I am working on a small, audio device with aux input and output (both headphone-level) that has some interesting requirements:
- In passive mode (dead battery) audio needs to go from input to output
- In active mode audio will normally go from input to output
- In active mode the MCU can block the audio from the input, and pass in audio from a DAC/amplifier
- SSRs that I have found are too big for my application. I'm also not sure what effect they could have on audio (quality is important)
- This is for a prototype, so working with .4mm BGA is somewhat out of the question
Here is what I have so far:
- This IC is pretty much what I think I'm looking for, however I (personally) would not be able to use it: https://www.fairchildsemi.com/products/analog-mixed-signal/switches/analog-switches/FSA550.html#product-availability
- The answer from jippie in this question MOSFETs or BJTs for using as switch for audio signals seems like it may work, however I'm concerned about the +6V input – would this need to be there in passive mode? It seems like the answer would be "no" but I'm not confident
- This is different from most BT headphone circuits, since in headphones, the aux input usually trumps the BT, whereas here it is not the case.
Can anyone recommend either an IC that performs as a normally-closed analog switch w/o power or a circuit/application through which I can achieve my goal?
Best Answer
There are normally-closed SSRs available in DIP-8 packages and smaller. For example, the Toshiba TLP4176G(TP,F) which has a 7mm x 4mm footprint roughly, but there are others.
That particular one has an on-state resistance of 15\$\Omega\$ typically (25\$\Omega\$ max). You may have to add a shunt element to improve off-state isolation (the 'off' MOSFETs will have a tiny bit of leakage, and some capacitance) but that is much less of a problem since you will have power available. There are in fact a few SSRs that have both NO and NC elements in one (somewhat larger) package.