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LT1014: Reverse Voltage

Gerald Choi
Expert 1400 points
Part Number: LT1014


Hi High Reliability team.

Can we get information which is protection reverse voltage on LT1104 as a below?

Best Regards

Gerald choi

  • 0
    TI__Guru* 93105 points

    Hi Gerald,

    Operational amplifiers outputs are not normally characterized for reverse voltage applied to the output. Since it is an output it is intended to deliver very specific current and voltage levels depending on the output load. Those levels depend on the circuit configuration, drive conditions, etc. The output is not intended to have current forced back into while is attempting to establish its own output current and voltage levels.

    If a low impedance voltage source is connected to the operational amplifier output, current can be forced in to and from the output depending on the voltage level and polarity. We don't have a modern simulation model for the LT1014 so I tested another operational amplifier that has a similar bipolar NPN-NPN output stage design. What I observe when I sweep the voltage source over a range approaching the +/-15 V supply range is that within a few volts of zero it appears the one, or the other output transistors go into breakdown; possibly emitter-base breakdown. Before that there is a cross-over region associated with the class AB output and then the current increases to several tens-of-milliamperes until the breakdown takes place. At that point the current becomes dangerously high and would certainly destroy the transistor associated with that output polarity. I tested this on several different bipolar operational amplifier models and found similar breakdown characteristics, but they did occur at different levels. 

    Back driving the operational amplifier does not appear to be a safe thing to do and will likely end in destroying the amplifier output if the current becomes too high. A similar situation exists with power operational amplifiers driving highly inductive loads where they can be damaged by back EMF. In such cases the applications always include fast rectifier diodes from the output pin to each supply pin. These diodes are normally reverse biased, but in the case of a back EMF event one, or the other diode, can become forward biased. That diode shunts the output stage transistor protecting it. If you think your application might develop this condition, then we can recommend some output diodes and transient voltage suppressor diodes (TVS) that could be added to protect the operational amplifier.

    Regards, Thomas

    Precision Amplifiers Applications Engineering