OPA209_10 Datasheet, PDF (14/25 Pages) Texas Instruments – OPERATIONAL AMPLIFIER

OPA209
OPA2209
OPA4209
SBOS426B – NOVEMBER 2008 – REVISED AUGUST 2010
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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.
It is helpful to have a good understanding of this basic ESD circuitry and its relevance to an electrical overstress
event. See Figure 35 for an illustration of the ESD circuits contained in the OPA209 series (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 they meet at 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,
high-current pulse as it discharges through a semiconductor device. The ESD protection circuits are designed to
provide a current path around the operational amplifier core to prevent it 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 above the normal operating voltage of the
OPA209 but below the device breakdown voltage level. Once 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 35 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.
Should this condition occur, 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.
Figure 35 depicts a specific example where the input voltage, V_IN, exceeds the positive supply voltage (+V_S) by
500mV 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 datasheet specifications recommend that applications
limit the input current to 10mA.
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
the power supplies +V_S and/or –V_S are at 0V.
Again, it depends on the supply characteristic while at 0V, or at a level below the input signal amplitude. If the
supplies appear as high impedance, then the operational amplifier supply current may be supplied by the input
source via 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 an uncertainty about the ability of the supply to absorb this current, external zener diodes may be
added to the supply pins as shown in Figure 35. The zener voltage must be selected such that the diode does
not turn on during normal operation.
However, its zener voltage should be low enough so that the zener diode conducts if the supply pin begins to
rise above the safe operating supply voltage level.
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