LX2201

Microsemi

Integrated Products Division

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

Page 9

Copyright

© 2000

Rev. 1.2b, 2005-04-13

USB Li-Ion Battery Charger

TM

®

APPLI CATI ON NOTE

When using input power supplies that are current limited

or have high source impedance, the battery charging current

and/or load current can pull the VDD pin down very close

to the battery terminal voltage. This may cause the LX2201

to enter an indeterminate state where it switches between

charge and discharge modes.

To prevent this, the PNP

differential amplifier circuit of figure 4 can be added; this

circuit adds an additional control loop that regulates the

charger headroom to 300mV by scaling back the battery

charging current.

The internal power switch within the LX2201 requires

the power supply have a minimal level of strength to switch

from discharge mode to charge mode. If the power source is

severely current limited, the LX2201 may not open the

discharge switch. In this case, a small amount of current

will be allowed to flow into the battery unregulated. If left

in this state indefinitely, the current will eventually raise the

battery terminal voltage to the point where the batteries

internal protection circuitry will open the circuit. Typically,

the LX2201 requires a minimum of 80mA in USB mode

and 200mA using the VDD input to force the switchover to

charge mode.

Alternatively, if weak power sources are

used, the switchover from discharge to charge mode can be

forced by momentarily setting VID0 and VID1 to a low

state, which will open the discharge switch allowing the

VDD voltage to rise so the LX2201 can enter charge mode.

To be compliant with the USB specification, the +5V

current must be less than 100mA in the low power mode

and less than 500mA in the high power mode.

If the

LX2201 is configured as shown in Figure 3, it is possible

for the system to consume more than the maximum

allowed USB current (in which case the battery charging

current will have been fully scaled back).

If it is not

possible to regulate the load current when charging from a

USB power source and strict adherence to the USB power

budget is required. In this case the system load can be

applied directly to the battery and the LX2201 will prevent

the load + battery from drawing more power than is

allowed for USB compliance. A better approach that

provides battery isolation may be used; in this approach,

the op amp circuit provides a current limiter that prevents

the system from exceeding the USB power limits.

The

battery is only discharged if the AC adapter or USB inputs

are removed or current limited.

•

It is important when laying out the LX2201 to

place 10µF ceramic capacitors close to the VDD

•

It is important to provide a low thermal impedance

path from the thermal pad on the bottom of the

LX2201 package to the ground plane of the circuit

board to maximize the heat dissipation.