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





RE: Back to back FETs
See:
www.maxim-ic.com/appnotes.cfm/appnote_number/638
RE: Back to back FETs
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
Are you sure that you'll need two?
RE: Back to back FETs
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
I guess that those substrate diodes are going cause problems for MacGyverS2000's planned back-to-back MOSFETs ?
RE: Back to back FETs
RE: Back to back FETs
But, you find that the Ron performance will be degraded.
TTFN
RE: Back to back FETs
RE: Back to back FETs
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
<nbucska@pcperipherals DOT com> subj: eng-tips
RE: Back to back FETs
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
http://www.fairchildsemi.com/products/interface/switch.html
RE: Back to back FETs
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
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
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
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
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
The search continues...
RE: Back to back FETs
RE: Back to back FETs
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
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
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
RE: Back to back FETs
RE: Back to back FETs
RE: Back to back FETs
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
TTFN