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Thomas Kuehl

Jan 15, 2016

opampportal

TLV2620

TLV2332

pampportal

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Related

What you need to know about operating an op amp when your
voltage is too low

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In my last blog, I described how a customer had been operating
their op amp circuit with a power supply voltage level that was too high for the
amplifier and the consequences of doing this. Recently, while meeting with other
customers, I’ve had similar conversations about voltage that is too low and I
want to share my advice to them in this blog post.

This may be surprising, but I have come across instances where a customer
discovered that the supply voltage they selected to power their application
turned out to be lower than the minimum rated operating voltage of the
operational amplifiers (op amps) used in their circuit design.

I expect this happens due to an oversight that occurs over the time it takes
to develop and design a product and put it into manufacturing. I can imagine a
scenario where a designer identifies and selects a particular op amp for its
electrical performance and ability to meet the low-end supply-voltage
requirement.

For example, let’s say that the minimum voltage requirement is 2.7V and you
select an op amp that meets that requirement. Then later, someone decides to use
a different op amp due to lower cost or availability, but no one noticed its
minimum supply voltage is higher, such as 3V. Initially the circuit performs as
expected in production. But later, when the customer receives another different
batch of the op amps, the circuit performance doesn’t meet the necessary levels.
An investigation into the performance issue brings the supply-voltage
discrepancy to light.

An undervoltage scenario does not sound too scary. That is because the bad
things that can happen when an op amp is exposed to a high overvoltage are
unlikely to occur. The main concern then becomes how the op amp performs
electrically when subjected to an undervoltage condition.

When examining the lower end of the supply-voltage scale shown in Figure 1,
you’ll see that for this complementary metal-oxide semiconductor CMOS op amp,
the minimum specified operating voltage is 2.7V. The op amp’s design team takes
into consideration its design and semiconductor process guardbands when
establishing the amplifier’s lower supply limit. The process guardband
compensates for normal semiconductor process variances that affect the op amp’s
internal operating points and provides assurance that every one of that
particular op amp operates at 2.7V,  (if observing the stated datasheet
conditions).

Figure 1: An example of the low
operating voltage end for a CMOS op amp

In fact, because of the guardbanding, the op amp actually continues to
operate with a supply voltage level somewhat less than 2.7V. Casual checks made
in the lab on some low-voltage CMOS op amps show that their lower supply
functional limit can be a few tenths of a volt to almost a volt below the
minimum specified voltage.

You can see why some op amps continue to operate with a small undervoltage by
viewing a set of curves commonly found in the datasheet. Figure 2 is a graph of
supply current, IDD, vs. supply voltage, VDD, from the TLV2620 data
sheet. The TLV2620
is a wideband CMOS op amp with an 11MHz unity-gain bandwidth. Its recommended
supply range is 2.7 to 5.5V, but it is usable to 6V on the high end.

Figure 2: TLV2620 CMOS op amp IDD vs VDD
and temperature

Figure 2 provides information on how the TLV2620 IDD is a function of both supply voltage
and temperature. Of particular interest for this discussion is the minimum
specified 2.7V VDD level indicated by the vertical red line in Figure
2. The temperature curves show that IDD has the same slope on both
sides of the 2.7V line. The op amp has only slightly less IDD at 2.3V
or 2.4V (shown as the blue line) than it does at 2.7V. Since there isn’t an
abrupt change in current at this lower voltage, the op amp is very likely
operational.

IDD drops quickly as VDD decreases further. Voltages to
the left of the “cliff” where the current falls off quickly are associated with
the “power up region” shown in Figure 1. The internal circuits are not fully
biased in that region and begin to shut down as the voltage continues lower.

Different op amps exhibit different behaviors in the region around the cliff.
Some such as the TLV2620
show an immediate steep drop-off in IDD, while others show a gradual
decrease in the operating current as VDD is reduced. In either case,
you have no way of knowing at what level the amplifier becomes fully functional.
That is until the op amp’s minimum specified operating supply voltage is
attained.

Other op amps such as the CMOS TLV2332 exhibit much more gradual IDD vs.
VDD characteristics. The TLV2332 is a low-voltage, medium-power op amp; its
IDD vs. VDD graph is shown in Figure 3. Its electrical
characteristics are specified for VDD levels of 3V and 5V; however,
its recommended supply range is 2V to 8V, indicating that it is fully functional
at 2V.

Figure 3: TLV2332 IDD vs VDD

Figure 4: TLV2332 open-loop gain vs.
VDD

Figure 3 shows how IDD follows VDD down and doesn’t
exhibit an abrupt cliff. Interestingly, the TLV2332 datasheet provides several graphs plotting various
DC and AC parameters against VDD. Figure 4 is an example, which plots
the large-signal differential-voltage amplification, AVD (also known
as open-loop gain, Aol) with VDD. Figure 4 also shows the dramatic
effect that VDD has on Aol. Even though it falls off significantly
with low VDD to about 50kV/V with a 2V supply level, it is still high
enough for less-critical applications.

This low supply-voltage capability is particularly useful in battery-powered
applications. If an application is powered from a 3V battery, it can discharge
down to 2V and the op amp will remain functional – albeit with reduced
performance.

Returning to my original undervoltage topic, it is important to understand
the performance implications, should you find yourself with a supply voltage
lower than the minimum rated operating voltage of your op amp. Certainly, it is
best not to use an op amp with a minimum supply voltage rating that is higher
than the application’s.

Once you discover this issue, immediately begin searching for a replacement
op amp specified for the lower supply-voltage level that also provides the
correct electrical performance. If you need help finding a suitable replacement,
share your requirements via a quick post to the TI E2E™ Community Precision Amplifiers Forum, and we’ll provide some
options.

Additional resources

Read the TLV2620
and the TLV2332
datasheets.

Learn more about TI’s precision amplifier portfolio and find technical
resources.

Watch on-demand training courses on input offset voltage and input bias
current in TI Precision Labs.

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