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How to drive 20V digital signals?
ebastler:
For an interface to a vintage computer, I need to drive eight digital signals, with signal levels 0V and 20V. (I also need to provide the complementary signals, so a driver that does both, like the venerable 4041, would be a plus.) Required output source/sink current is just a couple of mA per channel. The signals originate from a 3.3V microcontroller, so what I am looking for is essentially a level translator.
My prototype uses its own 20V supply, two ULN2803 darlington arrays to pull the outputs to ground, and 2k2 pullup resistors to bring them back up to 20V quickly enough. (Up to a microsecond of risetime is OK.) This works, but needs close to 80 mA for the pullups alone -- at any time there are eight positive or inverted signals pulled to ground against the pullup resistors' will.
I would like to power the final design from the vintage computer's built-in 20V supply, which is wimpy. So I am looking for a lower-power solution -- no more pullups, but proper push/pull output stages. Here's what I figured so far:
* Considered a two-chip solution, adding a UDN2981 (high-side switch) to the ULN2803 (low-side switch) I already use. But the voltage drop of the UDN2981 is uncomfortably high at nearly 2V.
* I had already mentioned the good old 4041 CMOS chip above. Two of these would conveniently drive the 8 channels and the inverted ones -- but operation at 20V would be marginal, and it lacks the level translation capability.
* I would have thought there are plenty of H-bridge or half-bridge arrays out there, to get eight push/pull output stages in one or two chips. But I have not found anything convincing so far. Again, low voltage drop is required for the output stages; I need to get "close" to 0V and 20V. Say 1V drop at either end at most, less would be better.
Any ideas? Did I overlook some nice quad or octal drivers/bridges, preferably with low-drop MOS output stages? Other simple circuit ideas? Thank you very much in advance!
RoGeorge:
What vintage interface/computer has had a 20V digital signals?
ebastler:
--- Quote from: RoGeorge on October 04, 2019, 07:03:28 pm ---What vintage interface/computer has had a 20V digital signals?
--- End quote ---
The LGP-30 from Librascope/General Precision. Tube flip-flops and inverters operating at a couple 100V, but the logic signals are clipped to -20V (logic low) and 0V (logic high). I have tied my microcontroller's ground to the -20V, to avoid the inconvenient negative voltage.
amyk:
--- Quote from: ebastler on October 04, 2019, 06:54:32 pm ---Again, low voltage drop is required for the output stages; I need to get "close" to 0V and 20V. Say 1V drop at either end at most, less would be better.[/li]
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Any ideas? Did I overlook some nice quad or octal drivers/bridges, preferably with low-drop MOS output stages? Other simple circuit ideas? Thank you very much in advance!
--- End quote ---
Give it a 22-23V supply, and lower your MCU ground a few volts too if needed. Now you can drive to (or even slightly above) 20V.
Trying to save a few watts when you're interfacing with a tube computer that draws 1.5kW seems futile, I would just go with the pullups if they're fast enough, and make sure their power dissipation is within limits.
DaJMasta:
I'd be tempted to try to increase the pullups to see how slow it could actually work at, but you could also try a high side equivalent of the darlington array, and it seems like there are fewer but something like the UDN2981 could work. You could also do it with discrete p channel fets, if needed, or go the standard CMOS route and use pairs of fets for totem pole output.
If you have the ability, you can try to reduce capacitance on your drive lines - even small decreases would speed up your rise time significantly with your current pullup solution.
Though if you can tolerate 1V on either end and want to get crafty........ make yourself a virtual ground at 1V, then run your 4041 at 19V supply >:D
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