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Back to back FETs

Back to back FETs

Back to back FETs

(OP)
Can anyone think of issues I might encounter by putting MOSFETs back to back (D to S, and S to D, G and G tied together), like an AC switch?  When the gate is enabled, I want a signal to pass through this "switch", regardless of which end has the higher potential.  I didn't think a single MOSFET would accomplish the goal due to the varying potentials.

As always, replacement circuit suggestions welcome (I'm using SOT-23 packages, so any replacement must be compact).

RE: Back to back FETs

Your description reminded me of those 4066 and similar 'CMOS Switches', but when I checked apparently not quite the same. They are using P- and N-Channel switches in parallel to perform the same function.

See:
www.maxim-ic.com/appnotes.cfm/appnote_number/638


RE: Back to back FETs

(OP)
Hmmmm, devices like the MAX4626 look useful, but pricey.  Two MOSFETs would be about 1/10th the price, even though they would take twice the board real estate.  Ron is surprisingly high on those buggers, must be the lack of silicon real estate for the FETS due to it being taken up by the control circuitry.

Good reading.  Never thought about Ron being in parallel, so that can only help things.  At $0.90 a pop, though, I may stick with the dual NFETs back to back... unless someone can give me a reason it won't work.

RE: Back to back FETs

I seem to recall that there isn't very much significant difference between the S and D ends of a MOSFET.  It is just a bulk semiconductor between those pins.

Are you sure that you'll need two?

RE: Back to back FETs

Actually, there is a difference.  A MOSFET is actually a 4-terminal device, with the substrate being the 4th terminal.  The substrate is usually connected to the source terminal on a 3-terminal package.

Which, will cause a problem if the source is raised higher than the drain during operation.  The substrate diode to the drain will be forward biased in that condition.  That's why the conventional MOSFET switches leave the substrate connection tied to the lowest circuit potential to ensure that the substrate is never forward biased.  Obviously, the p-channel substrate will need to be tied to the highest circuit potential.

This is why the Ron on MOSFET switches are so bad.  If you recall the MOSFET body effect, it's directly impacted by the reverse bias of the substrate relative to the channel, and acts to reduce the effective gain of the transistor.

TTFN

RE: Back to back FETs

Thanks for that correction.

I guess that those substrate diodes are going cause problems for MacGyverS2000's planned back-to-back MOSFETs ?

RE: Back to back FETs

Take a look at the Maxim/Dallas Semiconductor analog switches.  Pretty cheap and easy to use.

RE: Back to back FETs

I think there are 4-terminal varieties of MOSFETS, although I suspect they're probably relatively low performance ones.  Those would work if the substrates are connected correctly.  

But, you find that the Ron performance will be degraded.

TTFN

RE: Back to back FETs

A N Channel J-FET can be used.  A resistor connected gate to source gives zero volts and the device is on.  Pull the gate to, say, -15 V with an open collector driver and the device is off for a wide range of drain and source voltages.

RE: Back to back FETs

(OP)
Kinda forgot about the whole reverse bias deal... it sounded good while it lasted though, ;)

I suppose I could waste an extra pin and control a P and an N separately, but it defeats my purpose of saving board space AND a pin.  Not sure yet what I'd do for the negative voltage to control the PFET.  This is to selectively allow 2-way communication over a 0-5V bus line, so finding a bi-way switch that would also reamplify the signal back to 0-5V as it degraded along the line would be ideal.

RE: Back to back FETs

What is the data of your signal and of the input circuit ?

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Back to back FETs

(OP)
Short and sweet of it: a 0-5V binary data bus, two-way communication between micro #1 and #2.  A third micro somewhere along the line should be able to interrupt the communications by shutting off the line.  So, the switch should be two-way in nature, and ideally should have a single control line.

At the moment, I don't believe I'll need directional-specific control (disable one way, but not the other), but if it's possible with little to no extra, I may find a use for it.

RE: Back to back FETs

(OP)
felix, Now THOSE are some worthy components to check out.  Looks like the NC7SZ384 is going to be a potential winner.  I also have a new keyword to look for in the hopes I'm lucky enough to find something even better and/or cheaper.

