Can somebody explain why and how certain USB charger can supply higher voltage than the standard 5 volt? Apparently certain devices require higher voltage (e.g. ASUS Transformer tablet--requires 15 volts; or Motorola Droid requires 9 or 9.5 volt. Somehow there is a handshake process when the USB plug is connected to the right device--then the charger knows how to switch to the higher voltage to enable charging. Without this proper handshake, the charger seems to politely issue the standard 5V. What kind of handshake is this, and what's the electronic/logic to determine the demanded voltage?
Basically there are a number of options for doing this sort of higher voltage charging.
Older devices did that by using proprietary connectors where additional lines were used for higher voltages (the older Asus Transformer chargers are an example for that).
It would also be possible to have a specific wall ward outputting a higher voltage onto the 5V-USB-line and the device-side being set up to just accept that higher charging voltage. In case of a dumb wall ward there would be the risk that when plugging in any other "normal" USB device it could be damaged.
The cleverer solution is to have this sort of handshake going on where devices can agree higher charging voltages. Recently that was pushed and branded by Qualcomm as "Quick Charge 2.0". It basically works by having some ICs in both the charger and the charged device that are QC certified and have an agreed on handshaking mechanism. There is a patent describing the technology briefly.
On the charger side you will have a high-voltage dedicated charging port (HVDCP) interface IC (i.e. the CHY100) which basically checks the voltage on the USB-D-lines and adjusts the output voltage according to some lookup table (shown in the datasheet).
On the device side you will have a power controller IC (i.e. the FAN6100Q) that checks that the external charger is able to adjust its voltage and if so selects between different modes that result in different voltages requested from the QC-enabled charger.