1
\$\begingroup\$

Say, I have a SPST CMOS switch, does it break the connection between two terminals when the power is out (Meaning Vdd=Vcontrol=0)? Or does it ground one terminal?

If it doesn't completely break the connection, what can I use as a switch that does?

I'm switching between AC signals (line audio) and I'm converging output of two switches (only one which is ON at a time) at a common output. I need to make sure one switch is ON when power is off, so I'm using a JFET switch for that one.

This question is for the other switch. I would rather not use enhancement MOS. Since it will either require a relay-like configuration which is undesirable (I can't afford LED current) or a diode in series with MOSFET. Because otherwise it would ground the output half of the cycle when it's off. Also note that if I end up using a series diode, it'll require biasing a corresponding signal so that the diode won't rectify it when the MOSFET's turned ON. I can't do a basic pull-up because I don't want to attenuate my signal. So I'll pretty much have to use an OPAMP.

Summary:

How do CMOS switches behave when Vdd is grounded?

I need solid state SPST switch that breaks the connection completely (NOT grounding terminals) when power rail is grounded and it should be able to work with AC signals.

Edit-Edit: Updated images with much higher quality ones. This is a working schematic but it has problems such as DC biasing and headroom in a battery powered application. Vgs threshold needs to be very low in a real application since I won't have access to high voltages present below.

enter image description here enter image description here

|improve this question
\$\endgroup\$
  • \$\begingroup\$ Can you share your schematic? Without it it's hard to understand what circuit you're working with. My gut instinct is to say the body diode will screw you up, but without a schematic I'd have to think too hard to be sure of it. \$\endgroup\$ – The Photon Feb 7 '17 at 17:44
  • \$\begingroup\$ What do you mean by a CMOS switch? Are you talking about a discrete device (like a MOSFET) or do you mean some kind of IC? Most ICs on the market today include protection diodes that will short any input > Vdd+0.7 to the power rails \$\endgroup\$ – mbrig Feb 7 '17 at 17:48
  • \$\begingroup\$ There are some SPST switches that have guaranteed characteristics with Vcc/Vdd = 0. Consider using a SPDT MOSFET SSR to replace both switches. \$\endgroup\$ – Spehro Pefhany Feb 7 '17 at 17:55
  • \$\begingroup\$ @SpehroPefhany Thanks for pointing it out. I found an SPST switch that provides isolation when Vcc=0 but haven't had any luck with finding an SPDT switch that selects one terminal by default when Vcc=0. Can you refer to a specific device? \$\endgroup\$ – A__A Feb 7 '17 at 18:49
  • \$\begingroup\$ TLP4026G, LCC110, LBA110STR, LH1502BACTR, LBA716STR, AQW612EHA etc. etc. \$\endgroup\$ – Spehro Pefhany Feb 7 '17 at 19:13
1
\$\begingroup\$

You could use the OMRON G3VM-21LR10 mosfet relay: -

enter image description here

They are very small but a bit pricey. You can get ones with 1 ohm on resistance and 1 pF off-capacitance making them quite good for audio switching. They even make versions of the device that are changeover thus featuring a normally closed contact but I think they are 15 ohm on resistance.

Read the data sheet for full details.

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ Well, the device above aint far off; it needs 3 mA to trigger it on. Read the data sheet! \$\endgroup\$ – Andy aka Feb 7 '17 at 18:29
  • \$\begingroup\$ My bad. Up to 1mA would be a better way to put it. \$\endgroup\$ – A__A Feb 7 '17 at 18:41
1
\$\begingroup\$

Consider using a MOSFET output SSR that has both normally open and normally closed outputs. It may be described as a 'form C' or 'form A' + 'form B'.

The normally-closed function is achieved by using a depletion-mode MOSFET so no power is required in that state. A wide variety of such devices are available from less than 0.5 ohm Rds(on) up to tens of ohms or more (the higher resistance devices tend to be rated for hundreds of volts, probably overkill for audio line level).

Examples of such devices are: TLP4026G, LCC110, LBA110STR, LH1502BACTR, LBA716STR, AQW612EHA etc. etc.

There are also a few conventional analog switches which have isolation when Vdd/Vdd = 0 (for example the TS3A5017) , but in the general case the input protection diodes will usually cause issues. You would still need to use a depletion mode device such as a JFET together with such a device, of course.

|improve this answer
\$\endgroup\$
  • \$\begingroup\$ Well, this is the best solution probably. But unfortunately I can't seem to get my hands on these kind of devices. My supplier simply don't have them. So I'm thinking about using a power-down mode isolated SPST switch and a JFET. Do you have any tips for this application? Also these devices can't go below ground. So I will either have to bias the input or put in a charge pump before the device. At the end, I will end up with a circuit as crowded as I posted above anyways. So maybe I should just use the circuit above? \$\endgroup\$ – A__A Feb 7 '17 at 22:24
  • \$\begingroup\$ @A__A You may have to add some buffering to keep the distortion from being too bad with the JFET and the voltages might be too high for the analog switch. \$\endgroup\$ – Spehro Pefhany Feb 7 '17 at 22:27
  • \$\begingroup\$ I can't buffer the output, since it will simply ground the output in case of a power failure. And next device in line will buffer it anyways. Also what about biasing? I'll have to bias the signal either way, so maybe I should omit SPST and use a MOSFET instead. What would you do? \$\endgroup\$ – A__A Feb 7 '17 at 22:29
  • \$\begingroup\$ @A__A It would take more time than I can spend here to come up with a real design. \$\endgroup\$ – Spehro Pefhany Feb 7 '17 at 23:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.