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.
MICROCHIP • www.microchip.com
MOXIA ENERGY LTD.
• www.usbcell.com
TARGUS • www.targus.com