OPA172IDR Datasheet, PDF (18/29 Pages) Texas Instruments – 36-V Single-Supply 10-MHz Rail-to-Rail Output Operational Amplifiers

OPA172
OPA2172
OPA4172
SBOS618 – DECEMBER 2013
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When the operational amplifier connects into a circuit (such as the one Figure 45 depicts), 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 can turn on and conduct current.
Any such current flow occurs through steering-diode paths and rarely involves the absorption device.
Figure 45 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 can 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 or –V_S are at 0 V. Again, this question depends on the supply characteristic while at 0 V,
or at a level below the input-signal amplitude. If the supplies appear as high impedance, then the operational
amplifier supply current can 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 zener diodes to the
supply pins, as shown in Figure 45. Select the zener voltage so that the diode does not turn on during normal
operation. However, the 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.
The OPAx172 input terminals are protected from excessive differential voltage with back-to-back diodes, as
shown in Figure 45. In most circuit applications, the input protection circuitry has no effect. 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, limit the input signal current to 10 mA or less. If the input signal current is not inherently limited, an
input series resistor can be used to limit the input signal current. This input series resistor degrades the low-noise
performance of the OPAx172. Figure 45 shows an example configuration that implements a current-limiting
feedback resistor.
OVERLOAD RECOVERY
Overload recovery is defined as the time it takes for the op amp output to recover from the saturated state to the
linear state. The output devices of the op amp enter the saturation region when the output voltage exceeds the
rated operating voltage, either due to the high input voltage or the high gain. After the device enters the
saturation region, the charge carriers in the output devices need time to return back to the normal state. After the
charge carriers return back to the equilibrium state, the device begins to slew at the normal slew rate. Thus, the
propagation delay in case of an overload condition is the sum of the overload recovery time and the slew time.
The overload recovery time for the OPAx172 is approximately 200 ns.
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