OPA462: Supply voltage drop

Hi Charles,

What voltage levels are being applied to the OPA462 at the input_signal point? What is that voltage level when the +72 V power supply is first applied to the OPA462?

Do you have to leave the OPA462 E/D Com pin floating? Does the problem cease if you connect the E/D Com pin to a voltage between (V–) minimum, to (V+) – 6 V maximum, for VS < 106 V that the Electrical Characteristics table lists?

The OPA454 is a 100 V op amp having very similar function to the OPA462, and comes in the same DDA package. It is a different design on a different semiconductor process than the OPA462 and OPA455. A possible alternative is the new OPA593DNT, an 85 V high output current op amp. It too has various flags and E/D function, but comes in a smaller package than the OPA462.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Charles,

Charles Lambert said:

Whatever I try, the power supply of +77V drops down to around +30V

Why is the supply volate dropping? Have you activated a current limiter in the power supply? What power supply do you use?

Kai

Thank you for the answer.

When the board is powered, the input_signal voltage is close to the V+ power supply.

I have tried letting the E/D and E/D com pins floating which drops the power supply to around +30V.

I have tried tying E/D com to GND and setting a voltage divider between GND and V+ to get a +5V voltage on E/D which results in a power supply drop to around +50V.

It seems there is a shortage of OPA454/OPA593, so I won't be able to try these amplifiers. I would also like to stay in the same package and pinout in order to keep the original ADA4700-1 as a backup should it become available again.

I'm using a LT1172IS8#PBF switching regulator operated as a boost to make the +77V. There is no current limiter that I know of. The same diagram worked fine when using an ADA4700-1.

Hi Charles,

Do you have a way to monitor the OPA462 supply current when the +77 V gets pulled down to +30 V? It would seem that the OPA462 would have to be drawing significant current for that supply drop to occur. If that is the case, a feasible explanation is that a parasitic latch circuit is developing internal to the OPA462 and that is acting as sink path for the supply current.

It may be that having the input voltage up near the V+ rail during power up is a problem. When you mention that it is close to the V+ level on power up, is it exceeding the upper common-mode voltage (Vcm) limit of (V+) – 3 V? When an op amp's Vcm range is exceeded it's forced to conditions outside its linear operating range. Most op amps no longer function as an op amp because their output is driven into saturation and the loop control falls apart.

Would it be possible for you to set-up an O-scope where you monitor the OPA462 supply current, supply voltage, input_voltage and possibly output voltage upon power up? If we find that the Vcm is the issue, there may be a way to help remedy the problem.

By the way, have you observed this behavior with more than one OPA462 in your circuit setup?

Regards, Thomas

Precision Amplifiers Applications Engineering