USB-Powered Li+ Charger

6

_______________________________________________________________________________________

USB*

PORT

4.35V TO 5.5V

IN

MAX1811

SYSTEM

LOAD

BATT

SINGLE

Li+

CELL

*WHEN USING WALL ADAPTER, IN VOLTAGE RANGE IS FROM 4.35V TO 6.5V.

Figure 2. System Configuration

Detailed Description

Charger-Control Circuitry

The voltage/current regulator consists of a voltage con-

trol loop, a current control loop, and a thermal control

loop (Figure 1). Use the SELV input to set the battery

regulation voltage to a 4.1V or 4.2V single Li+ cell. The

current and thermal loops are internally compensated

and require no external compensation. The outputs

from all loops drive an internal linear regulator. The

thermal loop modulates the current loop by limiting the

charge current if the die temperature exceeds +125°C.

The MAX1811 is in current mode when the BATT volt-

age is below the regulation set point and in voltage

mode when the BATT voltage is near the regulation set

point. The CHG output indicates whether the part is in

current mode (CHG = low) or voltage mode (CHG =

high impedance). Battery voltages less than 2.5V acti-

vate a 43mA preconditioning mode (CHG = high

impedance). Normal charging resumes when the bat-

tery voltage exceeds 2.5V.

System Configuration

The MAX1811 is designed to operate seamlessly with a

universal serial bus (USB) port. In a typical design, the

USB connects to the MAX1811 input, and the MAX1811

drives the load and charges the battery when enabled.

Charge-Current Selection

The MAX1811 charges a single cell Li+ battery in either

100mA or 500mA modes. The MAX1811 expects the

system to poll the USB host to determine if the USB is

capable of providing 100mA or 500mA and regulates

the charging current accordingly (Figure 2). This is to

maintain compatibility with both powered and unpow-

ered USB hosts. A powered USB host is capable of

providing 500mA, and an unpowered USB hub is limit-

ed to only 100mA.

Drive SELI low to set the charge current to the 100mA

mode. Use a 10k

Ω pulldown resistor to ground on SELI,

if necessary, to ensure that the MAX1811 defaults to

the 100mA mode in the event that no logic signal is

present. Drive SELI high to increase the charge current

to the 500mA mode only if the polled USB port can pro-

vide the required current.

Thermal-Control Circuitry

The thermal loop limits the MAX1811 die temperature to

+125°C by reducing the charging current as neces-

sary. The MAX1811 can operate normally with the ther-

mal loop active. This is not a fault condition and can be

used continuously. The power dissipated by the inter-

The power dissipation rating for the thermally enhanced

8-pin SO package is 1.4W at +50°C ambient (assuming

ambient temperature at which most Li+ battery manu-

facturers allow charging. The 1.4W power dissipation

may never be reached due to the MAX1811’s thermal

regulation loop.

Applications Information

USB Output Voltage

The minimum voltage to a USB-powered device may

be as low as 4.35V when cable and connector drops

are considered (Figure 3). The MAX1811 is optimized

for operation at these low input voltage levels. USB

hubs may also provide as much as 5.5V. 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.