OPA2314 in constant current control circuit latch up

Hi David,

Somewhat as expected simulations don't reproduce this issue.  The simulations do confirm one of our initial worries that the circuit has the opportunity to produce very large currents which may damage the load or other circuitry in the condition where the load, and/or power supply voltage are removed and then reapplied.  The results are shown at the bottom of this message. 

The initial analysis of this circuit was a little off as the op amp output does not need to swing to the negative rail when the input is a 70mV.  This is because to create the desired 0.07V / 0.1Ohms = 700mA load current the op amp must drive the gate to a voltage near the 2V threshold voltage. 

Marek stated that the Vos and Aol specifications do not cover when the output is at the rails.  Perhaps you can explain why?

This behavior is typical of all op amps and is specified in the open-loop gain (Aol) specification table.  Basically as the output approaches the rail the output stage devices begin to get saturated (or cutoff) and as a result the Aol specification, which is defined as ΔVout / ΔVos, begins to degrade.  The open-loop gain and feedback factor (β) determine the loop-gain (Aol*β) which directly relates to the linearity of the circuit.  Once the output is fully saturated the virtual short between the inputs is broken and the offset voltage can be very high.  Different op amps will have different linear swing-to-rail limitations based on the internal design, for instance the OPA340 which Marek mentioned can swing linearly within 5mV of the negative rail with a light 100k load. 

Here's a few questions:

1.) Is there something that limits the current in the condition where the op amp output is forcing the gate to +5V after the MOSFET drain voltage is reapplied? 

2.)  What level of current flows during the condition above?  What voltage does the inverting input go to during this condition?

3.)  Could you take scope captures showing the output of the op amp, voltage across the 100mV resistor, and IN+ voltages when the power supply is reconnected?

4.)  Going back to your first post, you mentioned:  "+ If the VCC supply voltage is increased so that the Vds voltage is greater than 200mV, the OPA2314 behaves normally."  Could you elaborate on this test a little more?  With the 1.6V supply shown in the image, the Vds voltage should be well above the 200mV mentioned in your test.

The circuit in an active dc state along with simulated transient results are shown below.  SW1 is opened at 750us and then closed again at 2ms, this behavior repeats.  As expected when the 1.6V supply (or load) is removed the output of the op amp saturates to the positive rail.  At 2ms when the supply is reconnected there is a large burst of current limited only by the source impedance of the supply, on resistance of the MOSFET and the 100mOhm shunt resistor.  The current spike will occur until the overload recovery time of the op amp is exceeded and the 1k+4.7nF feedback network can discharge to the proper level.  If there is not already one in the system, you may consider adding a series BJT based current limiting circuit to prevent this condition from occurring.

Thanks for the additional information.  In the condition where the output remains stuck at +5V, the IN+ voltage is roughly 70mV and the IN- voltage is 783mA*0.1Ohms = 78.3mA, resulting in an 8.3mV differential voltage between the inputs. 

It's sounding like Marek's original suggestion regarding the degraded Aol is likely the cause of the issue.  If the degradation of Aol near the +5V rail causes the offset voltage to increase to >8.3mV then the IN+ voltage will look higher than the IN- voltage and the op amp output will stay stuck to the +5V rail.  Disturbing the system with an input transient will cause the output to move off of the positive rail and the offset voltage will reduce back to the expected datasheet levels allowing the loop to regulate again.  Increasing the FET VDS voltage increases the current flow and a larger differential voltage is produced between the inputs which overcomes the increased offset voltage, again allowing the loop to regulate.

It may not be an acceptable condition in your system, but if more current was allowed to flow through the FET after the load/supply is reconnected then the differential voltage between the inputs would increase which would likely overcome the shifted offset voltage, as you saw when increasing the VDS voltage. 

Otherwise, you may consider using the OPA313.  It is a very similar product to the OPA314 but with an open-loop gain specification that is valid to within 0.05V of the positive rails with a light load.  I agree that when the output saturates to the positive rail (within 5mV typically) the Aol and therefore offset voltages won't be specified, but because the discrepancy between the specification limit and the expected operating range is much reduced compared with the OPA314.  If you're interested, you can order some free samples from the link below: 

Hi David,

Let me discuss this with some of my colleagues to see if we can come up with suggestions for a test that will help you screen for products which may display this behavior.  

There's a few things in play regarding why some devices perform this way while others do not.  First, if the change in input offset voltage as the output approaches the rail is negative, then when the load is reconnected the IN- pin will always still seem higher than the IN+ pin and as a result the output will move off of the positive rail to try to reestablish the virtual short between the inputs.  Similarly, if the change in offset is positive but  is < 8.3mV then when the load is reconnected the IN- pin will be at a higher voltage than the IN+ pin and the output will move off the rail to regulate the output.  Unfortunately as noted earlier, the typical/maximum Aol specifications are not provided for this region so we don't have any information on what a typical device may do versus a device that exhibits larger amounts of this behavior.  We do think the OPA313 will perform better in this situation, but I understand your concerns since we won't be able to guarantee the performance in that region.  

A device like the OPA340 is fully specified for output voltages within 5mV from the rail and we're pretty confident you wouldn't see the same issues with this device.  Other similar products with different bandwidths, offset voltages, and prices are the OPA342, OPA343, OPA344