in/output protection

[CeeRox]

Hi!

Consider this scenario: A 5V TTL-signal goes from a connector to a MCU.
If eg. power supply, 12V is connected to this pin on the connector,
what would be the easiest way to protect the input of the MCU?

/C

 

[Jan Panteltje]

Hi!

Consider this scenario: A 5V TTL-signal goes from a connector to a MCU.
If eg. power supply, 12V is connected to this pin on the connector,
what would be the easiest way to protect the input of the MCU?

/C

1)
--- R -----------> MCU
|k
zener 5.1V
|
///

2)
+5V
|k
diode
|a
--- R ---------> MCU



2) will make the +5V higher if R is low enough, or there is little load.

 

[Spehro Pefhany]

Hi!

Consider this scenario: A 5V TTL-signal goes from a connector to a MCU.
If eg. power supply, 12V is connected to this pin on the connector,
what would be the easiest way to protect the input of the MCU?

/C

If it's an input to a CMOS chip (TTL levels), usually a series
resistor of a few K will suffice.

If it's an output you may need a small value resistor plus some kind
of clamping device such as a TVS or Zener diode.



Best regards,
Spehro Pefhany

 

[CeeRox]

Jan Panteltje skrev:

1)
--- R -----------> MCU
|k
zener 5.1V
|
///

2)
+5V
|k
diode
|a
--- R ---------> MCU



2) will make the +5V higher if R is low enough, or there is little load.

R would be at least 50 Ohm if the zener can handle 1 Watt continous
(7V).... There would be problems if a CMOS-output should have an input
protection for eg. 75V. But then maybe a VDR could be useful, if the
zener-diode would survive the time before the VDR gets "high-ohmic".

 

[Joerg]

Jan Panteltje skrev:




R would be at least 50 Ohm if the zener can handle 1 Watt continous
(7V).... There would be problems if a CMOS-output should have an input
protection for eg. 75V. But then maybe a VDR could be useful, if the
zener-diode would survive the time before the VDR gets "high-ohmic".

Don't do this. If the zener or diode clamps just a wee bit beyond the
substrate diode then the substrate diode will "win", take the brunt of
the current and the MCU goes up in smoke. They can only take so many mA
on the substrate diodes and after that it can latch up. If you are
really lucky it'll take the regulator and everything else with it to the
grave.

Use several 10K or provide Schottky diodes to VCC and GND. Or use a
regular double diode such as the BAV 99 (for low current cases) but a
second resistor between that and the MCU pin.

Another often unwanted effect is that a current into the upper substrate
diode can actually power your MCU side. If it doesn't present enough of
a load, maybe because it has entered a low power mode, that voltage
might exceed abs max and then bzzzt ... poof.

 

[CeeRox]

Joerg skrev:

Don't do this. If the zener or diode clamps just a wee bit beyond the
substrate diode then the substrate diode will "win", take the brunt of
the current and the MCU goes up in smoke. They can only take so many mA
on the substrate diodes and after that it can latch up. If you are
really lucky it'll take the regulator and everything else with it to the
grave.

Use several 10K or provide Schottky diodes to VCC and GND. Or use a
regular double diode such as the BAV 99 (for low current cases) but a
second resistor between that and the MCU pin.

Another often unwanted effect is that a current into the upper substrate
diode can actually power your MCU side. If it doesn't present enough of
a load, maybe because it has entered a low power mode, that voltage
might exceed abs max and then bzzzt ... poof.

If we skip the 75V case....what would be the difference with a schottky
diode..isn't there a risk that the substrate diode would win in that
case aswell..?

If we consider the case with a buffer curcuit between the MCU and the
connector, e.g. if the MCU has LV-TTL outputs and there is a need for
an 5V output. A 74LVC2G07 (powered with 3V3) and the outputs pulled up
to 5V could be used. Then a zener might be suitable...?

Buffer--- R -----------> connector
|k
zener 5.1V
|
///

 

[CeeRox]

CeeRox skrev:

Joerg skrev:


If we skip the 75V case....what would be the difference with a schottky
diode..isn't there a risk that the substrate diode would win in that
case aswell..?

If we consider the case with a buffer curcuit between the MCU and the
connector, e.g. if the MCU has LV-TTL outputs and there is a need for
an 5V output. A 74LVC2G07 (powered with 3V3) and the outputs pulled up
to 5V could be used. Then a zener might be suitable...?

Buffer--- R -----------> connector
|k
zener 5.1V
|
///

Skip the buffer disscussion... =P I wrote to quickly there. The main
problem is that the output should survive 12V continously, not just a
quick spike...

 

[Joerg]

CeeRox skrev:

The Schottkys win provide they are large enough for the expected
current. Schottkys begin to conduct around 400mV while the substrate
diodes on all but the most esoteric chips start at 600mV. But again you
can also use regular silicon diodes here if you spring for another
resistor between diode protector and MCU pin. This resistor can be much
smaller. Reason is that there will be very little voltage differential
between that point and the MCU pin.

Skip the buffer disscussion... =P I wrote to quickly there. The main
problem is that the output should survive 12V continously, not just a
quick spike...

Then we'd need to know: How much current must the output be capable to
drive and what's the type of load it will see?

Protecting power outputs that must supply lots of current is a lot more
tricky than to protect weak outputs or inputs.

 

[[email protected]]

Consider this scenario: A 5V TTL-signal goes from a connector to a MCU. ....
what would be the easiest way to protect the input of the MCU?

The 'easy' character may be disputed, but usually one would want to
feed the 'dirty' signal into a chip intended for such signals. The old
MC1489A receiver for RS-232 will take +/- 20V without complaint,
and convert it to a TTL output. It's an inverter, of course.

 

[Roger Hamlett]

What is the likely frequency range of the signal?. What is the allowable
input voltage range on the MCU?. What is the driver feeding the signal
(CMOS, or TTL, or something else)?. Could differential signalling be
used?. What is the likely noise enviroment?. How is the return path
handled?.
Different answers depending on all these factors, and a few others.
Ranging from Pi networks, an input resistor with a MOV, through various
types of ESD protected buffer, to opto-isolation.

Best Wishes