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Hi. My name's Chi Nguyen, and I'm an applications engineer intern from the comparators team at Texas Instruments. Today, we'll be discussing negative transient input voltages into comparators in high-current automotive electrical systems.

In high-current automotive electrical systems, a common challenge that occurs is a difference in ground potentials, also known as ground offsets. This can be caused when there are split ground planes to isolate sensitive analog circuitry from high-current switching nodes, or when the ground planes are not utilized. Hybrid electric and electric vehicles frequently have two different ground reference points that may sit at different potentials due to ground shifting from high-current electrical circuits. This can cause negative transient voltages on the input pins of comparators.

Let's begin with a basic overview of how a comparator works. A comparator gets its name because it compares the voltages applied through its inputs and sends its output voltage based on the input levels. If the input signal is greater than the reference voltage, the output will be high, or a logic 1, which approaches the positive power supply voltage. If the input signal is less than the reference voltage, the output will be low, or a logic 0, which approaches the negative power supply voltage.

A negative input voltage relative to the ground pin may come from these issues in automotive electrical circuits, where it violates the negative common mode voltage range and causes a scenario called phase reversal to occur. Under phase reversal conditions, the negative input current turns on internal parasitic transistors that steal current from other internal nodes, which leads to malfunctions and causes the polarity of the output to become incorrect. This phase reversal condition is shown in the figure below, which shows the transient analysis of a comparator in a noninverting configuration.

As the input signal goes below 0 volts, the output of the comparator goes low, as expected. However, as the input reaches about negative 570 millivolts, the output inverts and goes high, even though this is not the correct operation of the comparator. This is what we call phase reversal or phase inversion.

Fortunately, some comparators are designed to prevent phase inversion. As you see in the figure on the bottom left here, these comparators have internal ESD protection diodes connected from the negative supply pin to the input pins. This prevents large negative current spikes to flow directly into the pins and causing damage to the comparator. The ESD diode helps clamp any over-voltages to the negative supply rail by diverting current flow to ground, and thereby protects the comparator.

The figure on the right shows the voltage-current characteristic curve for a diode in the forward bias region. When we have a negative input voltage or a voltage below ground, the diode is in the forward bias region. The diode does not start conducting current until it reaches a certain voltage called the knee point. After this knee voltage point, the diode conducts current linearly with voltage.

The TLV1701 is a comparator that includes this low-side ESD diode feature. The TLV1701 is a high-voltage microPower comparator with an open collector output. The absolute maximum ratings table of this device state that the minimum input voltage allowed is 0.5 volts below the VS minus supply rail. Therefore, assuming that VS minus is 0 volts at ground, the input voltage needs to be limited so that the ESD diode doesn't conduct beyond negative 0.5 volts.

Depending on how large the negative input voltage is, one way to do this is to limit the input current to a safe level by using a current-limiting resistor in series with the input signal. In this case, for the TLV1701, footnote 2 of the absolute maximum ratings table states that the input signal can swing beyond negative 0.5 volts as long as the current is limited to less than 10 milliamps. Limiting the current to less than 10 milliamps will protect your device and prevent it from being damaged, but not prevent phase reversal. However, by limiting the input voltage to less than negative 0.5 volts, this will always prevent phase reversal, as well as limits the current to less than 10 milliamps.

Typically, the current should be limited to 1 milliamps or less at the highest expected over-voltage, so the resistor must be at least 1 kiloohms per volt. For example, if the maximum expected negative voltage is negative 5 volts, then the resistor should release 5 kiloohms or larger. However, not all devices follow this rule.

Some devices require the input to be limited to a voltage more positive or close to ground than negative 0.5 volts. So the current-limiting resistor may need to be larger than 1 kiloohms per volt. You will need to look at the absolute maximum ratings table of the device to see how much resistance you need to limit the current.

Let's take a look at how phase reversal is prevented using the TLV1701. The figure on the left shows a device set up in a noninverting configuration, along with a 10-kiloohm resistor in series with the input. The plot on the right shows the output response when the input signal is ramping on negative 1 volt below ground. The blue wave form is the signal at the noninverting pin, and the red wave form is the output pin, not the comparator.

Note that the voltage at the noninverting pin gets limited and doesn't get to negative 1 volt due to the current-limiting resistor and the ESD protection diode. This shows that the operation of the comparator works as intended and the output does not invert. That concludes this presentation. Thank you so much for your time.