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Challenges in USB battery charging

Posted: 20 Mar 2012 ?? ?Print Version ?Bookmark and Share

Keywords:USB port? universal charging? Battery?

As the USB port becomes increasingly ubiquitous, it is also starting to be accepted as a universal charging port. Unfortunately, this concept of universal is easier to say than it is to do. This article is an introduction into the common challenges a designer runs into in creating this highly desirable, omnipresent USB charging port.

Why is this taking so long?
So, what does it mean to provide a "fast" charge? This usually boils down to customer expectations. The common example is, "I charge my phone, MP3 player ...fill in blank, in x hours at home, but at work, with my laptop, with my monitor, with my new adaptor...fill in blank, it takes all day to charge!"

So we start with the "native" charger that comes with any device. This charging experience is the baseline for customer satisfaction.

The native wall charger for a device will very often have a special signature on the data pins to let a device know it is safe to charge with more current. In some cases, it also prevents the device from charging at all if the host is unknown. This signature may come in the form of a specific voltage placed on D+, or D-, or both.

Refer to the figure, which illustrates a common architecture for a wall charger using this methodology. Note that these configurations are implemented so the manufacturer can sell more accessories.

Figure: Common architecture for a wall charger. (Click on image to enlarge.)

Make no mistake: selling specialized accessories is definitely in the business plan for a portable product. For every chargeable product purchased, about 50% of us will go out and buy another charger. The reason is simple: we do not like carrying them around, so we leave a charger in the other places we frequent, such as in our office or in the car.

The "right" charging current
To begin an analysis of USB charging, you first need a system to help measure the current on Vbus and to measure and apply voltages on D+ and D-. This can be done by creating a board that both the peripheral and the host can plug into while exposing their D+, D+ and Vbus lines for analysis.

Jumping ahead, it is time to evaluate the charging current with a device connected via your interposer board. So let's assume we are all smart enough to determine what voltage the native charge places on D+ and D- and we recreate a discrete charging circuit to confirm our suspicions. We then apply the right voltages, just like the native charger on D+ and D-, but the charging current is not matching our previous results.

It is time to check your power. No, not just whether things are plugged in, but the level of power. Battery-power level plays a key role in charging. Many of us who have worked on cell-phone designs can tell you that a deeply discharged lithium-ion battery needs to be trickle charged before the real charging can start.

This, too, complicates knowing whether you have an optimal charging current. The peripheral that gets plugged into a USB port may have several different points of charging before it is full. It most likely has a low-charging mode for the aforementioned trickle charging. It also may have a different charging state for when the battery is nominally charged. Finally, it may have a charging state for a fully charged battery.

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