TVSs are usually just a Zener rated for high-energy pulses, so I use
just TVSs in my external connections.

The series resistor is an excellent idea too, to limit the inrush
current in the pin and prevent latchup and also to protect the TVS itself.

For input pins I use one series resistor from the connector to the
cathode of the TVS and a second series resistor from the cathode of the
TVS to the MCU pin.
When the pin is expected to run without nothing connected, I put also a
pull-up or pull-down resistor from the connector pin to a suitable voltage.

For not-too-high-speed and low-current output pins I use a pull-down
resistor directly to the MCU pin and the same 2 resistors plus one TVS
to the connector.

The secret is choosing the value of the resistors to get a good balance
between protection-level/drive-current/switching-speed. Don't forget
that the TVS/Zener diode ads some capacitance that together with
high-value resistors will create a low-pass filter.

This scheme can be used even for hot-plug setups.


For harsher environments you can use BJT transistors between the
connector and MCU pins. BJTs are very rugged and even if they fail it is
much less probable that the disturbance hits the MCU pin.


Best regards,

Isaac


On 12/10/2014 17:07, Phil Keller wrote:
>    I am working on a board design that has the end user attaching=20
> another PCB to the 5 volt I/O's of my board.  I am concerned with=20
> protecting the inputs of my board from both ESD and over voltage on the=20
> pins.   The classic approach for over voltage is to utilize a 5.1 volt=20
> ZENER (I chose the DDZ9689).  The classic approach to protect against=20
> ESD is to utilize a TVS diode (I chose the ESD5V3U2U).    Cost is not a=20
> significant issue since less than 200 boards will be built, but they do=20
> need to be 'forgiving'.
>
>    My circuit has the input pin connected to the TVS diode and a 200ohm=20
> series resistor that is connected to the ZENER (to ground) and the input=
=20
> of a CMOS buffer (NC7WZ17).
>
> - Am I on the right track or is this configuration not as robust as it=20
> could be?
> - What should I be concerned with?
>
> -Phil-



--=20
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