Datasheet

TPS65735

SLVSAI6–JUNE 2011

www.ti.com

2.7 Linear Charger Operation

This device has an integrated Li-Ion battery charger and system power path management feature targeted

at space-limited portable applications. The architecture powers the system while simultaneously and

independently charging the battery. This feature reduces the number of charge and discharge cycles on

the battery, allows for proper charge termination, and enables the system to run with a defective or absent

battery pack. It also allows instant system turn-on even with a totally discharged battery.

The input power source for charging the battery and running the system can be an AC adapter or USB

port connected to the VIN pin as long as the input meets the device operating conditions outlined in this

datasheet. The power-path management feature automatically reduces the charging current if the system

load increases. Note that the charger input, VIN, has voltage protection up to 28 V.

2.7.1 Battery and TS Detection

To detect and determine between a good or damaged battery, the device checks for a short circuit on the

BAT pin by sourcing I

BAT(SC)

to the battery and monitoring the voltage on the BAT pin. While sourcing this

current if the BAT pin voltage exceeds V

BAT(SC)

, a battery has been detected. If the voltage stays below

the V

BAT(SC)

level, the battery is presumed to be damaged and not safe to charge.

The device will also check for the presence of a 10 kΩ NTC thermistor attached to the TS pin of the

device. The check for the NTC thermistor on the TS pin is done much like the battery detection feature

described previously. The voltage on the TS pin is compared against a defined level and if it is found to be

above the threshold, the NTC thermistor is assumed to be disconnected or not used in the system. To

reduce the system quiescent current, the NTC thermistor temperature sensing function is only enabled

when the device is charging and when the thermistor has been detected.

Figure 2-4. Thermistor Detection and Circuit

2.7.2 Battery Charging

The battery is charged in three phases: conditioning pre-charge, constant-current fast charge (current

regulation), and a constant-voltage tapering (voltage regulation). In all charge phases, an internal control

loop monitors the IC junction temperature and reduces the charge current if an internal temperature

threshold is exceeded. Figure 2-5 shows what happens in each of the three charge phases:

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