placing a resistor across C2 would allow the input voltage to be reduced to below Vcc-2 if desired. || Z2 is not needed (as Curd says) if Vin is always >0. It has a BAD effect if Vin goes -ve as it fails to clamp the potentially damaging negative input. ie better not used & serves no useful purpose. BUT as above - the IC will tolerate the Vin without the zener! - bit Z1 does no harm. \$\endgroup\$ – Russell McMahon Aug 23 '17 at 23:47
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The absolute maximum ratings section of the datasheet indicates that the maximum on Vin is 36 V and that the maximum differential input is +/- 36 volts.

As such the device should be safe and not be damaged by a 24V input.

Correct operation is a different matter.
Note 3 of the electrical characteristics indicates that the device will operate correctly with inputs in excess of Vcc as long as you remain within the stated common mode range.

The common mode range is 0 to Vcc - 2 which means that with a 12V power supply you can only ever have a maximum difference between the two inputs of 10V.
So with a 12 V supply one input can go to 24V if and only if the other input is at least 14V at the time.

If you exceed that voltage difference (e.g. inputs of 12V and 24V) the device operation is not guaranteed however you are still within the absolute maximums so the device shouldn't be damaged.

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  • \$\begingroup\$ Thanks and please see my edit. Vcc will be 12 single supply. So I decided to use a 8.2V zener to ease the input. But there are many types. In my case the Input will be 12 to 24V pulses with max freq. 900Hz. Which type of zener would be good for this application? I attached the options in my edit \$\endgroup\$ – HelpMee Aug 23 '17 at 9:32
  • \$\begingroup\$ +1. You correctly noted that Vin may exceed Vcc. A look at the less than normal behaviour shows that operation with one input inside Vicm and one above it is acceptable. See addition to my answer. \$\endgroup\$ – Russell McMahon Aug 23 '17 at 11:26
  • \$\begingroup\$ See my revised answer due to your and ArtN's input. \$\endgroup\$ – Russell McMahon Aug 23 '17 at 11:49
  • \$\begingroup\$ Andrew - they seem to have a "different to usual" interpretation of what common mode voltage means. After looking at data sheets from 6 manufacturers I think the TI summary in my answer probably states what happens. ONE input must be in the 0 - (Vcc-2) range * BUT the other can be 0 - 36V without damage AND with correct operation! Overall this is a very unusual interpretation or terms and mode of operation. / Note that Iin can be very large, unlike usual devices. / [[* Diodes inc say lower limit is -0.3V. / Vcc-2 is for full temperature range. Vcc-1.5 at 25C]] \$\endgroup\$ – Russell McMahon Aug 23 '17 at 23:37
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Check Electrical characteristic table Vid

Positive excursions of input voltage may exceed the power supply level. As long as the other voltage remains within the common-mode range, the comparator provides a proper output state. The low input voltage state must not be less than –0.3 V (or 0.3 V below the negative power supply, if used).

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  • \$\begingroup\$ See my revised answer due to your and Andrew's input. \$\endgroup\$ – Russell McMahon Aug 23 '17 at 11:49
  • \$\begingroup\$ My answer further added to. A very unusual spec. \$\endgroup\$ – Russell McMahon Aug 24 '17 at 0:03

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