For the modern day embedded designs, ESD protection is a must. Especially, in the microcontroller designs where the input comes from external means like the sensors or there could be some signals which are terminated to a connector for driving external circuits or there could be power supply input to the board on which Microcontroller is mounted . For all these scenarios, ESD protection is a must to pass compliance tests. Forget about standard products requiring compliance, even if you are designing a hobby project, evaluation board, ESD is protection is must as this might help in reducing the failures due to static energy.
Let us return turn our discussion to protection in Microcontrollers. Microcontrollers do have protection on their pins. However, as this is internal and may not be sufficient to control the ESD at all times. Why we say at all times is because the response time to an ESD event could be higher and hence the ESD would have caused damage to the device by the time the diodes respond. The protection can be done in many ways:
- A simple series resistor to dampen the signal. This is most widely used and is also helpful in current limiting under fault scenarios. While this serve the purpose, the only issue is that this adds up input impedance to the pin of the microcontroller and should be carefully chosen such that it doesn't have an impact on the input that is connected to Microcontroller. Requires clear understanding of the input impedance of the Microcontroller pin, voltage requirements, minimal current required, etc
- An external ESD diode connected on the I/O lines provides the best protection and is widely used.
- The amount of ESD to be handled which depends on the compliance requirements of the board
- Space on the board which determines the package that can be selected
- Capacitance of the ESD diode
- Current handling capability
- Obviously, the working voltage requirement
- ESD for power or signal. If signal, what is the maximum frequency of the signal?
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