I’m designing a PCB that will charge an iDevice. Of course, this requires “special” D-/D+ voltages and I can’t find a convincing argument for/against the use of a Zener plus a resistor to reduce the voltage from 5V to xV (x = 2.0, for example). Two-resistor solutions have been used (e.g. Mintyboost) and there are some that use a resistor with a Zener (http://vusb.wikidot.com/hardware). What would you use, and why? Obviously there isn’t a huge current on the D-lines (<20mA), and the cost difference between resistor and Zener is a few cents for SMDs. But, if a Zener is involved, current has to be taken into consideration to select the resistor, correct? Perhaps I just convinced myself to use the pure-resistor solution…or maybe not just yet.
Also, a tangential question: is USB data transfer hindered/prohibited if D-/D+ signals are limited to something lower than 3V? I’ve read some nitty-gritty USB info regarding data transfer voltages/speeds (e.g. http://www.beyondlogic.org/usbnutshell/usb2.shtml), but I’d like to hear some real-world experience, if it’s out there.
Thanks!