OPA140
OPA2140, OPA4140
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SBOS498A –JULY 2010–REVISED AUGUST 2010
Figure 38 depicts a specific example where the input
voltage, VIN, exceeds the positive supply voltage
(+VS) by 500mV or more. Much of what happens in
the circuit depends on the supply characteristics. If
+VS can sink the current, one of the upper input
steering diodes conducts and directs current to +VS.
Excessively high current levels can flow with
increasingly higher VIN. As a result, the datasheet
specifications recommend that applications limit the
input current to 10mA.
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 the supply is not capable of sinking the current, VIN
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.
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 38. The zener voltage must be selected such
that the diode does not turn on during normal
operation.
Another common question involves what happens to
the amplifier if an input signal is applied to the input
while the power supplies +VS and/or –VS are at 0V.
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.
(2)
TVS
RF
+VS
+V
OPA140
RI
ESD Current-
Steering Diodes
-In
(3)
Out
Op Amp
Core
RS
+In
Edge-Triggered ESD
Absorption Circuit
RL
ID
(1)
VIN
-V
-VS
(2)
TVS
(1) VIN = +VS + 500mV.
(2) TVS: +VS(max) > VTVSBR (Min) > +VS
(3) Suggested value approximately 1kΩ.
Figure 38. Equivalent Internal ESD Circuitry and Its Relation to a Typical Circuit Application
Copyright © 2010, Texas Instruments Incorporated
17
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