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Teardown: Cell-phone charger: nice idea done right

-March 19, 2013

Back in November 2012, EDN Brand Director Patrick Mannion published a teardown of a cell-phone battery charger. The design was not well done and received bad reviews. Redemption! I recently acquired a Mophie juice pack powerstation mini smartphone and USB-device battery charger that not only negates the previous teardown fiasco but also demonstrates the ability to provide more than twice the battery life to your smartphone while still keeping a low-profile shape that slips right into your pocket.

Mophie, a California-based designer and manufacturer of mobile intelligent devices and accessories, developed the juice pack, the first ever “Works with iPhone” portable battery case certified by Apple. I contacted the company via e-mail for its help clarifying this design as well as the component types. It has not yet responded, so I give you my educated guesses. EDN readers are encouraged to weigh in with their own thoughts and expertise.

1. The micro USB and standard USB ports on the side of the case were quite secure. I tested them many, many times, inserting and extracting the connectors from the supplied cable.

2. I think a Microchip MCU is located under the shield shown. The microcontroller usually controls the housekeeping; provides charge control for the USB battery-charger IC, IC1; controls the four LEDs for the charge-status indicator; has I2C bus capability if needed for IC1; and controls the power MOSFETs. The Microchip part is unusual for a few reasons. It appears to have a Microchip symbol and only a date code, with no sign of a part number. There are eight pins on two sides opposing one another. On the other two sides are what look like eight pins—six of the eight are cut off and two are connected to the PCB, on each opposing side. Sometimes manufacturers provide early samples of a new die not yet fully qualified in a nonstandard package, but I am not sure.

3. The boost-converter IC is located under the shield shown and needs to be close to its inductor. The function of this IC is to take the battery output voltage and convert it to 5 or 5.1V to the Mophie USB port through the MOSFETs when it becomes a host to charge an external device. Its output capability is probably around 1A. This IC has two sets of five pins on opposing sides. The other two opposing sides have some sort of heat-sinking pins, which are soldered to the board, possibly indicating internal power MOSFETs.

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4. The USB Li-ion battery-charger IC is TI’s bq24040. It is housed in a 10-pin package with no shield (indicating a linear regulator) and can handle an ac-wall-adapter input or USB power source from the micro USB connector input. It probably has integrated MOSFETs and can charge at around 1A, meaning the battery’s maximum charge time if fully discharged would be about three hours.

5. The power MOSFETs are configured to act as a USB-standard power switch, probably also implementing temperature protection, short-circuit protection, and overcurrent protection. Control is through the USB port when acting as a host to charge an external device.

There are three USB specifications—USB 1.0, 2.0, and 3.0—but here we focus on USB 2.0, as it’s by far the most common. In any USB network, there is one host and one device. In this case, the Mophie juice pack is the host, and your smartphone/tablet/camera is the device. Power always flows from the host to the device, but data can flow in both directions. In this case, the USB port is a dedicated charging port.


Standard USB pin designations are shown for regular cable and OTG cable.

6. A USB socket has four pins, and a USB cable has four wires. The inside pins carry data (D+ and D−); the outside pins provide a 5V power supply. In terms of actual current (milliamps, or mA), there are three kinds of USB ports dictated by the current specs: a standard downstream port, a charging downstream port, and a dedicated charging port. The first two can be found on your computer (and should be labeled as such); the third kind applies to “dumb” wall chargers. In the USB 1.0 and 2.0 specs, a standard downstream port is capable of delivering up to 500 mA (0.5A); in USB 3.0, it moves up to 900 mA (0.9A). The charging downstream and dedicated charging ports provide up to 1500 mA (1.5A).

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