At first I was only slightly worried about the on resistance of 5 ohms, but for longer runs I'll be using a driver periodically along the line anyway.  Package size is where I want it and price is within reason at $0.145 (SOT-23) and $0.119 (SC70, never heard of this package, seems Fairchild is still waiting for a JEDEC registration number on it, but the dimensions on a 6 pin LEADLESS are 1 x 1.45 mm!!!  My eyesight isn't that good.).

I Never knew this type of product existed, which seems to be the growing trend these days (at least for me).  I only know what my problem is and a vague idea how to solve it, but ignorant of any prepackaged solution... there's only so many product announcements one person can view.  It has become painfully clear the last several months, however, that I need to renew my subscription to EDN, EE Times, etc.  I let the subscriptions pass years ago as I just didn't have the time to read them and still have a life outside of work.  I guess I need to reevaluate my time management again, as I've spent days looking for a solution I should have known existed.

felix, you deserve a star for that one :)

RE: Back to back FETs

MacGyver, from the questions and answers I was not able to understand what you were trying to achieve, and the stuff was getting pretty complex until nbucska asked the basic question again.
Other manufacturers are also making similar switches. Try to get a second-sourced part, as they haven't reached the same perennial status as logic gates.
The bus-switches are getting very popular as they do not contribute to delays, other than series resistance, and give an no-hassle bidirectionality.
With Internet, until something very special really interests me, I don't read the magazines anymore.  However I subscribe to the email news from the big manufacturers like National, Fairchild, and TI, and also from the Woodstock Wire.  There's more than enough for me in all these, and no paper to waste.

RE: Back to back FETs

I don't understand what you are trying to do. If this is
a communication line between two micros, you can just command them to neglect it ( it costs one pin ) unless
the communication is too fast.

What are the types of the micros? What is the comm. speed?
Why do you have to disconnect ?

<nbucska@pcperipherals DOT com> subj: eng-tips

RE: Back to back FETs

(OP)
I'm preventing micros from further down the line from receiving messages at all.  Without going into unecessary detail, processors further down the line should not be able to listen in on the communications conversation until processors further up the line have OK'd it.

This has stemmed from one of my usual hair-brained schemes, but it is actually a quite elegant solution (I think) to a nagging problem.  Hardware is minimal, comm protocols are reduced in size, which also reduces software and bandwidth.

RE: Back to back FETs

(OP)
I'm bringing this one back to the top as I'm looking for a related component.  I would like to periodically "rejuvenate" the comm line strength with a driver.  However, I'm not sure what keywords I should be looking for, or even if this item exists.

Essentially, it would be a two-way repeater.  Two-way communication lines 'A' and 'B' are connected together and sending messages back and forth to multiple processors along the line.  My protocol won't allow both buses to have the line at the same time, so no need to worry about collisions.  All I need is a driver that keeps the current level up, resharpens the waveform edge, and a really nice thing would be an output enable pin.

Should I be looking for a single chip solution, or can anyone suggest a minimal parts count discrete component solution?  The bus-switches mentioned before are great (and are being designed into the project), but they don't add any drive to the line, they just connect them together.

RE: Back to back FETs

(OP)
I like using things they weren't really intended for.  In this case, a voltage interface transceiver may be useful.  A piece such as Fairchild's 74LVXC4245 ( http://fairchildsemi.com/ds/74/74LVXC4245.pdf ) translates the differing voltages on two different buses (for example, one's at 3.3V while the other is at 5.0V).  The idea would work by simply saying both bus voltages are the same, but at 8 bits, it's way too large... I need something similar, but with only 1 bit (like in an SOT-23-5 package).

The search continues...

RE: Back to back FETs

Are your bus signals at the same voltage?  Which one?  Have you looked at single-gate packages, usually SOT23.  You could use two xx125 or 126-type of tri-state gate, connected back to back. (would be an equivalent to a lot of fets) I'm not sure if they're available in 5V technology though.

