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C. Ryan Land

Jun 5, 2015

Buffers

ESD

Logic

signal
switches

voltage
level translation

diodes

little
logic

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Related

Clamp diodes: A one-way street

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“Am I allowed to have an input signal when VCC = 0 V?”

“What happens if the input or output voltage goes below GND?”

Customers ask
these questions quite frequently – and for good reasons. Many standard logic
datasheets don’t specify explicitly if conditions are allowed where inputs go
above VCC or below ground (GND). The devices themselves have few (if
any) performance characteristics in the datasheet at those fringe voltages.

One of the most important features of most TI devices is the presence of
electrostatic discharge (ESD) protection. In fact, in some devices, ESD
protection circuitry takes up most of the space on the die. Typically, an ESD
circuit includes a diode that channels excess current to GND (through reverse
breakdown) or VCC (through forward biasing) when the input voltage
approaches unsafe levels. The intent is to protect against damaging conditions
in the internal circuitry.

However, the ESD diodes can have some unintended effects. For example, if
there is an input diode going from the input to VCC, an input voltage
at VCC = 0 will cause current to flow through the device to the
VCC pin. Likewise, a diode from GND to the input will cause current
to flow from GND to the input when the input reaches a certain level below
GND.

So how do we know when the input diodes are present, and how can we protect
against negative effects?

The absolute maximum ratings table (Table 1) holds the answers.

Table
1: Sample Absolute Maximum Ratings Table

On this analog switch, we can see that a voltage on the I/O pins is not
allowed to be higher than VCC, or less than -0.5V. Therefore, we can
conclude that there is a diode to VCC and a diode from GND on each
I/O port of the device.

Figure 1: ESD diode structure
of device with Absolute Max Table similar to Table 1

Table 2 offers another example where the absolute maximum rating is
structured a bit differently.

Table
2: Sample Absolute Maximum Ratings table with different limits on inputs vs.
outputs

We can see here that the diode is present from GND to both the input and
output, and the ESD diode to VCC is present at the output. When the
device is in a high or low state, there is a risk of conducting current through
the VCC diode. This device has a special feature called a
high-impedance state that prevents current backflow through the diode. It
also has protection circuitry for when VCC = 0, called
IOFF.

However, from the VCC + 0.5V statement, we know there is a diode
going from the output to VCC, and there is protection circuitry to prevent
issues in certain configurations.

Figure 2: ESD diode structure
for device with Absolute Maximum Ratings similar to Table 2

But what are the limits? The specs IIK and IOK describe the current limits
into the device (positive values) and out of the device (negative values)
through ESD diodes. Consider the first case (Table 3), where we had two GND
diodes.

Table
3: Sample Ratings for Clamp Currents in Absolute Maximum Ratings

The current in the GND diode is limited to 50mA. For the second case with the
VCC diode, the “output clamp current” would have a +/- symbol,
indicating that current can flow in both directions (into the VCC
diode or out of the GND diode).

It is best to avoid situations where ESD diodes can turn on. Most data sheets
specify an exception to the input and output voltage rules – so if you limit the
diode clamp current with a series resistor on the input or output, your device
will be at much less risk of damage.

Learn more on designing with standard logic from the following application
notes:

“Latch Up,
ESD and Other Phenomena”

“Understanding
and Interpreting Standard-Logic Datasheets”

“Voltage
Translation Between 3.3-V, 2.5-V, 1.8-V and 1.5-V Logic Standards”

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