MAX1811 USB-Powered Li+ Charger Detailed Description 500mA, and an unpowered USB hub is limited to only Charger-Control Circuitry 100mA. The voltage/current regulator consists of a voltage con- Drive SELI low to set the charge current to the 100mA trol loop, a current control loop, and a thermal control mode. Use a 10kΩ pulldown resistor to ground on SELI, loop (Figure 1). Use the SELV input to set the battery if necessary, to ensure that the MAX1811 defaults to the regulation voltage to a 4.1V or 4.2V single Li+ cell. The 100mA mode in the event that no logic signal is present. current and thermal loops are internally compensated and Drive SELI high to increase the charge current to the require no external compensation. The outputs from all 500mA mode only if the polled USB port can provide the loops drive an internal linear regulator. The thermal loop required current. modulates the current loop by limiting the charge current if Thermal-Control Circuitry the die temperature exceeds +125°C. The MAX1811 is in The thermal loop limits the MAX1811 die temperature to current mode when the BATT voltage is below the regula- +125°C by reducing the charging current as necessary. tion set point and in voltage mode when the BATT voltage The MAX1811 can operate normally with the thermal is near the regulation set point. The CHG output indicates loop active. This is not a fault condition and can be used whether the part is in current mode (CHG = low) or continuously. The power dissipated by the internal power voltage mode (CHG = high impedance). Battery voltages FET is determined by (V less than 2.5V activate a 43mA preconditioning mode IN - VBATT) x ICHG. The power dissipation rating for the thermally enhanced 8-pin SO (CHG = high impedance). Normal charging resumes package is 1.4W at +50°C ambient (assuming a 1in2 PC when the battery voltage exceeds 2.5V. board radiating area), which is the maximum ambient System Configuration temperature at which most Li+ battery manufacturers The MAX1811 is designed to operate seamlessly with a allow charging. The 1.4W power dissipation may never be universal serial bus (USB) port. In a typical design, the reached due to the MAX1811’s thermal regulation loop. USB connects to the MAX1811 input, and the MAX1811 Applications Information drives the load and charges the battery when enabled. USB Output VoltageCharge-Current Selection The minimum voltage to a USB-powered device may be The MAX1811 charges a single cell Li+ battery in either as low as 4.35V when cable and connector drops are con- 100mA or 500mA modes. The MAX1811 expects the sidered (Figure 3). The MAX1811 is optimized for opera- system to poll the USB host to determine if the USB is tion at these low input voltage levels. USB hubs may also capable of providing 100mA or 500mA and regulates provide as much as 5.5V. At high input voltages (5.5V) the charging current accordingly (Figure 2). This is to and low cell voltages (2.7V), the MAX1811’s thermal loop maintain compatibility with both powered and unpowered may limit the charge current until the cell voltage rises. USB hosts. A powered USB host is capable of providing USB* IN BATT SYSTEM PORT MAX1811 SINGLE LOAD 4.35V TO 5.5V Li+ CELL *WHEN USING WALL ADAPTER, IN VOLTAGE RANGE IS FROM 4.35V TO 6.5V. Figure 2. System Configuration www.maximintegrated.com Maxim Integrated │