RE: Back to back FETs

(OP)
Same voltage (5V) on both sides, I just mentioned the bus level translators as they would do what I wanted if 5V was provided as the supply for both sides... couldn't find any other transceiver style of component other than bus translators.

I also considered the back to back buffer idea, but couldn't get past they thought of them getting into a race condition of some sort.  Connecting the output of buffer 1 to the enable of buffer 2 (and vice versa) seems fine on paper, but real-world components might not like that idea as much.  Not to mention one buffer will try to keep the line low/high while a processor down the line tries to drag it in the other direction.  Or am I missing the obvious?

RE: Back to back FETs

I'm missing something.  An LVXC4245 would be fine if it was working at 5 volts on both sides?  Then why just a regular 245 bidirectional driver can't do the job?

Output of buffer 1 to enable of buffer 2?  what do you mean?

Take a look at NC7WZ241.  I think it does exactly what you're looking for.  If not please explain differently what you're trying to do.

RE: Back to back FETs

(OP)
Yes, the LVCX4245 would work fine for this app, if it was significantly smaller... it's an 8-bit transceiver, but I only need 1 channel (I'll accept 2 channel, as the package would probably still be relatively small).

I'm definitely missing the hookup of a dual buffer with output enable, then.  Since this is a single wire transmission, the output of buffer 1 needs to hook to the input of buffer 2, and vice versa.  Like this:

CODE

    |--|
    |  |
    |  |\
----|--| >---|-----
    |  |/    |
    |        |
    |        |
    |   /|   |
    |--< |---|
        \|   |
         |   |
         |---|

At logic 0, we're fine, as both buffers are in a high-impedance state.  When a logic 1 (5V) comes down the line, weimmediately run into a snag... a processor will be trying to drive the line high, but so will the opposing buffer.  If a 0 is transmitted again, the buffers will continue to drive the line high indefinitely.

The dual active-high/active-low enable operation of the NC7WZ241 doesn't help resolve the issue.  A logic 1 will never get through the active-low buffer, and a logic 0 will never make it through the active-high buffer.

Honestly, I'm quite interested in how they make minimal size transceivers (SOT-23) with a minimum of logic gates... seems tricky.

RE: Back to back FETs

I don't get it.  The data controls the tri-state oputput as well?

RE: Back to back FETs

(OP)
I don't want it to, but in that configuration it does.  I will remove power from the buffers altogether when they're not needed, but otherwise they should be powered and doing their job.  It's a two-way communication line that I would like to ensure has enough power... I do not yet know if the edges will need periodic sharpening, so a schmitt trigger input to the buffers is ideal.

RE: Back to back FETs

Hi, check out SN74LVC1T45.

RE: Back to back FETs

(OP)
At first thought, back to back buffers seems the way to go.  Further thought says the input of one needs to tri-state the output of the other.  Even further says that just won't work.  I was going to post a small mass of gates necessary to make this work, then decided by the time I go through with all of that, the circuit would be quite large.

Luckily enough, this circuit will not need to be used very often, so I can skip populating those pieces on the boards that won't need it.  That being the case, I'm leaning towards two possible solutions.

1) A small processor, like a PIC 12F series, with interrupt-on-change capability.  It may introduce a minor delay in the signal, but if coded correctly, the delay should be constant.  I would have complete control over both sides of the bus, and although I probably wouldn't have Schmitt trigger inputs, the output signals would have sharp edges again.  With oscillator and support components, the circuit should be less than $2.  Board space should be around 0.5" square, even less if I use both sides of the board (the oscillator will be larger than the processor ).

2) A small PLD or the like with a setup similar to the above.  I don't know if they make PLDs as small (or as cheap) as the PIC mentioned above, but it's worth a shot to look for one.  I would need to purchase more programming equipment (that stinks), but there is potential for the unit to be really cheap (only potential, as the cheapest PIC is $0.55!).




I hope you guys don't think I'm being to picky about this stuff.  I just want to make sure this circuit works the first time around and do it as inexpensively and in as small a space as possible.

RE: Back to back FETs

A OTP bipolar fuse PLD should be inexpensive?

TTFN

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