USB Port-Powered Li-Ion/Li-Polymer Battery …

1 2005 Microchip Technology 1AN971 INTRODUCTIONThe Universal Serial Bus (USB) allows many computerperipherals to be easily swapped without having to turnoff the computer. Today, a variety of handheld, Battery -operated peripherals provide USB ports to facilitatedata transfer to and from a host computer. With theintroduction of the new USB specification, CD/DVDplayers, MP3 players, cameras, personal dataassistants and even cell phones can transfer data atrates up to 480 Mbps. These peripherals are, in some instances, self- powered . As a result, many of these peripherals do nottake full advantage of the USB port.

2 Often overshad-owed by the data interface is the power capability thata USB port provides. Microchip's MCP73853/55 andMCP73861 advanced, fully-integrated, single-cell Li-Ion/Li-Polymer charge-management devices allowthese peripherals to utilize the full power of the , SINGLE-CELL, LI-ION Battery CHARGERMany of the self- powered peripherals use a separatepower supply for Battery charging even whileconnected to a USB port. With the MCP73853/55,harnessing the power of the USB port for Battery charg-ing becomes extremely simple. A USB port transfers signal and power over a four-conductor cable, depicted in Figure 1.

3 Each USB portprovides a limited amount of power over the cable. Theamount of current provided from a USB port is definedin terms of unit loads, where one unit load is defined as100 mA. The number of unit loads that a port cansupply is an absolute maximum, not an average overtime. A port may be configured as either low-power (atone unit load) or high-power (providing up to five unitloads). All ports default to low-power. The transition tohigh-power is under the software s control. It is theresponsibility of the host software to ensure thatadequate power is available before allowingperipherals to consume high 1:USB USB high-power port is specified for a maximumcurrent of five unit loads (or 500 mA), with a voltage of5V, +5%.

4 The USB specification allows for voltagedrops in the USB connectors and cables of up to350 mV when delivering 500 mA. The maximum cablelength is specified as 16 feet. Shorter cables willintroduce less voltage drop. The minimum voltage seenat the peripheral device will be Limiting the current draw to 400 mA produces anabsolute minimum voltage of , providing ofheadroom to fully charge a single-cell, Li-Ion extremely low dropout voltage (200 mV at 400 mA)of the MCP73853/55 makes them an attractive andsimple approach for a USB Li-Ion/Li-Polymer 2 and Figure 3 illustrate USB- powered , single-cell Li-Ion/Li-Polymer Battery chargers.

5 The USB con-troller communicates with the host to determine if theperipheral is connected to either a low-power or high-power port. The controller then sets the appropriatecharge current setting. The MCP73853/55 employstypical charge current settings of 85 mA and 400 mAfor low-power and high-power, respectively. These set-tings ensure that the absolute maximum rating of theUSB port is not exceeded. Optionally, the high-powercharge current setting can be adjusted lower by placinga resistor between the MCP73853/55 and the USBcontroller. The appropriate value resistor can bedetermined from the formula:Author:Scott DearbornMicrochip Technology +D-GNDVBUSD+ IREG IREG ---------------------------------------- -----=Where:IREG is the desired high-power charge currentsetting in ampsRPROG is in kilo-ohmsUSB Port-Powered Li-Ion/Li-Polymer Battery ChargingAN971DS00971A-page 2 2005 Microchip Technology 2:USB- powered , Single-cell, Li-Ion/Li-Polymer Battery charger with Cell Temperature 3.

6 Small PCB Area USB- powered , Single-cell, Li-Ion/Li-Polymer Battery ChargerENSTAT1 STAT 2 VSETVSS3 VDD1 VDD2 VSS2 TIMERPROGTHE RMTHRE FVBAT3 VBAT2 VBAT1 CTIMERRPROGRT1RT2+-Single Lithium-IonCellVSS11234 MCP738531415165678910111213 SUSPEND100 mA 500 mAUSB CONTROLVBATVBUSGNDD+ D-CTIMER+-Single Lithium-Ion CellSUSPEND100 mA 500 mAUSB CONTROLVBATVBUSGNDD+ D-VDD1 VBAT1 ENTIMERSTAT11234 MCP73855 VSETVSS2 VBAT25678910 PROGVSS1 2005 Microchip Technology 3AN971 ADDITIONAL USB SUPPORTED FEATURESIn addition to the maximum permissible current draw onthe USB power port, additional considerations must beadhered to as well. Any peripheral connected to a USBport must support a Suspend state as defined by theUSB specification.

7 A peripheral can enter the Suspendstate from either power state (low power or highpower). The allowed Suspend current is a function ofunit load allocation. Ports operating in the Low-powermode are limited to 500 A of Suspend current. Portsoperating in the High-power mode are limited the circuits of Figures 1 and 2, the peripheraldevice s system power, including the USB controller, isderived directly from the Battery . In this manner, theMCP73853/55 controls the charging of the Battery ,while maintaining the current draw from the USB portbelow the specification limits. The Suspend state isentered when the USB controller pulls the enable pin ofthe MCP73853/55 low.

8 Disabled, the MCP73853/55consumes less than 1 A of current. The peripheralSuspend current can be computed as the current fromVBUS through the bus pull-up and pull-downtermination resistors plus 1 current must also be considered whenever aperipheral is connected to a USB port. The VBUS powerlines at the port are bypassed with a minimum low-ESRcapacitance of 120 F. The maximum load of theperipheral device is limited to 44 , in parallel with10 F of capacitance. This ensures that the VBUS voltage does not get pulled below its minimumoperating voltage when a peripheral is connected to theport.

9 The maximum droop allowed on VBUS is 330 MCP73853/55 requires a minimum input capaci-tance of 1 F, although F is recommended. This iswell below the maximum USB specification. In addition,the MCP73853/55 controls the slew rate of the chargecurrent when transitioning from preconditioning to fastcharge, and when a new charge current setting isrequested ( , from low-power to high-power). Theseunique features prevent large slugs of current fromflowing to the charger output capacitor and Battery ,causing the VBUS voltage to droop important specification to consider when con-necting to the USB power bus is that no peripheral shallsupply (source) current on VBUS at any time.

10 TheMCP73853/55 employs integrated reverse dischargecircuitry that prevents current to flow from the batteryonto CHARGE CYCLE TIMES WITH WALL ADAPTERThe USB high-power port is specified for a maximumcurrent of five unit loads, or 500 mA. UtilizingMicrochip s MCP73861, charge currents up to canbe realized from an alternate power source (such as awall cube).Figure 4 depicts the MCP73861 in a system thataccepts input power from a USB port or alternatepower source. This solution requires only a handful ofexternal components, in addition to the this manner, charge currents up to can be real-ized when the alternate power source is present.