Does all 74HC series have ESD diodes on their CMOS outputs going to the rails? - Page 1
Forum section: electronics / projects

author: julian1
author_rank: Frequent Contributor
timestamp: October 24, 2022, 08:22:05 pm
content: And is there any reason to think, they could not reliably dissipate the inductive transient from a small signal relay, at switch off?

author: Gyro
author_rank: Super Contributor
timestamp: October 24, 2022, 08:30:00 pm
content: Yes, they do typically have ESD diodes on the outputs, however they are there to protect the device and prevent it from latching up under specific Human body and Charged Device model limits. They are not suitable for relay coil reverse transients.Aside from the very limited current handling, the protection diode structures would steer the coil transient currents through the IC supply rails rather than constraining them to a tight loop between the coil and a proper protection diode across it.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 24, 2022, 08:53:20 pm
content: At least most of the Chips have "diodes" to the supplies. There may be a few special ones with open drain outputs or bidirectional ones that may have a few pins without.Wether those diodes are suitable to dampen the inductive kickback of a relay depends. Often the output would also switch to the corresponding rail and the current could thus also flow though the FET turned on in parallel. Quite often the path to the supply would also be parallel to the relay coil. It is not ideal, but it often works.One problem is that the diodes are more like parts of substrate isolation and current flowing there can effect adjacent functions. So while there is current flowing through the diodes, the function is not longer guaranteed. E.g. the transition level of adjacent pins may change. Too much current could trigger a latchup (trigger a parasitic SCR to short the supply and possibly quite litterally blow up the chip).

author: TimFox
author_rank: Super Contributor
timestamp: October 24, 2022, 08:54:02 pm
content: As Gyro said, you need to put the diode directly across the coil terminals for good protection of the rest of the circuit (not only the 74HC device, but anything else on that power rail).

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2022, 09:02:50 pm
content: The "output diodes" are inherent to the MOSFET transistor structure and are also called "parasitic diodes". They can't be used foranything.The input protection diodes are a different story, they've been added on purpose.Do the job right.

author: julian1
author_rank: Frequent Contributor
timestamp: October 24, 2022, 09:44:38 pm
content: Thanks for the comments.Fet body diodes are sometimes 'freewheeled' in power supply switching contexts, so I am not certain there is no precedent to their use.On the otherhand with integrated cmos - there is a whole lot more going on with substrate connections, and anti latch up behavior. And perhaps more importantly - nothing is specified.The PN ESD will always activate somewhat, unless external diodes with a lower forward-drop are used.To turn the question around, if external protection is used, should schottkys (eg bat54) be mandatory?.Or can the difference in forward drop, as current increases be relied on, so most of current is steered through the external PN diode (eg. mmbf4148 )I suppose I should measure the cmos output ESD/body-diode forward-drop with a current source, to get a relative sense of its size versus external discrete options.

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2022, 09:48:55 pm
content: Quote from: julian1 on October 24, 2022, 09:44:38 pmYes. InpowerMOSFETs, where they are also characterized in the data sheet. Not in logic ICs.

author: coppice
author_rank: Super Contributor
timestamp: October 24, 2022, 09:51:49 pm
content: Quote from: julian1 on October 24, 2022, 08:22:05 pmIn most CMOS ICs those ESD protection diodes are only rated for something like 2mA. Is your inductive kick that limited?

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2022, 10:06:34 pm
content: Quote from: coppice on October 24, 2022, 09:51:49 pmWhere'd you get that number? Or is it a typo? 20 mA is the number to my knowledge for 74HCxx.

author: coppice
author_rank: Super Contributor
timestamp: October 24, 2022, 10:40:50 pm
content: Quote from: Benta on October 24, 2022, 10:06:34 pm2mA is pretty typical for current medium geometry CMOS parts, like MCUs, which are generally more robust to ESD than older designs. ESD gets taken more and more seriously as time passes. 2mA is a continuous rating. An inductive kick takes a while to dissipate, so you should probably be looking at that, rather than its peak capability during an HBM (human body model) ESD strike.

author: Benta
author_rank: Super Contributor
timestamp: October 24, 2022, 10:48:45 pm
content: The OP asked about 74HC. Here you go (p.3):https://assets.nexperia.com/documents/data-sheet/74HC_HCT00.pdf

author: coppice
author_rank: Super Contributor
timestamp: October 24, 2022, 11:17:35 pm
content: Quote from: Benta on October 24, 2022, 10:48:45 pmThe 74HC user guide seems to indicate the output may pull a continuous 20mA through the clamping diode. It sounds like it may be OK, as long as the peak current is not too extreme..

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2022, 07:27:28 am
content: The current from the inductive kickback is normally no larger than the current to drive the relay. As the 74HC can also no provide much more than 20 mA one should normally be below this limit. A typical example would be a small reed relay that needs some 5 to 10 mA.Even if they have an internal diode, part of the current may still flow through the driver, as the 74HC outputs are ususally push pull. So when the relay is turn off the FET parallel to the diode in question would be on. Some 50 Ohm R_on and 10 mA would be only 500 mV - so much of the current would flow through the normal output FET and not a parallel diode (internal or external).

author: julian1
author_rank: Frequent Contributor
timestamp: October 25, 2022, 08:00:27 am
content: Quote from: Kleinstein on October 25, 2022, 07:27:28 amThanks. That's a very good observation.

author: Gyro
author_rank: Super Contributor
timestamp: October 25, 2022, 09:23:52 am
content: Admission time. I have driven low coil current latching relays that way using well decoupled 74AC logic (for drive current capability). I didn't consider it best practice though and it was a one-off personal project.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2022, 09:41:57 am
content: Driving 2 coil latching relays is a bit more tricky. Some, possibly many of them have the coils coupled and there is not just the normal inductive kickback, but also some transformer like coupled pulse of usually opposite polarity to the normal drive. So it would be carefull to drive such a relay from a 74HC gate. It work however with low power 1 coil versions, using 2 gates in a kind of bridge configuration.