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Charging Li-ion batteries in portable apps (Part 2)

Posted: 06 Sep 2010 ?? ?Print Version ?Bookmark and Share

Keywords:USB port charging? handheld devices? USB power sources?

The USB specification requires that the host be enumerated before current above 2.5mA is pulled from the VBUS supply. However, there is a provision in the specification for dead, weak or no-battery cases. It states that, "a device with a dead, weak or no battery is allowed to draw up to 100mA for up to 100ms sometime after attaching, in order to determine if it will likely be able to connect." This may be a problem if the device is unable to power up with 100mA in less than 100ms.

To address this, the USB specification added a specific provision for battery charging. It states, "If a portable device is not able to power up and connect with less than 100mA, such a device with a dead or weak battery is allowed to use the 100mA from the host to first charge its battery to its weak battery threshold. Upon reaching its weak battery threshold, the device is immediately required to power up and connect."

Above the weak battery threshold, it is assumed that the battery is high enough to supply the host and, therefore, the host is active. Each application defines its own weak-battery threshold. The hardware enables for the bq2407x, along with a simple voltage detector, enable the designer to simply fulfil this requirement.

The voltage detector must be set with the applications weak battery threshold. For example, the TPS3836 is available with several thresholds. Also, there are voltage detectors available with an adjustable threshold for the ultimate flexibility. For this application, the important voltage detector feature is an active low RESET (Low when VIN THRESHOLD) push/pull output so that it can be isolated from the host outputs. Once the host comes up, it can either disable the voltage detector or the pullup. The pulldown strength of the host outputs must be stronger than the resistors that separate the voltage detector output from EN1 and EN2.

With consumer demands for fewer cords to power and connect their devices, along with the desire for flexibility to charge from the computer or an AC outlet, many current and future released handheld devices are required to charge from USB sources and AC-wall adapters. As a result, handhelds must follow USB specifications. These requirements have presented several new challenges for battery charging.

View the PDF document for more information.

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