STBC21 datasheet(13/54 Pages) STMICROELECTRONICS

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STBC21

Detailed description

Doc ID 023256 Rev 1

13/54

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 VCC

pin rises above the threshold (VCC_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 VPRE_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 VRECH 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.

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