In need of a readily wired-up Lightning receptacle, it turned out to be less expensive to buy a readymade adapter and throw away the parts not needed, rather than assembling it from components. So I ordered two such adapters for Apple devices recently, which break out a Lightning connection into a charging (Lightning) port as well as a 3.5mm audio output.
Needless to say, the audio quality of this adapter was very poor. It exhibited a very audible hum on the analog output, regardless of whether it was connect to a power supply or not. As Lightning is an all-digital port, the culprit has to be the digital-to-analog (DAC) converter hidden inside the Lightning plug. Let’s take a closer look.
On one side of the PCB, there’s a SO16 IC labeled AB19CL1J0Y.1-82 (maybe that’s also supposed to be AB19CL1JOY.1-82 for the joy of listening to a lot of hum and a little music?).
On the reverse side, there’s an SO8 package labeled B1SGS (the 2313 on the top is probably a date code; note how the pads on the PCB footprint are misaligned to the IC) next to a six-pin IC labeled 191203 (maybe i91203) and a few passives (two resistors and three 1uF 0402 caps - I wasn’t aware you could cram in as much capacitance into this small form factor).
After a bit of probing around the PCB, I removed the components to see how they were wired up. Interestingly, some pads of each of the ICs remain unconnected (specifically, pin 2 of the SO16, pins 1 and 4 of the SOT23-6 device, and pins 2, 3, 6, and 7 of the SO8). I have sketched this quickly in KiCAD.
It seems like both of the smaller ICs are in charge of handling the Lightning authentication/charge control, which means that the DAC and output amplifier are integrated into the SO16 case. Still, with a dozen interconnections between all of them, the most convenient option is likely to just use the full board as is to get a 5V supply from a lightning connector.
Bottomline: I got the cable with the Lightning receptacle, which I was after, with all the logic required to pass the 5V supply. Unsoldering the Lightning plug and the speaker connector turned this PCB into a nice compact form factor, which can be easily integrated into existing gadgets.