I have an ongoing question on the Raspberry Pi SE site, in which I'm attempting to figure out how much current is available to devices connected to the HDMI socket of each Pi model. I think I've figured out every model aside from the Pi Zero.
There are no schematics available for the Zero but, following some extreme squinting, I think that the most likely candidate for HDMI power management is the component labelled D6. It's immediately below the white printed 'HDMI' label to the right of the mini HDMI socket, which is towards the bottom left corner of the board.
The part is marked 'F03 | CD'. Some stumbling in the dark on Google turned up the Diodes Incorporated DFLS130LQ / '1.0A SURFACE MOUNT SCHOTTKY BARRIER RECTIFIER', which has identical markings.
Is it plausible that this part might limit current to pin 18 of the mini HDMI socket? If so, what is the maximum current that it would allow to pass?