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self-powering device?

The self-powering device?

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The self-powering device?

Timothy
Claycomb

Apr 14 2014 13:50 PM

Comments 4

As a
newly hired employee recently out of college, previously known as “John the intern”, I thought my learning days were over.
However, I quickly found out that is not the case and had a lesson on
electro-static discharge (ESD) diodes of a device and the unintended effects
they may cause in an application.

I needed
to do some testing on the OPA192, so I grabbed my circuit board, headed to the lab and
connected everything up. I noticed some very odd behavior when I observed
the output on the oscilloscope. The device was in a unity gain configuration, so
I expected the output to be equal to the input. Figure 1 shows the input
(channel 1) and output (channel 2) of what I saw.

After
some troubleshooting, I realized I made a rookie mistake. I forgot to turn on
the power supplies! This got me thinking, how can there be an output without
powering the device?

I
researched this phenomenon and found Figure 2 in the OPA192 datasheet
that shows the internal ESD diodes of the device.

I found
that if an input signal is greater than the supply voltage by approximately one
diode drop then the positive supply diode would conduct and route current
around the device and to the power supplies. With the power supply turned off,
all that is present is the internal impedance of the supply. Figure 3 shows a
simplified version of Figure 2 with an equivalent model of the power
supplies.

Routing
the input signal to the supply will create a voltage drop across the impedance
of the power supply and this will cause a voltage to be present on the
power supply pins. Table 1 shows the voltage measured at the output, positive
supply and negative supply for different input voltages.

As you
can see, with a positive voltage at the input the V+ supply has a measurable
voltage. This is because ESD diode D1 is conducting and creating a voltage on
the positive power supply. Similarly, with a negative input, a voltage is
present at V- supply because ESD diode D2 is conducting. The voltage at the
supplies turns on the internal transistors of the device and it begins to
operate as it would with the supplies turned on. This is why I saw a distorted
sine wave at the output with no power to the device.

In a
typical system, there are multiple devices that share the same power supply.
These devices may turn on unexpectedly because of the voltage at the power
supply pins caused from turning on the ESD diodes of the device. Figure 4 shows
a block diagram of how sharing the same supply can potentially power other
devices, such as analog to digital converters (ADC) or logic gates.

Furthermore, having an unexpected output similar
to the one shown in Figure 1 is just as problematic. The output might be
connected to a comparator that interfaces with a microcontroller (MCU). If the comparator and MCU are powered
with a separate supply, the output of the amplifier could cause the comparator
to output false data to the MCU. Figure 5 shows a block diagram of how having an
unwanted output to a comparator can cause a MCU to receive false data.

Both of
these situations will give false results in the system, which can cause the
entire system to fail. The effects of this vary but can be as drastic as a car
turning off while driving or disabling a piece of medical equipment.

If your
design can have an input with the device not powered, you may want to consider
placing a switch between the input signal and input pin of the device.
Eliminating any possible input to the device will prevent the internal ESD
diodes from conducting and powering the device (and possibly other devices) with
the input signal.

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$post.CommentCount)

OPA192

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Comments

Stephen
Scott

Apr 27 2014 09:15 AM

The article mentions false inputs to a MCU. In the case that I observed,
the voltage from the input signal was sufficient to prevent the MCU from
performing a power-down-reset when the power was turned off.

Martin
Steube-Henkel

Apr 27 2014 10:53 AM

In Addition this usecase may cause an EOS of the diodes. The current is not
limited by series resistors. The d/s do not mention about the current rating
of the diodes. Tim, any Figuren about that?

Amalinda

Apr 27 2014 19:49 PM

Every newbie had this problem! I think everyone jurst figured a way around
it without trying to figure why. good effort.

Timothy
Claycomb

Apr 28 2014 10:55 AM

Hi Martin,

I do not know of any figures relating to the EOS of a device. Most
datasheets recommend limiting the input current to 10mA to prevent any damage
to the device. The OPA192 datasheet gives this limitation in the Absolute
Maximum Ratings. You could place a resistor in series with the input signal to
limit the current but that will not eliminate the possibility of turning on
the ESD diodes with the input signal.

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