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Thomas Kuehl

Oct 23, 2015

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Op amp supply limits: are they critical?

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A few years ago, a customer asked me a rather surprising question: “I realize
I am exceeding the maximum supply-voltage rating, but how long will your OPA373 operational
amplifier continue to operate if it is powered by a 12V supply?” The operational amplifier (op amp) in question has an absolute
maximum supply rating of 7V.

I told him that it would only be a matter of time before he would begin to
have field failures and all of the op amps subjected to the overvoltage would
eventually fail earlier than he might expect. The customer assured me that he
was in the process of correcting the error and that future production would use
an amplifier that could operate safely with 12V.

Applying a voltage to a circuit that exceeds the op amp supply, or any other
component’s maximum rating is inviting trouble. A small overvoltage probably
won’t trigger an immediate failure, but as an applied overvoltage becomes
larger, the possibility for spontaneous, catastrophic failure increases
dramatically. Integrated circuits (ICs) have some supply voltage guard banding
to accommodate normal process variances. However, they are in place to assure
that the product always operates when using supplies set within the specified
power-supply range.

Figure 1 illustrates an example of an op amp power-supply range that includes
a region where safe operation is assured, and unsafe regions where its lifetime
may be reduced or cut very short. The op amp in this case is a classified as a
low voltage op amp, fabricated on a complementary metal oxide silicon (CMOS)
semiconductor process. The amplifier’s supply increases on the vertical scale
from 0V to a level beyond its absolute maximum voltage, where catastrophic
failure is inevitable.

Figure 1: CMOS op amp power supply
regions

Instantaneous failure can happen when the insulating dielectric materials
separating the IC elements cannot withstand the potential difference applied
between them. Circuit points where V+ and V- (GND) are physically very close
might exhibit the highest electrical field intensity and potential for
breakdown. There may be other circuits where the voltage difference is less, but
because of very high field intensity, breakdown might occur first. Should that
happen, high current flows through the minute metal traces and/or silicon,
resulting in very high localized current and power densities, and near
instantaneous super-heating in the immediate area. The associated metal and/or
silicon melt and are redistributed on and within the IC structure. They solidify
quickly, creating unintended conductive short circuits and/or open circuits. In
addition, damage to semiconductor junctions can occur if the current flow
through junctions is excessive. The IC is damaged and rendered useless and may
now draw excessive current.

So what about the customer case where the amplifiers continued to function
despite having the V+ maximum rating exceeded by a full 5V? Immediate breakdown
did not occur and they continued to function in the field. Even so, the customer
should not expect the devices to operate correctly long-term and they may
eventually incur catastrophic failure. Delayed failure of an IC can take
moments, months or even years to happen.

Although an applied voltage may not be high enough to trigger instantaneous
failure, it may be high enough to move the transistor operating conditions from
safe and normal to unsafe and abnormal. The higher voltage conditions and
increased field intensities can negatively affect the precisely constructed
semiconductor structures and junctions resulting in excessive operating current
levels, unintended current paths (latches), increased leakage currents and
increased self-heating. The amplifier circuit may still function, but it may not
operate with the same electrical performance.

For example, CMOS op amps may enter an unintended bias condition called hot
carrier injection. It occurs when excessively high voltage is applied between
the drain and sources of the MOS field effect transistors (MOSFETs). Accelerated
electrons are injected into the transistor’s gate oxide causing a shift in its
threshold voltage. The shifts continue with continued exposure to the high
electric field conditions. The MOSFET electrical characteristics become affected
over time and in turn, the overall performance of the op amp may degrade. Always
avoid exposing any op amp, CMOS, bipolar or junction FET (JFET) even for a
moment to a moderate-to-high overvoltage supply level.

When operated within the specified supply range, an amplifier IC can operate
continuously and reliably, often for many decades. Just how long its lifetime
might be is predictable by applying the Arrhenius equation. My colleague Marek
Lis covered this subject at length in another Precision Hub blog post. However, for overvoltage
conditions, things are not as straightforward, and it is nearly impossible to
determine how long the amplifier will last.

As odd as it might seem, I have encountered instances where customers had
powered their amplifier circuit with too low of a supply voltage, yet they
observed mostly normal operation from the circuit. This supply condition falls
somewhere in the power-up region of Figure 1. In my next post, I’ll examine
amplifier operation at this other end of the supply-voltage range.

Additional resources:

Learn about TI’s precision operational amplifier portfolio.

Find useful analog design formulas in the Analog Engineer’s Pocket Reference.

Watch on-demand training courses on stability and more in TI Precision Labs – Op Amps.

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