BQ24450 Datasheet, PDF (9/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
www.ti.com ..................................................................................................................................................................................................... SLUS929 – APRIL 2009
As charging proceeds, the voltage at the VFB pin increases further to V_REF. At this point, the voltage regulating
amplifier prevents the voltage at the VFB pin from rising further, maintaining the battery voltage at V_BOOST. [(4) in
Figure 3].
V_BOOST = V_REF × _A + R_B//R_C) ÷ R_B//R_C
I_CHG keeps flowing into the battery. As the battery approaches full charge, the current into the battery decreases,
while the battery terminal voltage is maintained at V_BOOST.
At (5), the charging current I_CHG reduces to a value I_TAPER such that the voltage across R_ISNS becomes less than
V_ISNS (25mV typical)
I_TAPER = V_ISNS ÷ R_ISNS
Q6 at the output of the current sense comparator turns OFF. The internal current source pulls the BSTOP pin
HIGH, latch L1 is forced to SET, in turn forcing L2 to SET. The reference voltage on the voltage sense
comparator is now 0.9V_REF. STAT1 turns OFF, and the voltage on the battery settles to:
V_FLOAT = V_REF × _A + R_B) ÷ R_B
As long as the peak load current is less than I_MAX-CHG, it will be supplied by Q_EXT, and the voltage across the
battery will be maintained at V_FLOAT. But if the peak load current exceeds I_MAX-CHG, the battery will have to
provide the excess current, and the battery terminal voltage will drop. Once it drops below 0.9V_REF, at (6) in
Figure 3, a new charge cycle is initiated. The battery voltage V_BAT at this point, V_RCH, is given by:
V_RCH = 0.9V_REF × _A + R_B) / R_B
12
3
4
5
6
7
V_UVLO
V_IN
I_MAX-CHG
I_TAPER
I_CHG
V_BOOST
V_BI
V_FLOAT
V_RCH
V_BAT
PGOOD Q7
ISNS Q6
STAT1 Q8
STAT2 Q9
STATE #
OFF ON
OFF ON
ON
OFF
ON
STATE #1
STATE #2
ON OFF
ON OFF
ON OFF
Figure 4.
STATE #3
OFF ON
OFF ON
OFF ON
#1
#2
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