CPLD replacement for 74141 decoder - Page 1
Forum section: electronics / projects

author: jmw
author_rank: Frequent Contributor
timestamp: October 18, 2019, 05:42:10 pm
content: Russian clones of the 74141 are still cheap at the moment, but at times I have thought about building a BCD to decimal decoder in a PLD. The logic part is easy enough, but the high-voltage output driver is not. The 74141 is rated to withstand 60 V on its outputs, whereas no CPLD will tolerate that on a tri-stated pin. Is there a more cost-effective way to do this than 10 discrete driver transistors per digit?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 18, 2019, 06:10:49 pm
content: The main difficulty is the high voltage drive, not the logic. Just 10 discrete transistors and resistors is probably the best bet. For the logic one may not even need an CPLD. A more normal 74 series chip (74HC42 should work) should be sufficient with NPN transistors in base circuit.The transistor solution is just using quite a few parts and board space.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 18, 2019, 07:36:49 pm
content: You're not saying anything about the end application. Do you just want a generic drop-in replacement, or do you have a specific use in mind (such as driving LEDs)?If you're going to drive LEDs/LED displays, one suggestion would be to use an integrated LED driver as the output stage. You can find many with multiple outputs. Benefit is a lot of them are constant current drivers. Then the logic decoding can be done with pretty much anything you see fit (discrete logic, cpld, MCU, ...)

author: jmw
author_rank: Frequent Contributor
timestamp: October 18, 2019, 08:02:53 pm
content: Application would be driving Nixie tubes, as was typical of the 74141 and hence the point about high voltage drive. The hoped for advantage of a CPLD would be the high pin count so you can cram 6-10 decoders in a single package without multiplexing. But if you need 10x(# of digits) in discrete low-power MOSFETs to protect those pins it's probably not saving money or space...

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 18, 2019, 08:07:49 pm
content: You could take a look at this:https://www.maximintegrated.com/en/products/power/display-power-control/MAX6921.html

author: jmw
author_rank: Frequent Contributor
timestamp: October 18, 2019, 09:43:52 pm
content: https://www.ti.com/product/SN75468Hmm, this seems usable - 7 channel NPN Darlington array that can stand up to 100 V.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2019, 07:12:36 am
content: The leakage current could be a problem with the SN74468. The transistor array also does not save that much space compare to small transistors (e.g using duals). Normal transistors are probably easier to get in case of a later repair.The maxim parts have the advantage of having a kind of SPI input, so less input lines. It depends on where the signal comes from, if this is good or bad.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 19, 2019, 09:54:37 am
content: Bear in mind that due to the voltage drop over the nixie, the driver doesn't have to stand off the full voltage.

author: TurboTom
author_rank: Super Contributor
timestamp: October 19, 2019, 10:22:05 am
content: Since nowadays usually one will drive the decoders from a microcontroller, the decode function isn't really necessary. For a pending mini-nixie wrist watch project, I had a closer look at theSupertex/Microchip HV5623. This should easily get the job done for a full four-digit clock with a few lines to spare (or three full decimal digits plus two decimal dots).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 19, 2019, 11:14:24 am
content: Quote from: TurboTom on October 19, 2019, 10:22:05 amThe Supertex chips are nice, but way more expensive than a bunch of HV transistors like MMBTA42

author: TurboTom
author_rank: Super Contributor
timestamp: October 19, 2019, 11:57:02 am
content: Quote from: mikeselectricstuff on October 19, 2019, 11:14:24 amYes that's correct but in a situation where PCB area and general compactness of a design matters more than price (i.e. "fun project" of a nixie wrist watch in my case, of which probably never more than ten pieces will be made), the supertex/microchip driver may still be the most reasonable choice.