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INA128: Short Circuit Output Protection

Brandon Magnan
Prodigy 250 points
Part Number: INA128

In my design I have the INA128 running off of a single ended 12V supply.  I am running the output into a 3.3V part, so I would like to add protection on the output of the INA128 to make sure that my 3.3V device wouldn't get damaged if for some reason the INA128 rails the output to 12V.

I want to just place a 3.3V zener diode on the output, but I am not sure if that will harm the device if the device is trying to output 12V and I am clamping it to 3.3V.  From what I can tell the datasheet seems to say that it can have the output short circuited to ground continuously (under the absolute max ratings).  I'm wondering if this condition would cause any damage to the device though.  On page 6 of the datasheet they mention the short circuit current value as typically 6mA.  I'm assuming this isn't a value I need to design to (for instance adding a current limiting resistor to prevent the output from driving that much current), but I want to make sure.

  • 0
    TI__Guru* 93105 points

    Hello Brandon,

    The INA128 is relatively low power instrumentation amplifier and its output short-circuit current capability is limited. The Electrical Characteristics table list the typical shot-circuit current as +6/-15 mA. Since your application is a single +12 V supply you need consider is the output current sourcing capability, the -15 mA. The minus sign in front of the 15 mA indicates current out of the device, or sourcing. 

    If the output attempts to move to its maximum positive level and the 3.3 V zener is in place, the zener will turn on around +3.3 V. That will establish +12 V - (+3 V), or 8.7 V across the INA128 output transistor. That voltage in conjunction with the 15 mA sets the power in the output stage. That is simply Po = (8.7 V) (0.015 A) = 0.13 W. There is also the quiescent  power, which is Pq = (12 V) (700 uA) = 8.4 mW. Therefore, the total INA128 power dissipation would be about 0.138 W under this condition.

    The concern in short-circuit mode is the transistor junction temperature (TJ) and that it remains below the Absolute Maximum rating of 150 °C. That can be calculated from the relation:

    Pd = Po + Pq

    TJ= TA + Pd ΘJA

    for the 8-pin SOIC package ΘJA is specified as 110 °C/W

    TJ = TA + (0.138 W) (110 °C/W) = TA + 15.2 °C

    Therefore, as long as the ambient temperature is kept approximately 15 °C below the maximum TJ temperature, the device should be okay sourcing the 15 mA through the 3.3 V zener diode.

    Regards, Thomas

    Precision Amplifiers Applications Engineering