I/O input protection requirements and design
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Author:  apalrd [ Tue Dec 13, 2016 5:00 pm ]
Post subject:  I/O input protection requirements and design

I've noticed in the prototype ECU discussions thread, a lot of opinions on how to protect analog I/O, but no decisions on how much protection is necessary. I'd like to collect ideas on what the requirements should be for I/O protection, including power supply.

First thoughts: Power input
The normal arrangement for an OEM vehicle is to power the ECU logic directly from the battery (through a fuse), and power all of the engine loads either directly through the ECU (e.g. H bridges, steppers, high side drivers) or through one or more relays, which are turned on by the ECU. In this way, the ECU can protect loads from reverse battery and long-term high voltage by turning off the relays.

My thoughts on ECU input power:
-For 12v nominal, input must handle +16v forever, for 24v nominal, +28v forever
-ECU must remain operable to 6v cranking condition
-Reverse voltage to -16v (12v nom) or -28v (24v nom) for 5 minutes, ECU must not power up in this condition, no negative voltage may be passed to ECU-supplied sensors or actuators
-Overvoltage protection to +26v (12v nom) or +38v (24v nom) for 5 minutes, ECU remains powered on but can shutdown in software - Double jump protection
-Transient protection (<1sec events) - Need to absorb this to some level on the ECU input power without damage or shutdown
Descriptions of some automotive transients (from Littelfuse): ... n_note.pdf
I think we can design for a lot of these on the power input without too much difficulty.

For outputs - I think the majority of load drivers (MOEFETs and IGBTs) needs at most an ESD capacitor at the pin.

For general purpose analog inputs - What voltage do we need to protect, and how long?
I think +-16v while not powered on is the most we would see on an analog pin. This would be a short to battery or reverse battery.
Freescale/NXP app note on injection current for IO protection ... AN4731.pdf
We should be able to use a 16K+ resistor in series to provide enough protection from shorts (and limit to 1ma injection current), and an ESD cap at the connector pin to protect from ESD. The series resistor can also be part of an RC filter.

For the battery voltage sense, we would need to protect the 5v ADC input to the requirements of the power input (80v? load dump spike worst-case?), this could be done with a voltage divider then a Bi-TVS or an op-amp buffer.

For digital inputs - Hall-effect frequency inputs, cam sensors, short to ground discrete switches - We can isolate these using a transistor as a buffer, buffer chip, or a series resistor like the analog inputs. For open-drain sensors, a 5v strong pull up and series resistor should be adequate.

For VR inputs - MAX9924 specifies VCC+0.3 absolute limit and +40ma injection current. They recommend a 10K input resistor and filter cap between the two pins. This seems like it would protect well enough. We could also add a load resistor between the two pins (before the 10K input resistors) to reduce the output voltage of the VR sensor.

Your thoughts?

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