7.3.7 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 39 for an illustration of the ESD circuits contained in the OPAx187 (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 diodes meet at an absorption device internal to the op amp. 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, high-current pulse while discharging through a semiconductor device. The ESD protection circuits are designed to provide a current path around the op amp core to prevent damage. The energy absorbed by the protection circuitry is then dissipated as heat.

When an ESD voltage develops across two or more amplifier device pins, current flows through one or more 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 OPAx187, but less than the device breakdown voltage level. When 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 (see Figure 39), the ESD protection components are intended to remain inactive and do 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 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.

Figure 39 shows a specific example where the input voltage (V_IN) exceeds the positive supply voltage (+V_S) by 500 mV or more. Much of what happens in the circuit depends on the supply characteristics. If +V_S can sink the current, one of the upper input steering diodes conducts and directs current to +V_S. Excessively high current levels can flow with increasingly higher V_IN. As a result, the data-sheet specifications recommend that applications limit the input current to 10 mA.

If the supply is not capable of sinking the current, V_IN may begin sourcing current to the operational amplifier, 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 power supply +V_S or –V_S is at 0 V. Again, this question depends on the supply characteristic while at 0 V, or at a level less than the input signal amplitude. If the supplies are 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.

If there is any uncertainty about the ability of the supply to absorb this current, add external transient voltage suppressor (TVS) diodes to the supply pins, as shown in Figure 39. Select the TVS voltage so that the diode does not turn on during normal operation. However, make sure that the TVS voltage is low enough so that the TVS diode conducts if the supply pin exceeds the safe operating supply voltage level.

1.

NOTE 1:

V_IN = +V_S + 500 mV.

2.

NOTE 2:

TVS: +V_S(max) > V_{TVSBR (min)} > +V_S.

3.

NOTE 3:

Suggested value is approximately 1 kΩ.

Figure 39. Equivalent Internal ESD Circuitry Relative to a Typical Circuit Application

The OPAx187 input pins are protected from excessive differential voltage with back-to-back diodes, as shown in Figure 39. In most circuit applications, the input protection circuitry has no consequence. However, in low-gain or G = 1 circuits, fast-ramping input signals can forward-bias these diodes because the output of the amplifier cannot respond rapidly enough to the input ramp. If the input signal is fast enough to create this forward-bias condition, the input signal current must be limited to 10 mA or less. If the input signal current is not inherently limited, an input series resistor can be used to limit the signal input current. This input series resistor degrades the low-noise performance of the OPAx187. Figure 39 shows an example configuration that implements a current-limiting feedback resistor.