Datasheet

R
F
Op-Amp
Core
R
I
R
L
V
(1)
IN
I
D
-In
Out
+In
ESD Current-
Steering Diodes
Edge-Triggered ESD
Absorption Circuit
+V
S
+V
-V
-V
S
OPA1611
OPA1611
OPA1612
www.ti.com SBOS450B –JULY 2009–REVISED JULY 2011
When the operational amplifier connects into a circuit direct current path is established between the +V
S
such as the one Figure 34 shows, the ESD protection and –V
S
supplies. The power dissipation of the
components are intended to remain inactive and not absorption device is quickly exceeded, and the
become involved in the application circuit operation. extreme internal heating destroys the operational
However, circumstances may arise where an applied amplifier.
voltage exceeds the operating voltage range of a
Another common question involves what happens to
given pin. Should this condition occur, there is a risk
the amplifier if an input signal is applied to the input
that some of the internal ESD protection circuits may
while the power supplies +V
S
and/or –V
S
are at 0V.
be biased on, and conduct current. Any such current
Again, it depends on the supply characteristic while at
flow occurs through steering diode paths and rarely
0V, or at a level below the input signal amplitude. If
involves the absorption device.
the supplies appear as high impedance, then the
Figure 34 depicts a specific example where the input operational amplifier supply current may be supplied
voltage, V
IN
, exceeds the positive supply voltage by the input source via the current steering diodes.
(+V
S
) by 500mV or more. Much of what happens in This state is not a normal bias condition; the amplifier
the circuit depends on the supply characteristics. If most likely will not operate normally. If the supplies
+V
S
can sink the current, one of the upper input are low impedance, then the current through the
steering diodes conducts and directs current to +V
S
. steering diodes can become quite high. The current
Excessively high current levels can flow with level depends on the ability of the input source to
increasingly higher V
IN
. As a result, the datasheet deliver current, and any resistance in the input path.
specifications recommend that applications limit the
If there is an uncertainty about the ability of the
input current to 10mA.
supply to absorb this current, external zener diodes
If the supply is not capable of sinking the current, V
IN
may be added to the supply pins as shown in
may begin sourcing current to the operational Figure 34. The zener voltage must be selected such
amplifier, and then take over as the source of positive that the diode does not turn on during normal
supply voltage. The danger in this case is that the operation. However, its zener voltage should be low
voltage can rise to levels that exceed the operational enough so that the zener diode conducts if the supply
amplifier absolute maximum ratings. In extreme but pin begins to rise above the safe operating supply
rare cases, the absorption device triggers on while voltage level.
+V
S
and –V
S
are applied. If this event happens, a
(1) V
IN
= +V
S
+ 500mV.
Figure 34. Equivalent Internal ESD Circuitry and Its Relation to a Typical Circuit Application
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