1-Channel CD4066BM: Why higher Ron ? - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: October 12, 2021, 10:46:00 am
content: Just looked at Farnell for "analogue switch, 1 channel". (I am omitting the "MAX4510" and "DGxx" etc because I am after the simple, cheap 4066 type).It is important that a supply of 13V is OK.I notice this then limits me to the BU4S66G2-TR - but this has a channel resistance of typically 950R, compared with about 150..250R for a 4066.Looked at Texas, 1 channel SPST. Vdd of 16V has 0 hits for 1 channel devices, nearest are 12V parts (https://www.ti.com/switches-multiplexers/analog/products.html?pqs=paqs&familyid=520#p1143=1:1%20SPST&p480=1&p3306=16;12)Question: Are single-channel 4066 made which are like 1 channel of a CD4066BM (3V-18V, 240R) ?

author: Ian.M
author_rank: Super Contributor
timestamp: October 12, 2021, 11:20:08 am
content: You are reading the datasheet wrong. Yes, the BU4S66G2 has an on resistance of typ. 500 ohms, max 600 ohms at 5V Vdd but at 5V Vdd a T.I. CD4066 has an on resistance of typ. 470 ohms, max. 1050 ohms, so at best is fractionally better but is quite likely to be significantly worse. Anyway why do you care about the on resistance at 5V Vdd when your supply is 13V? You probably should be averaging the 10V and 15V typ. on resistances from the datasheets for a typ. value near 13V and using the 10V max. on resistance for worst case design.Also, what's wrong with using an ordinary 4066, even if 3/4 of it isn't needed? If you need a lower on resistance, parallel channels. With the current supply chain issues, using a 'jellybean' widely second sourced part could save you a lot of grief vs designing in a single source specialist part, then when it suddenly goes out of stock, running around trying to find a pin compatible replacement and probably having to pay through the nose for it, or having to source parts of dubious provenance, that have been improperly stored, from a parts scalper.Also your hate-on for DG-series parts is irrational - some are only slightly more expensive than the BU4S66G2 you are looking at with vastly superior performance.Here's a Mouser parametric search with five options under £1:https://mou.sr/3v2OSqn

author: 741
author_rank: Frequent Contributor
timestamp: October 12, 2021, 11:57:28 am
content: Firstly, thanks for your helpI was simply going by the Farnell search's quoted values. I am aware the search can give funny values, but without checking all DS, it is the only way I know to narrow down a wide field of choices - after all, that is why Farnell take the trouble to show it.I do not 'hate' DGxx! I am sure they are great parts, much lower, flatter Ron. But certainly pricier.The reason I want '1 channel' is physical size. This is a retro-fit on a small number of PCBs. A SOT23-5 would be easiest to slide in here. I can revise the PCB on the next iteration, if there is one.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 12, 2021, 12:02:30 pm
content: If you only need one switch and the capacitance is not critical, then connect in parallel all the 4 conduction gates of a 4066, for a lower on resistance.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 12, 2021, 01:02:42 pm
content: The databases at the distributors are a bit limited. The numbers may not be all accurate and like in this example be for different supply voltages. It is a general problem and hard to fix as the DS are so different. Use the values given in Farnel or Mouser / Digiky table as a first hint, but check the DS too.Some of the DG.... series and similar chips are also no that expensive. Another point to watch for is the input side logic level. The Cd4066 with 13 V supply needs some 9 V for a high level.In some cases just a MOSFET or JFET can be used instead of single switch.

author: 741
author_rank: Frequent Contributor
timestamp: October 12, 2021, 03:02:34 pm
content: About 20 years ago I recall we were switching a current (from a current source) for some application.We tried some 'nicely specified' analog switch (possibly as DGxx) and when that did not give the results we wanted, we tried a CD4066.To my surprise, in this particular case, the 4066 gave best results for our application - can't recall the details now - maybe feedthrough, maybe time to fully turn on - something perhaps related to the frequency we were gating the switches at?I seem to recall the 'posh' switch never actually attained it's nice low Ron before we had turned it off again.Of course, when the switches were OFF the current souce must have been frantically ramping up...

author: nfmax
author_rank: Super Contributor
timestamp: October 12, 2021, 03:37:49 pm
content: 4000 series CMOS is the Secret Sauce of electronics design. Too often ridiculed as hopelessly outdated, it has lots of benefits: very low power, handles high supply voltages, and slow enough to stay out of trouble (and not to cause it). Still available, with lots of useful functions.

author: TimFox
author_rank: Super Contributor
timestamp: October 12, 2021, 03:46:22 pm
content: Also note that theRONfor the 4066 series switches is much lower at 10 or 15 V total supply voltage than at 5 V, and the variation inRONwith "signal voltage" (voltage at the switch with respect to midpoint of the power supply voltages) is much worse at 5 V.

author: Benta
author_rank: Super Contributor
timestamp: October 12, 2021, 07:31:59 pm
content: Perhaps you should look at Toshiba TC7S66 instead? Typical on resistance at 12 V is 90 ohms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2021, 02:15:06 am
content: Or TC4W53FU,LF. Which is an SPDT to be exact, but I think it's in a series with the other usual styles. Or just leave one side open.Tim