8

bq21040

SLUSCE2A – APRIL 2016 – REVISED MAY 2016

www.ti.com

Product Folder Links: bq21040

Submit Documentation Feedback

Copyright © 2016, Texas Instruments Incorporated

8 Detailed Description

8.1 Overview

The bq21040 is a highly integrated single cell Li-Ion and Li-Pol charger. The charger can be used to charge a

battery, power a system or both. The charger has three phases of charging: Pre-charge to recover a fully

discharged battery, fast-charge constant current to supply the buck charge safely and voltage regulation to safely

reach full capacity. The charger is very flexible, allowing programming of the fast-charge current. This charger is

designed to work with a USB connection or Adaptor (DC out). The charger also checks to see if a battery is

present.

The charger also comes with a full set of safety features: Temperature Sensing Standard, Over-Voltage

Protection, DPM-IN, Safety Timers, and ISET short protection. All of these features and more are described in

detail below.

The charger is designed for a single power path from the input to the output to charge a single cell Li-Ion or

Li-Pol battery pack. Upon application of a 5VDC power source the ISET and OUT short checks are performed to

assure a proper charge cycle.

If the battery voltage is below the LOWV threshold, the battery is considered discharged and a preconditioning

cycle begins. The amount of the current goes into the battery during this phase is called pre-charge current. It is

fixed to 20% of the fast charge current.

Once the battery voltage has charged to the VLOWV threshold, fast charge is initiated and the fast charge

current is applied. The fast charge constant current is programmed using the ISET terminal. The constant current

provides the bulk of the charge. Power dissipation in the IC is greatest in fast charge with a lower battery voltage.

If the IC reaches 125°C the IC enters thermal regulation, slows the timer clock by half and reduce the charge

current as needed to keep the temperature from rising any further. Figure 7 shows the charging profile with

thermal regulation. Typically under normal operating conditions, the IC’s junction temperature is less than 125°C

and thermal regulation is not entered.

Once the cell has charged to the regulation voltage the voltage loop takes control and holds the battery at the

regulation voltage until the current tapers to the termination threshold. The termination current is set to 10% of

the fast charge current. The CHG terminal is low (LED on) during the first charge cycle only and turns off once

the termination threshold is reached, regardless if termination, for charge current, is enabled or disabled.