OPA551: Output short circuit protection method?

Antenna_Head

Part Number: OPA551
Other Parts Discussed in Thread: LM317, LM337

The OPA551 can source and lot of current from relatively high voltage supplies. It includes a current limit feature as well as thermal shutdown with flag.

However, it is clear from the Abs Max ratings and the SOA curve they refer to that it is still possible to damage the device if it drives a continuous short circuit (fault).

The circuit in question uses 12V/Gnd as bias to the OPA551. According to SOA curve with 12V from Vs to Vo, the part will tolerate ~60 mA before exceeding the SOA for the SOIC package.

This '551 is driving a transducer through a wiring harness and we need to mitigate the fault where the wires i the harness are shorted to ground.

Do you have any suggestions on the best way to protect the part from a short circuit on its output?

over 6 years ago

0 Thomas Kuehl over 6 years ago

TI__Guru* 93105 points

Hello Antenna_Head,

The OPA551 does have internal thermal shutdown protection, but that may not be the best way to protect the device during an output short circuit condition. There are external methods that can be employed to reduce the maximum current the OPA551 can output, but they do come at a cost.

A low value power resistor can be added directly after the OPA551 output pin, inside the feedback loop. Should the output point after the Op amp and power resistor encounter a short circuit condition, the power resistor would dissipate some of the power. Having the resistor in the loop would assure that the output after the resistor is the correct level. However, the output level directly at the OPA551 output pin must swing to a higher level to make up for the voltage drop that would occur as the result of current flowing through the added output resistor. That may require the supply voltage be set higher than 12 V.

Another technique is to add an active current limiter to the Op amp power supply rail. The older 3-terminal voltage regulators such as the LM317 can be easily set up as a current limiter. You can see an example below taken from the TI LM317 datasheet. You simply set the current limit to a safe, maximum level. The drawback is the drop-out voltage of the regulator. A similar arrangement can be used with the LM337 3-terminal regulator when a negative supply current must be limited. The supply voltage would need to be increased to make up for the voltage drop that occurs from the input to the output of the regulator.

I hope this helps.

Regards, Thomas

Precision Amplifiers Applications Engineering