Tuesday, 15 April 2014

How does a USB 2.0 Wall Charger negotiate current output?


I'm trying to use a mobile phone charger for my projects, and want a high current output. I have read this, however my measurements show something else.


Here is a link to the Battery Charging Specification Rev. 1.2.




1.4.7 Dedicated Charging Port


A Dedicated Charging Port (DCP) is a downstream port on a device that outputs power through a USB connector, but is not capable of enumerating a downstream device. A DCP shall source \$I_{DCP}\$ at an average voltage of \$V_{CHG}\$. A DCP shall short the D+ line to the D- line.



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I verified on three different chargers, and all read \$R_{DCP\_DAT}\$ as ~1.5 Ohms. Now, if there is a short between the D+ and D-, there is no detection on the charging port side, and the charger should always output \$I_{DCP}\$ {0.5 - 5.0A max} on the VBUS line - is this correct?


I tested the current output of three chargers, but they are all completely different.


Charger 1 - Nokia


Rated current output: 1.3A


Measured current output: 1.34A



Charger 2 - Asus


Rated current output: 2.0A


Measured current output: 0.7A - 1.1A (unstable)


Charger 3 - HTC


Rated current output: 1A


Measurent current output: 0.1A


If all these dedicated charging ports have no current negotiation, how come only one charger is showing it's rated output?


N.B All three chargers can charge a mobile phone in a reasonable amount of time.




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