Opamp input voltage above its supply voltage?

[Flyback]

Hello,
Is it ok that the LT6220 opamp has voltage on its input pins that is above its supply voltage?, as in the attached schematic.
Its just that the datasheet says on the front page that this is ok, but then in the “absolute max” section it says the maximum input voltage is +/-Vs.
These are contradictory statements.
If it is ok, then how high above the supply voltage are the inputs allowed to go?

LT6220 opamp datasheet:
http://cds.linear.com/docs/en/datasheet/622012fc.pdf

 

[JimB]

Did you not have a thread with a very similar theme just a couple of months ago?

JimB

 

[Ian Rogers]

But!!! The overdrive function will kick in at @4v so the output will reflect a different output to the expected one..

I'm struggling to work out if the opamp will differentiate anything in those conditions..

 

[Flyback]

the opamp isn't supposed to be working in the above schematic, its just conditions that it will see at times, and we wonder if it will damage the opamp?...for example damage the esd diodes on page 13 of datasheet that go from input to supply.

 

[crutschow]

the opamp isn't supposed to be working in the above schematic, its just conditions that it will see at times, and we wonder if it will damage the opamp?...for example damage the esd diodes on page 13 of datasheet that go from input to supply.

Yes it may.
You can add resistors in series with the inputs to limit the ESD diode current to a safe value under the overvoltage conditions.

 

[Flyback]

thanks, as you know , unfortunately, the datasheet doesn't say what is the safe current, or safe surge current thru the esd diodes

 

[KeepItSimpleStupid]

Why not use an Over the Top OP amp? https://www.linear.com/product/LT6016

 

[crutschow]

thanks, as you know , unfortunately, the datasheet doesn't say what is the safe current, or safe surge current thru the esd diodes

I think if you limit the current to less than a mA you should be safe.

 

[dougy83]

thanks, as you know , unfortunately, the datasheet doesn't say what is the safe current, or safe surge current thru the esd diodes

Page 14 states

If the input voltage exceeds either power supply by 700mV, diode D1/D2 or D3/D4 will turn on to keep the output at the proper polarity. For the phase reversal protection to perform properly, the input current must be limited to less than 5mA.

And on page 15

input stages are also protected against a large differential input voltage of 1.4V or higher by a pair of back-to-back diodes, D5/D8, to prevent the emitter-base breakdown of the input transistors. The current in these diodes should be limited to less than 10mA when they are active.

The circuit you posted won't work due to the inputs being well outside the operating range of the opamp. You can bring the input voltage down by adding a resistor between each of the inputs and ground.

 

[Flyback]

sorry I don't need the cct to work when inputs are above supply..i just need it to survive.

 

[Flyback]

K.I.S.S......you appear to have hit the nail right , smack, bang on the head!....fantastic!....that was just what I was looking for.....does down to 1.5v and doesn't care if inputs go far above the supply.
K.I.S.S., you are a genius in my eyes.

now we will be able to monitor current from our 3 nimh cells with a supply of 3v3 to the LT6016 and not worry to death over what happens when the battery is at 4.8V.
Thankyou

 

[KeepItSimpleStupid]

Your welcome. It just so happens I was exercising my brain with the single supply OP amp stuff.

 

[Flyback]

I have now implemented the LED driver , which regulates its input current, with LT6016.
The LT6016 supply is 9v when the led driver is enabled, but when the led driver is disabled, the LT6016 supply rail may go below the battery voltage, to which its inputs would still be connected, but thanks to K.I.S.S., a "over-the-top" amp has been used, and this can easily handle input voltages being well above the voltage on its supply pin...the ltspice simulation and schem show the led driver regulating its input current to 1 amp.

 

[tcmtech]

K.I.S.S......you appear to have hit the nail right , smack, bang on the head!....fantastic!....that was just what I was looking for.....does down to 1.5v and doesn't care if inputs go far above the supply.
K.I.S.S., you are a genius in my eyes.

So being able to read, comprehend and retain basic info from a 22 page data sheet set makes a person a genius nowadays?

I think I just heard Stephen Hawking drive his chair into a open elevator shaft from the 10th floor on purpose just so he won't live to be associated with the new group of geniuses about to hit the world.

 

[dougy83]

So being able to read, comprehend and retain basic info from a 22 page data sheet set makes a person a genius nowadays?

You're just jealous

 

[tcmtech]

I went to public school. I can't read period. In fact the only thing I ever learned to do is make snarky comments.