LX2205

Microsemi

Analog Mixed Signal Group

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 10

Copyright

© 2007

Rev. 1.0a, 2007-03-02

1A Li-Ion Battery Charger with Power Source Management

TM

®

TH EORY OF O PERAT ION

Battery Current w ith 100m A USB Current Lim it

-80

-60

-40

-20

0

20

40

60

80

100

120

0

40

80

120

160

System Load Current (m A)

USB Limit

Battery

The isolated battery topology reduces charge time from

the USB port when the appliance is turned on while also

charging. Because the system power rail can be a higher

voltage than the battery voltage, the system will require less

power from the USB source which leaves more power

available to charge the battery quickly. For example, if the

system draws 1W, and the USB input is 5V, the system

draws 1W/5V = 200mA from the USB source; this leaves

300mA to charge the battery. In a topology where the load

connects directly to the battery (as is done with

conventional non-isolated linear chargers), if the average

battery voltage is 3.7V, the system will draw 1W/3.7V =

270mA from the USB source, this leaves only 230mA to

charge the battery. In this case the LX2205 will charge the

battery 30% faster.

When the SUSP pin is pulled high, less than 25μA is

pulled from the USB port. The rest of the LX2205,

however, continues to function normally. If an adapter is

applied, the effect of SUSP is negligible. If the system is

running off both USB and the battery and SUSP is logic

high, the system load will be transferred 100% to the

battery.

SHDN

When SHDN is logic high, the LX2205 is placed in a

total shutdown mode; no current will flow between SYS,

USB and BAT. Quiescent current is 25μA during

SHDN mode.

UCL

The UCL pin is used to select the current limit level for

the USB input. When the UCL pin is a logic high the

maximum current level as determined by the CUS resistor

is selected; when the UCL pin is a logic low, the current

limit is set to 20% of the maximum level.

When the power is removed from both MDC and

USB, the battery current flows from BAT to SYS and thus

provides power to the load without the need for any

external switches or monitoring circuits. There is a circuit

equivalent of an ideal diode (patent-pending bi-directional

MOSFET) in the LX2205 between the BAT pin and SYS

pin. This ideal diode appears as a low impedance to high

forward current and presents a high impedance to low

discharge currents or potentially reverse charge currents.

For small forward currents, the ideal diode regulates a

small forward voltage.

This small voltage allows the

power sense circuitry to determine whether an input power

source is present even if the current through the ideal diode

is less than 1mA.

This feature makes it virtually

impossible to charge the battery (unregulated) in reverse

through the ideal diode and also enables the battery to

assist a current limited input power source without

chattering even if only a few mA of battery current are

required.