BQ24450 Datasheet, PDF (8/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
SLUS929 – APRIL 2009 ..................................................................................................................................................................................................... www.ti.com
DETAILED OPERATION AND APPLICATION INFORMATION
A Simple Dual-Level Float-Cum-Boost Charger
Figure 3 shows the bq24450 configured as a simple dual-level float-cum-boost charger. Figure 4 shows the
sequence of events that occur in a normal charge cycle. At (1) in Figure 4, power is switched ON. As long as the
input voltage V_IN is below the undervoltage lockout threshold UVLO, Q2 is ON, disabling the driver transistor Q1.
As the input voltage V_IN ramps up and rises above UVLO Q2 turns OFF. This enables Q1 and thus the external
transistor Q_EXT. At the same time, Q7 turns ON, latch L1 is forced to RESET and latch L2 is SET (see Figure 1
for the internals of the Charging State Logic).
The voltage regulating amplifier tries to force the voltage at the VFB pin to V_REF by turning Q1 and thus Q_EXT fully
ON, but the current limiting amplifier limits the charging current I_CHG to I_MAX-CHG such that the voltage across
R_ISNS is V_ILIM – 250mV typical. Thus I_MAX-CHG is given by:
I_MAX-CHG = V_ILIM ÷ R_ISNS
As I_CHG flows into the battery, the battery terminal voltage increases. The voltage at the VFB pin is the battery
voltage scaled by the resistive divider formed by R_A and R_B//R_C (because Q8 is ON). At (3), the voltage on the
VFB pin exceeds 0.95V_REF, and the output of the voltage sense comparator goes HIGH. This forces L2 to
RESET, and STAT2 turns ON. The battery voltage V_BI at this point when STAT2 indicates boost is given by:
V_BI = 0.95V_REF × _A + R_B//R_C) ÷ R_B//R_C
Other than STAT2 changing state at this point, there is no externally observable change in the charging
conditions. I_MAX-CHG continues to flow into the battery.
External
Supply
R_ISNS
Q_EXT
ISNS ₁
Q6
ISNSP
3
ISNSM IFB DRVC
2
4
16
DRVE
15
250mV
Q1
+
+
25mV
Q5
V_BAT
I_CHG
VRLA
Battery
R_A
COMP
14
V_REF
VFB
13
R_B
11
R_C
5
IN
Voltage
Reference
V_IN
V_REF
Q2 Q3
Q4
CE
12 _Connect
to IN
V_REF
8
BSTOP
0.95V_REF
0.90V_REF
V_VFB
Charging State Logic
6
GND
Q7
Q8
7
STAT1
10
9
Q9
Figure 3.
8
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