Electronic – How does a USB 2.0 Wall Charger negotiate current output

chargerusb

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.

enter image description here

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.

Best Answer

there is no detection on the charging port side, and the charger should always output IDCPIDCP {0.5 - 5.0A max} on the VBUS line - is this correct?

I don't understand this completely, because the BC spec is confusing to read, but yes, dedicated chargers (DCP) short the D+ and D- together to indicate what they are. This doesn't indicate any particular current available, though, it just says that it's a DCP. Different chargers supply different amounts of current.

The charger has no brain in it; it just supplies 5 V until the current draw is too great, and then its voltage starts to droop:

DCP Required Operating Range

It's the "portable device" (PD) which has to be smart about limiting its own current draw from the DCP to stay within the dark region of the plot. So it can try to draw up to 1.5 A, but if the charger voltage drops below 2 V at 0.5 A, then you can't draw any more than 0.5 A from it.

For a dedicated charger or USB charger, the current limit is determined by loading the adapter. When the adapter’s output voltage starts to collapse, it is an indication that the current limit of the device is reached. - MAX8895 datasheet

Related Topic