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Okay, the title sounds odd, I'll admit. I need my project to work without power. That is, a complete power failure may happen, and no additional power (such as a battery or supercap) is available mainly due to size and cost restrictions. The ground will always be connected during a power failure. The power failure may also include, for example, a short circuit placing the 3.3V buck regulator into a protection mode, causing a loss of power.

Here is the scenario. I'm developing a video processor which has multiple video inputs (two in most cases.) It uses an analog switch (ADG733 or TS3A24157, depending on version) to select the desired video feed, all software controlled. With power, it selects either one. However, when no power is present, I don't get a good video signal, because the mux goes into some kind of null state, with both inputs kind of conducting but very weakly.

The result is without power, a video picture which looks like this:

enter image description here

The output amplitude is large enough to allow a TV card to sync onto it, but not large enough to provide a clear picture.

So, I need some kind of circuit which activates when there is no power and passes through either video input (one or the other, but not both) as a sort of fail safe for when power is lost or a bad hardware problem occurs.

I was thinking of a PMOS, as a negative gate potential will turn that device on, but wasn't sure how to make that work properly.

As this is video, we are talking about low on resistance; preferably less than 10 ohms. The analog mux probably has quite a high resistance when powered down, so as long as the circuit which provides the backup has considerably lower resistance, it should be possible to override it.

Also, it should be small, and low cost to implement. The current board is just 50mm x 39mm.

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You could use a depletion-mode FET like CPC5602 or DN3135. But the obvious way to connect it would require the use of a negative gate voltage to open the circuit. The negative voltage doesn't have to supply much current however, so you can probably arrange a simple charge pump with a square wave and a couple diodes and caps. When the square wave stops, the transistor will turn on.

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  • \$\begingroup\$ I suppose I could use my MCU to generate the square wave. All I would require would be a few extra diodes to form a basic charge pump capable of inverting the 3.3V supply. Neat idea! But I wonder if the -2.5V (including all diode drops, assuming a good Schottky like BAT54) will be enough to keep the pFET off. \$\endgroup\$ – Thomas O Sep 28 '11 at 20:22
  • \$\begingroup\$ falstad sim link: bit.ly/nMPnT5 Remember, diode forward drop goes down with lower currents. The only current drain will be diode leakage. And Shottkys can leak a bit, especially when hot. \$\endgroup\$ – markrages Sep 28 '11 at 22:06
  • \$\begingroup\$ I did not know you could stack voltage inverters like that. I guess you learn something new every day. Even under a 1k load that circuit maintains enough of an output to pull the fet way out of conduction, so it's perfectly capable of doing what is needed. Now I just gotta find a good source of low cost n-ch depletion mode fets. \$\endgroup\$ – Thomas O Sep 28 '11 at 22:22
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Use relay. Those work fine without power. Since you don't care which input is selected, you can use either latching or non-latching.

Edit: If you are worried about area, consider replacing the analog switch with the relay. And look into solid-state relays.

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  • \$\begingroup\$ This is a very small design; a relay would be cumbersome and too big. Also, it would eat up considerable power when the unit has power. \$\endgroup\$ – Thomas O Sep 28 '11 at 17:52
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    \$\begingroup\$ @ThomasO, if you have specific size restraints please explicitly state what size you can fit on your design in your question. This answer is perfectly valid for the question in its current form. \$\endgroup\$ – Kortuk Sep 28 '11 at 18:06
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    \$\begingroup\$ @Kortuk "That is, a complete power failure may happen, and no additional power (such as a battery or supercap) is available mainly due to size and cost restrictions." I admit I could have made it clearer, so I will edit my question. \$\endgroup\$ – Thomas O Sep 28 '11 at 18:11
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    \$\begingroup\$ @ThomasO, Yes, I assumed you just did not have space for long term power. No room for low power relays is a different matter. \$\endgroup\$ – Kortuk Sep 28 '11 at 18:28
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    \$\begingroup\$ I found a latching relay that was 10mm x 6.5mm, although it wasn't exactly cheap. search.digikey.com/scripts/DkSearch/… \$\endgroup\$ – W5VO Sep 28 '11 at 18:45

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