Linear Technology’s damage evaluation on amplifier

Article By : Tim Regan and James Mahoney

A curve tracer shows the current vs voltage characteristic when overdriving specific pin combinations.

In long life, high reliability systems, supplied power is provided only to essential circuitry. As a result, many of the unpowered circuits may have voltages applied to inputs and outputs without proper supply biasing.

So, Linear Technology has given an advice for what lies within the pins of several common amplifiers used in these applications. Most of the amplifiers are the radiation hard amplifiers. Other amplifier, the LT6016, is particularly robust with over, under and reversed polarity voltage conditions and is included for reference.

With no power applied to the amplifier, forcing a voltage between two pins will cause a current to flow. The magnitude of this current differs from pin to pin and device to device. A curve tracer is used to show the current vs voltage characteristic when overdriving specific pin combinations.

Also shown is a plot of a normal supply connection voltage sweep which indicates the amplifier’s start-up characteristic. A note added to this plot states at what point to expect a supply overvoltage condition where the supply current begins to increase rapidly.

Referencing these curve trace plots will provide an indication of the magnitude of current flow for a particular voltage applied and also any clamp voltage at the device pin. From these it is hoped that an educated assessment of the risk of damage can be made.

The usefulness of these plots can be shown through an example. An RH/LT1013 op amp is powered off with its supply pins at circuit ground while other circuitry is active and presents ±10V potentials through resistors to the –IN pin and the OUTPUT pin.

Another consequence of this condition however is loading of the powered amplifier caused by the output clamping of the unpowered amplifier. In this example the powered amplifier must be able to sink the 2.5mA of current to output the –10V level expected. If the powered amplifier is from another RH/LT1013 package, it is able to sink this much current so operation should be as expected. Other lower power amplifiers may not have the output current capability and the circuit output will be in error caused by the loading interaction from the unpowered amplifier.

Use of these plots can provide a good starting point for the evaluation of the risk of circuit damage and/or potential erroneous operation of systems subjected to abnormal supply connections.

Read more: Download the full application note

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