7.3.5 Phase-Reversal Protection

The OPAx145 has internal phase-reversal protection. Many FET-input and bipolar-input op amps exhibit a phase

reversal when the input is driven beyond its linear common-mode range. This condition is most often

encountered in noninverting circuits when the input is driven beyond the specified common-mode voltage range,

causing the output to reverse into the opposite rail. The input circuitry of the OPAx145 prevents phase reversal

with excessive common-mode voltage; instead, the output limits into the appropriate rail (see Figure 6-28).

7.3.6 Electrical Overstress

Designers often ask questions about the capability of an operational amplifier to withstand electrical overstress.

These questions tend to focus on the device inputs, but may involve the supply voltage pins or even the output

pin. Each of these different pin functions have electrical stress limits determined by the voltage breakdown

characteristics of the particular semiconductor fabrication process and specific circuits connected to the pin.

Additionally, internal electrostatic discharge (ESD) protection is built into these circuits to protect them from

accidental ESD events both before and during product assembly.

A good understanding of this basic ESD circuitry and its relevance to an electrical overstress event is helpful.

See Figure 7-3 for an illustration of the ESD circuits contained in the OPAx145 (indicated by the dashed line

area). The ESD protection circuitry involves several current-steering diodes connected from the input and output

pins and routed back to the internal power-supply lines, where the power supply is connected to an absorption

device internal to the operational amplifier. This protection circuitry is intended to remain inactive during normal

circuit operation.

An ESD event produces a short duration, high-voltage pulse that is transformed into a short duration, highcurrent

pulse that discharges through a semiconductor device. The ESD protection circuits are designed to provide a

current path around the operational amplifier core to prevent the amplifier from being damaged. The energy

absorbed by the protection circuitry is then dissipated as heat.

When an ESD voltage develops across two or more of the amplifier device pins, current flows through one or

more of the steering diodes. Depending on the path that the current takes, the absorption device may activate.

The absorption device has a trigger, or threshold voltage, that is greater than the normal operating voltage of the

OPAx145 but less than the device breakdown voltage level. After this threshold is exceeded, the absorption

device quickly activates and clamps the voltage across the supply rails to a safe level.

When the operational amplifier connects into a circuit, such as the one Figure 7-3 shows, the ESD protection

components are intended to remain inactive and not become involved in the application circuit operation.

However, circumstances may arise where an applied voltage exceeds the operating voltage range of a given pin.

If this condition occurs, there is a risk that some of the internal ESD protection circuits may be biased on, and

conduct current. Any such current flow occurs through steering diode paths and rarely involves the absorption

device.

applications limit the input current to 10 mA.

and then take over as the source of positive supply voltage. The danger in this case is that the voltage can rise

to levels that exceed the operational amplifier absolute maximum ratings.

Another common question involves what happens to the amplifier if an input signal is applied to the input while

level less than the input signal amplitude. If the supplies appear as high impedance, then the operational

amplifier supply current may be supplied by the input source through the current steering diodes. This state is

not a normal bias condition; the amplifier most likely will not operate normally. If the supplies are low impedance,

then the current through the steering diodes can become quite high. The current level depends on the ability of

the input source to deliver current, and any resistance in the input path.

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OPA145,, OPA2145

SBOS427E – JUNE 2017 – REVISED OCTOBER 2020

Copyright © 2020 Texas Instruments Incorporated

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