STBC21 Datasheet, PDF (13/54 Pages) STMicroelectronics – LI+ battery charger with thermal regulation

STBC21
5
Detailed description
Detailed description
The STBC21 is designed to charge single-cell lithium-ion batteries using the constant
current/constant voltage algorithm. The charger can deliver up to 1.1 A (programmable) of
charge current with a final float voltage accuracy of 20 mV.
Once the battery is connected, the normal charge cycle begins when the voltage at the V_CC
pin rises above the threshold (V_{CC_OK}).
The process generally starts with a pre-charge phase in constant current mode, where a
reduced charge current (see Appendix B) is supplied to the battery. A programmable timer
(TIMER-1) during this phase allows possibly damaged batteries to be spotted in case the
voltage doesn't reach the expected value within a given time.
Note that, if the battery voltage is below 1.0 V, the charger first attempts to charge (0 V-
charge mode) for 16 seconds (TIMER-4) to test if the battery is dead. After the TIMER-4 has
elapsed, in case the voltage doesn't cross the 1.0 V threshold, a battery alarm is flagged.
Otherwise the charger keeps on charging.
When the battery voltage reaches 3.0 V, the charging current reaches its maximum value
(fast-charge phase) programmable by an external resistor.
When the battery voltage approaches the final float voltage (programmable, typ. 4.2 V), the
STBC21 enters constant voltage mode and the charge current decreases as the battery
becomes fully charged. The fast-charge and the following constant voltage phases have a
dedicated timer set through bits in NVM to make sure the current reaches its termination
value within a given time. If during this phase the battery voltage falls below V_{PRE_CH} or
rises above 4.23 V (±20 mV), the operation is stopped and an alarm sounds.
The STBC21 terminates the constant voltage phase when the charging current reaches the
termination current threshold (value set in ISET3 register, see Appendix B). In addition, a
further step (end of charge) can be activated through bits in NVM. In this case, the charging
process in constant voltage mode continues for a predefined amount of time (TIMER-3): this
behavior is useful to increase the battery duration.
After charge termination the battery may be discharged by the leakage. When the battery
voltage reaches V_RECH with a lag time of 100 ms the charge automatically restarts. This
feature can be disabled setting a bit in NVM.
If, during the charging phase, one of the timeouts elapses, the charge is stopped and a flag
is set. See Appendix B for more details on timers. The STBC21 charging process works as
depicted in Figure 4.
Doc ID 023256 Rev 1
13/54