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OPA2626: Damage voltage on Vout when unpowered

Part Number: OPA2626

The OPA2626 looks like a good solution for our need as a high speed 50ohm output driver to a scope/ADC. In our design the output will be connected to BNC connector to the outside world to allow driving a scope or external ADC.

I am trying to protect the OPA2626 output pin from several potential fault conditions:

  • For short circuit conditions I am assuming the mid point (aka 2.5V) is the worst case power dissipation in the OPA2626 output stage (I cannot find any data on this in the datasheet) such that the OPA2626 must dissipate 0.12W (2.5V into 50ohms), hence at worst case 172degC/W I should get about 22degC increase from the ambient. If this assumption is correct the short circuit protection should be fine?
  • I would also like to protect from reverse voltages applied to the BNC connector by mistake. For this I have added back to back Schottky diode protection. Ideally I would like to ensure these diodes protect the output before the OPA2626 protection diodes do, however I cannot find this information in the datasheet (In fact the Electrical Over-stress section 8.3.2 mentions them but does not indicate when their protection will kick in, presumably similar to the input +/-0.3V)?
  • Finally our product may loose power while an output is still driving the OPA2626 output by mistake. In this case the over voltage Schottky diode protection clamping at 5V will be useless as the 5V rail will be off. Can you provide details of the damage voltage on the output pin when the power is not enabled (this is briefly discussed by analog devices engineers here www.analog.com/.../protecting-off-amps.html)

Thanks.

  • Hi Oliver,

    1. You are correct, the OPA2626 will keep cool driving a 50 ohm load.
    2. You are correct also that the output protection diodes are the same voltage as the input protection.
    3. The protection diode will not be useless; it will be forward biased. This condition will simply load the signal and supply power to your circuit through the ESD diode, which may be a concern if you have latch-up sensitive devices on that supply, but the bipolar amplifier will be fine.

    Best regards,

    Sean
  • Thanks for the quick reply Sean. I just have one follow up question.

    If the output diode protection will protect the OPA2626 from voltage on the OUT pin when the power is remove, what is the maximum voltage that can be applied when the power is off? I am assuming the "Power Supply ESD Cell" would only handle high voltage transients for ESD, but prolonged voltage (ie connecting OUT to a 50Vdc supply) that could damage the "Power Supply ESD Cell" ?

  • Hi Oliver,

    The diode acts like a normal diode up until 10mA, and then it will fail short.

    You can add a "make before break" current limiting resistor in series with the amplifier output if you know how much voltage you will have at Vout. Since the diode is directly connected to the Vcc pin, a forced voltage over 6V at the amplifier output will also exceed the absolute maximum power supply rating and will break the device. Your minimum resistor value is then (VOUT-6V)/10mA. However this will impact your normal output swing significantly at high current loads.

    Best regards,

    Sean

  • Thanks Sean. We already have 50ohm source impedance (so we can be 50ohm matched to the load), this only buys another 0.5V on top of the 6V at 10mA, but we cannot go higher than this since we need to be 50ohm matched.

    Instead I have protected the input via 2 back to back Schottky diode that have their own Zenner that fails at about 150mA (0.5W clamped at 3v3) which should be suitable for our protection as most potential faults will be form equipment sourcing only a few 10's of mA. I think given that the the Schottky diodes kick in a bit earlier than the OPA2626 ones, and the 50ohm series resistance between the OPA2626, our protection should then protect the OPA2626.

    Thanks for the support.