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Headphone driver short circuit monitoring?

Posted: Wed 2019-07-03 5:42
by msat
Hello all!

I am slowly getting acclimated with all things VS1005 (lots to learn here!), while simultaneously making some design decisions. I have a potential concern with my project where a mono line level plug may inadvertently get inserted into headphone jack, shorting the ring to the sleeve. I do see that there's short circuit protection but it's unlikely to kick in under battery power (and I'd prefer to not subject the battery to the abuse). If possible, I'd like to monitor it in software so that I could disable the drivers and notify the user via the UI. I came across bit 12 of ANA_CF2 called ANA_CF2_VCMST with the description "Ground buffer short circuit monitor", and was wondering if this could be utilized for the task, and a quick description of how it operates. I assume bit 12 enables the function, but how is it monitored? I've tried searching various documents but couldn't find anything. Maybe I missed it? Help appreciated!


Re: Headphone driver short circuit monitoring?

Posted: Wed 2019-07-03 16:39
by Panu

You're right. We usually always put at least 10R resistors in series with the audio signals so that the core doesn't reset when your power supply droops when the audio output suddenly draws 1000 milliamps...

That said, if your power supply successfully feeds the needed amount of milliamperes so that the core continues execution, the automatic LEFT, RIGHT, GBUF driver shutdown will happen and you can monitor this from the overload bit of the driver status register.

With 10R series resistors, there's usually never any problems. The maximum output level suffers a little, but hey, the good side of it is that your product is more EU hearing protection norm compliant :D 8-)


Re: Headphone driver short circuit monitoring?

Posted: Thu 2019-07-11 18:46
by msat
Thanks for the response, Panu.

I did indeed see the low value resistors in some of the dev board schematics. In my case, it would be kind of handy if the driver section shut down, but it sounds like I might not be able to rely on that happening. Maybe instead I can send a test DC signal to the right headphone and measure the voltage at CBUF and the right output via the SAR ADC and check for improper connection that way. The thing is, my device will have two 3.5mm jacks - one for headphones, and a mono for line input. I'd like to attempt to detect a cross connection (line device plugged into headphone jack, specifically), so I'm trying to figure out a method to do it electronically. Of course, this technique would go out the window if someone did something like use a stereo cable, but that's an undesirable edge case I may just be willing to tolerate. Any thoughts?