tlv3492 comparator... how to protect inputs

Hello Maurice,

Internal to the TLV3492 are ESD current steering diodes from each input to each supply pin. There diodes are off during normal operation. If one, or the other, diode is inadvertently turned on by an input over-voltage the device could be damaged. Therefore, external protection must be provided to keep this from happening. Here is an approach to protecting the TLV3492 inputs from the +/-25V high input voltage:

A resistor is placed in series with each input and a zener diode placed from each input to ground (cathode at pin, anode to ground). The zener voltage should be selected such that the diode turns on before the TLV3492 maximum input voltage is exceeded. If the supply voltage is 5V, then the diode should have a zener voltage of about 5V. The MMSZ4689T1 is a low current zener having a nominal 5.1V zener voltage and range of 4.85V to 5.36V (Iz=10mA). The series resistor is selected to drop the voltage from 25V to 5.1V, that drop is divided by the maximum current that you select to run through the diode. For example, if the diode current is set to 1mA and the drop is 20V, then the series resistor would be 20k-Ohms. The zener would then clamp a positive voltage to a safe level when the applied voltage exceeds a value greater than the zener voltage. When it comes to a negative input over-voltage the zener diode will clamp the input at the forward bias voltage level (0.6 -0.7V) of the diode. The internal ESD steering diode will be in parallel with the forward-biased zener and current will flow through it as well. However, the series resistor will limit the current through both of them.

The zener has fairly high capacitance in the tens-of-picofarads, or more. That capacitance in conjunction with added series resistance will slow down intended input transitions. Increasing the allowable current through the zener will reduce the required series resistance required and shorten the time constant. Keep in mind that the TLV3492 is a nano-power comparator and although not lightning fast it has a propagation delay time of about 6us. That is faster than the time constant of the 20k-Ohm series resistor/ zener capacitance (~20us). If that is a concern it would be worthwhile to seek a lower capacitance zener than that used as an example.

Regards, Thomas

PA - Linear Applications Engineering

Hello Maurice,

The normally off ESD steering diodes inside the TLV3492 can carry up to 10mA of current on a continuous basis; somewhat more on a momentary basis. The thing to keep in mind is where the input current is goes during an over-voltage event - the supply pin. Most power supplies are single quadrant power sources. They supply can supply a positive or negative voltage and only source current. Most often they do not sink current. Therefore, if you start over driving the TLV3492 input and the steering diodes steer current to the supply pin, the supply can not sink that current. Often what happens is the voltage on the supply pin begins to rise with the increased input over-voltage. There is always the risk that the maximum supply voltage of the comparator could be exceeded. However, the added series resistance placed in series with the input would likely limit the current to a safe value.

We often recommend adding a zener diode to each supply pin that has a zener voltage above the circuit's operating voltage, but lower than its absolute maximum supply voltage rating of the device. Should the input over-voltage momentarily attempt to raise the supply pin voltage above the absolute maximum the zener would conduct and provide a path to ground for the input current. In a single supply configuration the zener diode could only be added to the positive supply. A higher current Schottky diode (BA44) could be added from each input to ground such that it shunts the internal ESD diode that protects against negative-going ESD events. The Schottky will turn on at a lower forward voltage than the internal silicon diode and divert the input current.

If your supply is capable of sinking current, then the supply diodes should not be necessary.

Regards, Thomas

PA - Linear Applications Engineering