USB 1.x/2.0 standard pinout
Lithium-ion Polymer Battery
LiPo batteries are usually composed of several identical secondary cells in parallel to increase the discharge current capability. The voltage (nominal cell voltage is 3.7V) of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination.
Example, a circuit produces +5V and +3.3V to power portable devices It allows the port to maintain communications while supplying power, e.g., to charge a Li+ battery.
IC1 - MAX1811 (Li+ battery charger)
IC2 - step-up DC-DC converter, boosts VBATT to 5V and delivers up to 450mA. The low-battery detection circuitry and true shutdown (limits battery current to less than 2μA) capability protects the Li+ battery.
MAX1811 Functional Diagram
Pin 1 : VBUS (red) : +5V
Pin 2 : D- (white) : data -
Pin 3 : D+ (green) : data +
Pin 4: GND (black) : ground
The USB Battery Charging Specification of 2007 defines new types of USB ports, e.g., charging ports. As compared to standard downstream ports, where a portable device can only draw more than 100mA current after digital negotiation with the host or hub, charging ports can supply currents above 0.5A without digital negotiation. A charging port supplies up to 500 mA at 5 V, up to the rated current at 3.6 V or more, and drop its output voltage if the portable device attempts to draw more than the rated current. There is no upper limit for the rated current of a charging downstream port, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A). The charger port may shut down if the load is too high. The Battery Charging Specification 1.2 of 2010 makes clear, that there are safety limits to the rated current at 5 A coming from USB 2.0.
Charging ports exist in two flavors: charging downstream ports (CDP), supporting data transfers as well, and dedicated charging ports (DCP), without data support. With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals. Therefore, current draw may not exceed 900 mA during high-speed data transfer. On a dedicated charging port, the D+ and D- pins are shorted. Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPod and iPhone chargers indicate the available current by voltages on the D- and D+ lines. When D+ = D- = 2V, the device may pull up to 500 mA. When D+ = 2.0 V and D- = 2.8 V, the device may pull up to 1000 mA of current.
USB 2.0
One unit load is 100 mA.
A device may draw a maximum of 5 unit loads (500 mA) from a port.
A low-power device draws at most 1 unit load, with minimum operating voltage of 4.4 V.
USB 3.0
One unit load is 150 mA.
A device may draw a maximum of 5 unit loads (900 mA) from a port.
A low-power device draws at most 1 unit load, with minimum operating voltage of 4 V.
Lithium-ion Polymer Battery
LiPo batteries are usually composed of several identical secondary cells in parallel to increase the discharge current capability. The voltage (nominal cell voltage is 3.7V) of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged), and Li-poly cells have to be protected from overcharge by limiting the applied voltage to no more than 4.235 V per cell used in a series combination.
Example, a circuit produces +5V and +3.3V to power portable devices It allows the port to maintain communications while supplying power, e.g., to charge a Li+ battery.
IC1 - MAX1811 (Li+ battery charger)
- Pulling SELI terminal low sets the charging current to 100mA for low-power USB ports, and pulling high sets 500mA for high-power ports.
- Pulling SELV high or low configures the chip for charging a 4.2V or 4.1V Li+ battery. To protect the battery, IC1's final charging voltage exhibits 0.5% accuracy.
- /CHG light up an LED during charging.
- MAX1811 has preconditioning that soft-starts a near-dead battery cell before charging. It is available in 1.4W thermally enhanced 8-pin SO package.
- Battery voltages less than 2.5V activate a 43mA preconditioning mode (/CHG = high impedance). Normal charging resumes when the battery voltage exceeds 2.5V.
- At high input voltages (5.5V) and low cell voltages (2.7V), the MAX1811’s thermal loop may limit the charge current until the cell voltage rises.
- The low-battery trip point is set by an external resistive divider between VBATT and GND, connected to LBI. Connecting the low-battery output (LBO) to shutdown (SHDN) causes IC2 to disconnect its load in response to a low battery voltage.
- The n-channel FET at LBO eliminates on/off oscillation (caused by internal source impedance of a Li+ battery) by adding hysteresis to the low-battery detection circuitry. When VBATT goes below 2.9V, LBO opens and allows SHDN to be pulled high, turning on the FET. With the FET turned on, the parallel combination of 1.3MΩ and 249kΩ eliminates oscillation by setting the battery turn-on voltage to 3.3V.
MAX1811 Functional Diagram
