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Richard Zarr

Mar 25, 2016

SN65220-Q1

INA219

analog
blog

LMP8480

ESD
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Related

Circuit-protection basics

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The world is a hazardous place, and electronic systems are just as
susceptible to mechanical failures as they are to unseen electromagnetic
radiation. Let’s take a look at how to protect circuit designs from some of
these visible and invisible perils.

A world inhospitable to electronics

It isn’t enough that the complexity of systems is ever-increasing; designers
now need to worry about protecting their circuitry from both man-made and
naturally occurring phenomenon such as electrostatic discharge (ESD), lighting,
electromagnetic interference (EMI), mechanical failure (such as equipment drops)
and many more. However, the good news is that for a small additional cost, you
can circumvent many of these issues and protect the end user’s investment by
protecting the circuitry.

The first, most obvious problem is the users themselves. People can become
very large capacitors, especially in dry climates indoors during winter or
desert conditions elsewhere. ESD, or electrical overstress (EOS), is a leading
cause of electronic death for many systems.  ESD may occur when a buildup
of electric charge on an object or person discharges into an electronic system.
The charge naturally wants to dissipate, and that can happen extremely rapidly.
A classic example of an electrostatic discharge is when you are scuffing across
the floor, and you touch a grounded conductor such as a screw on a wall switch
or another person. The electric field intensity gets so high between these two
points that it ionizes the air, allowing a flow of electrons. It takes about
70,000V to breach one inch of dry air … and only several hundred volts to damage
some semiconductors.

By placing a simple low-cost device called a transient voltage suppressor
(TVS) diode at any location where a connector leads into the circuitry can help
prevents ESD damage to the system. Many of these TVS devices are designed for
specific applications, such as the SN65220-Q1
designed for USB  ports. Others are designed for higher-speed interfaces
where capacitive loading can be an issue.

Another problem with users is that they are prone to dropping things like
handheld equipment or smartphones. Most systems can handle mechanical shock, but
some cannot, such as a rotating media hard drive in a laptop. Excessive G-force
can damage hard drives and in many cases cause data loss. As strange as it
sounds, detecting a fall is extremely beneficial: adding a simple accelerometer
can detect the beginnings of a fall via a system management processor which can
quickly park drive heads in a position to prevent damage and data loss.

Following a crash or other system disruption, additional damage may occur
when there is an upset to the normal operating parameters (a latch-up condition
caused by an electromagnetic wave, ESD or other event) or damage to the device
caused mechanically which can cause open or short circuits. A simple but very
effective method for protecting the system is to monitor the power supplies by
sensing the output voltage and current. If the voltage sags and the current
rises, it’s a good bet that something is drawing current that shouldn’t, such as
when a paper clip pokes into a keyboard causing a short circuit.

Monitoring the voltage is fairly straightforward, but monitoring current is a
bit harder. If you sense the return current in the power supply, it is
ground-referenced and easy to measure (Figure 1). But if you want to sense
current in the supply (high) side, you must do a differential measurement across
the sensing resistor. Most designers want to use a very small value for this
resistor so as to not impact efficiency. The problem is that you need high gain
– along with a very low input offset and a high common-mode voltage – to
accurately measure the current that the power supply is providing.

Figure 1: Low-side sensing with a precision amplifier to
protect a load from overcurrent conditions

Fortunately, there are devices designed to measure the current in the high
side of a power supply. One example is the INA219,
which includes a programmable gain amplifier (PGA), analog-to-digital converter
(ADC) and I2C digital interface.

Figure 2 shows a typical high-side application. By simply reading registers
within the device, you can obtain the output voltage, current and power. If the
readings  go out of range,  a system monitor can turn off the power
supply before damage occurs. If a faster response is required, an analog output
version such as the LMP8480,
along with a high-speed comparator, can shut down the power supply.

Figure 2: High-side intelligent sensing example using the
INA219 with a digital interface

Conclusion

Protecting circuitry from the environment can be pretty easy, and the choices
available to designers for protection devices have increased significantly. So
if you don’t want your designs ending up as a paperweight, remember to build in
circuit protection from the power supplies out to the connectors… till next
time!

Additional resources

Check out this blog series on getting started with current sense amplifiers.

See TI’s broad precision amplifier portfolio.

Preview and log in to download Best of Baker’s Best --Precision Amplifier e-book by my
colleague, Bonnie Baker.

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in to Comment

Rookie

You sure those gain setting resistors are labeled correctly?

Apr 3, 2016

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Richard Zarr

Hi Rookie,

Yes, You are correct the values have been swapped... Rf=249, R=1.00K.
Nice catch.  I'll have the figure fixed.

Apr 4, 2016

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Richard Zarr

FYI... figure 1 has been updated with the feedback resistor values in the
correct place.   Rf=249K, R=1.00K, A= 250.

Apr 4, 2016

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