OPA2277-EP: max current into input terminals?

In general, SPICE models cannot be assumed to accurately show abnormal behaviour.

Section 9.2.2.2 of the datasheet says:

The inputs of the OPA2277 are protected with 1-kΩ series input resistors and diode clamps. The inputs can withstand ±30-V differential inputs without damage. The protection diodes conduct current when the inputs are overdriven. This may disturb the slewing behavior of unity-gain follower applications, but does not damage the operational amplifier.

Hi Francisco,

you could install a diode clamp into your voltage divider.

Can you post a schematic?

Kai

The problem with the above datasheet excerpt is that it's not clear how the diodes are configured. At one point we assumed them to be input-to-rails (same as several other op-amps). But looking more closely the diodes are not input-to-rails but across inputs. The non-EP datasheet has a figure showing the diode configuration. 

While your suggestion certainly will work what I'm trying to get is an answer to how much current can one push into the op-amp inputs so it can be evaluated in the future, in any circuit. Here's a ltspice file showing the behavior I described. Obviously there's a feedback network in real life that I've left off to show the current being sunk by the op-amp input and not the back-to-back diodes across inputs.

Hi Francisco,

Can you please share a more detailed schematic showing the actual in-circuit environment of the part, including the feedback path? This can be done via email if required.

Thanks,

Jon

Sorry I cannot. I'm not looking to evaluate or fix a particular design but come up with the op-amp max input current characteristic. 

Hi Francisco,

TI doesn't characterize op amp products or make any assurances about performance under conditions outside the valid operating range, such as when the supplies turned off and an input voltage is applied. As Clemens said, the SPICE model will not accurately reflect the response outside of the operating conditions either.

As a general rule of thumb, however, input currents in the 10s to 100s of uA are unlikely to result in enough power/heating to cause significant damage to an amplifier (depending on architecture).

There's not really a general solution to the question, though, because it depends on the current path. Say the IN- pin was connected to ground via a 2k resistor, such that the amplifier was in noninverting gain. One potential path for the current is through IN+, through the 1k current limiting protection resistor, through one of the clamping diodes, back through the other 1k current limiting resistor, and out IN- and through the 2k resistor to ground. But remove the 2k resistor and place it in a buffer configuration, and that current path is no longer viable. I suppose it could trace the same route and then go through the feedback resistor, then off towards the load, but that depends on what's downstream.

Either way, the diodes across the inputs are for clamping the differential voltage. The ESD diodes are separate from this. If there was enough voltage present at the inputs to turn these on, expect a drop of perhaps 0.5V across the diode, with the current being redirected to the V+ pin. With low enough voltage it is rather unlikely that anything within the amplifier will turn on. It depends what the rail at the V+ pin looks like when the power is off - most likely it's not a true ground, but rather a high impedance, so you'd once again have to assess potential paths for the current.

Ex: 80V * 5k/25k = 16V -> 15.5V across the 100k resistor -> ~155uA through the ESD diode, assuming V+ has a path to ground

Or with my hypothetical 2k resistor to ground, you'd see 16V across 100k+1k+1k+2k and the differential diode, so perhaps ~149uA. And if you wanted to get really granular you could assume some of the current goes through the ESD cell and some goes through the differential clamp, but again it's all very dependent on the actual implementation.

So to summarize, 

• Don't trust the SPICE model for the unpowered condition
• TI cannot make assurances for the unpowered amplifier, we can only discuss the possible/most likely behavior
• A typical rule-of-thumb estimation is that 100uA or so is unlikely to cause damage
• The actual current will depend on the path to ground

I hope this helps.

Cheers,

Jon

Jon you mention ESD diodes. These may be the answer. Can you elaborate on the configuration of said diodes? Am I right assuming there's a diode connecting each input to each rail (i.e. 2 diodes for every input)? If you have a current rating for said diodes then that answers my question. 

Perfect that answers the question. I forgot to mention that my external voltage is of a transient nature. And as pointed out current will be well below 1mA. We're used to see a max input current spec in other devices such as the ones below. But not the 2277 or 2277-EP. Yet all have the same ESD ratings. Any idea why the 2277 doesn't have such spec listed?

Also:

Hi Francisco,

Good question. If I had to take a guess, I'd say it's related to the fact that the OPA277 was actually originally a Burr-Brown part. You can still find the old BB datasheet online with a little digging. The part would have been released only about a year before TI acquired BB in 2000, so my guess is that the ESD diode current limits were not determined through the original testing, and since it had already passed ESD TI saw no reason to retest it again to determine those current specs. As a disclaimer this is mostly conjecture on my part, but it seems relatively feasible. The OPA227 was also a BB part, and would have released around the same time, but its old BB datasheet gives a max current value of 20mA. Unfortunately, I'm not sure if anyone who would have been a part of that original BB testing is still around TI, so it could be quite difficult to find the reason for the different test methods.

Cheers,

Jon

Jon, 

Was able to found a 1999 BB datasheet for the 2277, see attached. Interesting enough there's zero mention of the device having any ESD capabilities. I then found a 2005 datasheet (BB but with TI markings) which includes ESD ratings. Is it possible for you to double check that this device has in fact ESD protection diodes? Seems weird the 1999 BB datasheet would not highlight the device ESD capabilities. 

OPA2777_datasheet.pdf