Single-ended 3.3V line driver w/short circuit & ESD protection  - Page 1
Forum section: electronics / projects

author: forrestc
author_rank: Unknown
timestamp: September 20, 2019, 03:53:16 am
content: I have an application here I need to provide a 3.3V signal which travels over some distance *outdoors* in a cable. Some distance being up to around 100M.Due to historical reasons, now nearly 20 years old, this is a single ended, ground referenced, signal. Yes, I know balanced is better. Yes I know that if I could only change to get away from single ended everything else would work better, etc. But this particular choice was made long ago and I really don't have the flexibility to change it.In the past, I've just used a output of a 74HC series CMOS gate with a 300 ohm series resistor and some protection diodes to knock down any ESD and to limit short-circuit current. Recently, it has become apparent that for various reasons I need to reduce the output impedance from that 300 ohms to something a lot less. I'm still determining how much less, but we're down in the under 100 ohms range.I'd like to just drop a ESD-protected, short-circuit-protected line driver in place with probably some additional ESD protection diodes to improve ESD resistance. There are a couple drivers out there that fit this bill but all that I've found are 5V. I've considered using the + output from a RS-422 driver and not connecting the - output, but that seems kludgy to me. The other option is building something out of discretes which is doable but a pain.I'm hoping someone has a suggestion as to a drop in driver which might fit the bill. Or some other circuit element I haven't considered (aka something not labeled 'line driver' which is in effect a short circuit protected line driver).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 20, 2019, 11:29:43 am
content: Curious, what's at the far end? How fast is this supposed to go?Guessing it must be pretty slow (kbps tops?), given the line length, weak drive strength, and amount of filtering required to get a reasonable logic signal out of it. I hope the receiver is high impedance.If you like belt-and-suspenders approaches, I'm fond of the complementary bipolar inverter. It is what it sounds like: make a CMOS inverter, two transistors; but sneak in BJTs instead, and the requisite biasing (base dividers) of course. In this case, one last step: emitter resistors, so the current is limited.The output drive is rail-to-rail, with an on-resistance a little higher than the emitter resistance, until the current limit is reached.Speed is way more than needed, and can be ~10 ns with faster transistors and speed-up caps in the base dividers. Slow is probably the better way here, and can be done by just loading the output with a cap to ground (since, it's current limited, no problem eh?). Follow that with a nice sized ferrite bead (say 330 ohm, >= 0603) and you've got TX side filtering handled.The collectors have a lot of compliance voltage up and down (no body diode to get snagged on), so you can use whatever surge protection is appropriate. Say, a single SMAJ5.0A, more than enough. At peak surge voltage (10-20V), there's a fair amount of stress on the transistors, but everything's hanging by resistors, so it rides through no problem (just don't expect it to survive very long if cross-wired to, say, +/-12V or more).What about ground loop, is that any concern..? Is there isolation? Could ground loop offset or surge be a problem? That's more of a RX problem but at a high enough level (e.g. induced lightning surge), it can even be a TX problem.Ed: also, is this a one-off, or are there, like, hundreds or thousands of these links? If scale or size matters, a discrete approach will be annoying, and an IC preferred. I'm not sure offhand what kinds of device may serve the same purpose (*maybe* but probably not an LVDS transmitter? LED driver?) but it would be interesting to hunt around.As you note, RS-422 transmitters are basically full logic drivers, they're made to dump lots of current, and use large transistors to handle the relatively high power dissipation. (And yes, leaving off the opposite phase pin is okay.) Would be close enough to use that with a series resistor, really; 74HC is close to 40 ohms by itself, so you can still improve on that while keeping the driver happy by using an external resistor of comparable value.Tim

author: Marco
author_rank: Super Contributor
timestamp: September 20, 2019, 01:20:41 pm
content: You could just add a a PNP/diode/resistor current limiter in the supply line of the CMOS driver if you have a bit of headroom.