OPA1641_15 Datasheet, PDF (14/29 Pages) Texas Instruments – High-Performance, JFET-Input AUDIO OPERATIONAL AMPLIFIERS

OPA1641
OPA1642
OPA1644
SBOS484B – DECEMBER 2009 – REVISED AUGUST 2010
Although the output current is limited by internal
protection circuitry, accidental shorting of one or more
output channels of a device can result in excessive
heating. For instance, when an output is shorted to
mid-supply, the typical short-circuit current of 36mA
leads to an internal power dissipation of over 600mW
at a supply of ±18V. In case of a dual OPA1642 in an
MSOP-8 package (thermal resistance q_JA =
180°C/W), such a power dissipation would lead the
die temperature to be 220°C above ambient
temperature, when both channels are shorted. This
temperature increase would destroy the device.
In order to prevent such excessive heating that can
destroy the device, the OPA164x series has an
internal thermal shutdown circuit, which shuts down
the device if the die temperature exceeds
approximately +180°C. Once this thermal shutdown
circuit activates, a built-in hysteresis of 15°C ensures
that the die temperature must drop to about +165°C
before the device switches on again.
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. Figure 35 illustrates the ESD
circuits contained in the OPA164x 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.
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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 OPA164x 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.
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