I am working on a design for an automotive application. Because its for testing and prototyping I expect the inputs to accidentally, or perhaps intentionally, be attached directly to the battery/alternator on occasion. I want the board to function properly during this time.
What I'm shooting for and my assumptions:
- I want all pins to operate normally with up to 18VDC applied
- High channel count, trying to minimize components and board space
- All inputs MCU/ADC/MUX inputs are high impedance
- Signals are low frequency (<1kHz, most below 100Hz) and are a mix of digital an analog where some of the analog is buffered at the sender and some are not (switches, serial, thermocouples, resistance sensors, 0-5v pressure senders. etc)
- Some components have built-in ESD and OVP protection, but not all. I want to assume these features aren't present so that I can have a common solution for all channels.
- Most devices have an abs max pin input voltage of VCC+0.3V
- For devices that have OVP I want to avoid activating the intrinsic pin diodes for fears of undesirably, or poorly defined, behaviors.
I have made the example diagram below that shows 20V zener diodes near the inputs for ESD protection, current limiting resistors/lpf, and schotkey diode clamps. Because I haven't determined what regulator I am using yet I am going to assume that it doesn't have good current sinking ability, and have put another zener on the 3v3 line to help sink clamp diode current to ground.
What I like about the approach is:
- Relatively low component count
- The use of clamp diodes, as opposed to zeners, means I can use the same part for each input even if the voltage is 5v0 as opposed to 3v3.
What I don't like:
- The behavior feels ill defined. I'm not confident that I know the clamp diodes will activate before the intrinsic diodes. For the diagram below I went out of my way to make the intrinsic diodes have a higher Vf than the BAT54S, but there's no way to know this for sure.
- My choice of 3v3 zener is poor (D6, near the regulator), it dumps too much current to ground all the time. I'm concerned that using something with a higher cutoff would result in coupling input signals into the regulated power lines.
- I'd prefer smaller packages for the zeners and clamps than SOD-123 and SOT23, respectively, given the high channel count.
- I'm concerned that this might not be a good approach if the input was differential (and biased to mid rail) rather than single ended.
- Are my concerns well founded?
- Is this a good approach, if not what would be better?
- Given that this is an automotive application, can you suggest additional or better approaches for both input power and signal protection that I haven't considered?
Notes on schematic: V1 and R2 are an unknown voltage source and source impedance. I'm making the assumption that the user connect the input directly to a large battery being charged by an alternator.