ICs are now available that can automatically detect what type of equipment is connected to the phone or other mobile platform's USB port, and they can distinguish between a charger or headset. ICs designed to address the Chinese telecommunications standard, for example, require that an overvoltage protection device be incorporated into handsets to intercept potentially dangerous signals from external changers. This leads to features such as over- and undervoltage protection (UVP), detection of D+/D- short (to indicate if a charger or something else is connected), support of standard USB device connection, and control of a MOSFET switch. ICs that address such requirements can use an external MOSFET or, more often, one or more MOSFETs incorporated right into a single package together with the control circuitry to provide the isolation switching functionality. A typical OVP or UVP load switch IC will monitor the voltage bus for over- or undervoltage conditions and set a flag pin if the voltage reading is under or over specified limits (e.g., less than 3.3V or more than 6.0V) while turning off the MOSFET load switch.
Undervoltage monitoring, more often called undervoltage lockout (UVLO), is used to ensure that, once the battery voltage goes beneath a specified level, the system turns off in a controlled manner so as not to go unstable, oscillate, or enter a brownout condition. ICs used in mobile, PC, and other applications now provide UVLO. These modern ICs provide power supply signal conditioning by using MOSFETs (typically integrated in the package), as noted earlier, to provide both OVP and UVLO. There are also more sophisticated versions that add such things as in-rush current limiting, overcurrent protection (OCP), reverse current protection, fault management, slew rate control, and thermal protection. In-rush current limiting, for example, is useful to avoid gradual damage to components, reduce power loss, and reduce transient effects in the systems. In certain ICs, a virtual diode is provided to protect against reverse current conditions without the typical diode voltage drop. Integrated fault management allows automatic restart, current limit latchoff, or setting of a hard limit without extra hardware or software. Temperature monitoring is also becoming more important as portable features expand, leading to more power usage and thus higher operating temperatures. Integrating these features into load switches provides simple cost and space savings.
Detection and connection
As the USB port becomes more universal, it has become convenient and even necessary for the phone and other mobile platforms to determine what is connected (such as chargers, audio headsets, UARTs, TTY, and user definable accessories), as well as to switch and route the various signals to make the necessary system connections, all in addition to providing protection. Now there are ICs that provide multiple functions in a single package. These are supersets of the types of products mentioned earlier, so now there is a range of ICs from simple OVP to multifunction ICs that include detection and switching. A good example of a multifunction IC is the Fairchild FSA9285 with 28V OVP load switch, 3-to-1 multimedia switch, USB accessory detection, and auto-configuration to allow USB data, charger, audio (stereo and mono), and UART data to share the USB port.
Managing charging is a fact of life for mobile applications. With the rapid design cycles and proliferation of new platforms such as tablets, portable PCs, handheld games, cameras, navigation systems, and of course cellphones, it is critical for designers to have a variety of options to protect, detect, and connect mobile equipment with charging sources and other subsystems. The range of ICs on the market can help ensure that mobile designs are safe, compliant, compact, cost-effective, and environmentally friendly.
Bill Boldt is senior business development manager with Fairchild Semiconductor.