Why don’t all devices that draw less than 500 mA come with a Universal Serial Bus (USB) socket? Finding a charger these days hasn’t gotten any better, yet USB sockets are everywhere. Only a fraction of these devices, from cameras to cell phones to wireless headphones, has a USB socket, and it would be nice if that number would increase.
So why bring the topic up here? Because you or one of our other readers will likely design or select the design for these low-power commercial or consumer products. The USB alternative may not always be the cheapest, but it will definitely make your customer happier. Besides, you can include all sorts of neat twists.
UNEXPECTED USB POWER Take Moixa’s USBCELL (Fig. 1). Not only does it use a USB socket as a power source, but its 100-mA nickel-metal-hydride (NiMH) battery can provide power to a host of other devices.
The USBCELL contains its own charging unit, which is managed by a Microchip PIC. An LED indicates charging status, blinking at the 90% mark. It takes five hours to charge via USB and seven hours using a 250-mA NiMH charger.
Meanwhile, the Targus Rechargeable Bluetooth Laser Notebook Mouse comes with a USB cable that’s only used for recharging (Fig. 2). It also employs a standard mini-USB connector that’s used on plenty of other devices.
Unfortunately, my HP camera doesn’t have this type of connector. While the docking station is USB-based, the charger is not. Still, the Targus mouse is very handy for ultra-mobile personal-computer (UMPC) work (see “A Must-Have UMPC Add-On” at www.electronicdesign.com, ED Online 17995). It also eliminates one more charger I need to carry on trips.
USB IN YOUR DESIGN Most cell phones with a USB connector come with the usual charger, which has a USB plug on the end. Devices like the Targus mouse that are designed for a laptop or PC environment typically come with just a standard USB cable.
I am still looking for an adapter that would plug into a USB port that would charge a non-USB device like my LG VX9900 EnV cell phone. Most cell phones do not use that much power when charging, although a dc-dc converter may be needed.
All of this USB power does bring up an interesting dilemma for USB hubs that are sometimes too smart for their own good. Typically, the hubs provide power with a current limit up to the 500- mA rated maximum. Unfortunately, some shut down power to a trickle until a USB device is recognized. But most of the power-only USB solutions simply ignore the USB connection.
So where does this approach fit for products other than portable multimedia devices? It can show up in quite a number of spots. As long as the power requirements aren’t high, just about any device can be powered by USB.
Don’t forget that dc-dc converters are an option for moving up or down the voltage range. Likewise, many microcontrollers that come with built-in power regulators can handle a range of voltages that include 5 V, thus enabling a USB interface to power and charge a unit.
There is an advantage to taking this route for many applications, since USB is first and foremost a communication link. USB microcontrollers can be extremely tiny. They can provide monitoring facilities even if they don’t interface to the rest of the product directly.
I actually received my first USBCELL from a Microchip press briefing. I’m now looking forward to seeing what other interesting USB trinkets will show up at the next conference that I attend.
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