Hauppauge, New York—Using a USB (Universal Serial Bus) as a charging source is becoming increasingly commonplace, and should be a no-brainer, right? Long story short: no, it’s not, not at all. As the wags sometimes say: "if only. . . "
It turns out that the "USB as charger" role has lots of subtleties and traps for the unwary. A trio of related parts from SMSC, the USC1001-1, USC1001-2, and USC1002, called "USB Port Power Controllers with Charger Emulation" ICs, embody circuitry and "smarts" to tackle these issues.
Among the issues are:
•Need for fast charging
•Getting to universal charging over the USB port: Recognition and trickle charge of legacy devices, fromApple, RIM, and others, with multiple charging profiles
•Emerging standards and applications: USB-IF battery charging specifications (BC1.1 and BC1.2); as well as media tablets
•Power allocation and higher-current devices: power-supply capacity is not designed to provide 10W+ for charging from the USB port
•"Wake-up on USB attach": it is difficult to wake the system for charging with today's components
•Thermal limit shutdown: to stop charging at temperature/load limit
To meet these, the SMSC parts provide:
•Fast charging via an integrated power switch
•Compatibility with USB data-line handshaking
•Support for emerging standards and applications, by providing programmable charging profiles
•Power allocation through the use of current rationing to limit the power
•"Wake on USB attach" since detection does not rely on the main power
•Thermal limit shutdown by implementing dynamic thermal management
These USB port power controllers combine the power switch and pass-through switch for flexibility on the signal and power lines, and also enable the ability to emulate a wall charger's signature on those lines.
Why is this needed? USB wall chargers, despite their standard outward appearance, exist because they are tailored to the device for which they were built. Peripheral devices—when recognized—will draw charge as much as their circuitry will allow, but will charge slower if they do not see special USB signature. A standard computer USB port only provides 500mA, and RIM, Apple & other portable devices have special wall chargers.
The UCS100x devices integrate 2.5A Vbus power switch, and automatically present nine charging-emulation profiles (including most Apple devices, BC1.2 CDP & DCP and more; the vendor says that 22 brands were tested, and that number is growing). To make life easier for the designer, no host Intervention is required for an expanded list of device support. The table shows the key features and differences of the parts.
"Wake on USB attach" is the most intriguing feature I saw in these parts. Note that it is difficult to provide charging from a host that is asleep or off: laptops don’t want to charge—they don't want to kill their battery while they are off or sleeping. The UCS100x devices provide an "attach detect" signal that does not rely on the main power to be active.
Once an "attach" is detected, the host device is be notified, and the host can decide to wake up or not, and to charge or not. The UCS1002 can also limit the energy or current given out, so the battery will not be drained; thus, a battery-operated product can operate as an auxiliary power source.
The UCS100x backs off the charging limit as it approaches the thermal limit. It will reduce charging current or power dissipation, which prevents thermal shutdown, and it will keep charging where other devices have stopped completely.
Packaging, price, and availability: The UCS1001-1 and UCS1001-2 are $0.80 in 10k quantities; the UCS1002 is $1.15. All three are housed in 20-lead, 4 x 4 mm QFN packages, and are sampling now.
For more information: go to http://www.smsc.com.