BQ24450 Datasheet, PDF (10/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
SLUS929 – APRIL 2009 ..................................................................................................................................................................................................... www.ti.com
An Improved Dual-Level Float-Cum-Boost Charger with Pre-Charge
The problem with the charger circuit shown in Figure 3 is that even with deeply discharged batteries, charging
starts at full current level I_MAX-CHG. This can sometimes be hazardous, resulting in out-gassing from the battery.
The bq24450 can be configured to pre-charge the battery till the voltage levels rise to levels safe enough to
permit charging at I_MAX-CHG.
In the circuit of Figure 5, the CE pin is used to detect the battery voltage. As long as the voltage at the CE pin is
below V_REF, the enable comparator turns ON Q3 and Q4. This turns OFF Q1 and turns ON Q5, permitting a
pre-charge current I_PRE to flow from the PRE-CHG pin through R_T into the battery.
I_PRE = (V_IN – V_BAT) ÷ R_T
Once the battery voltage rises above a safe threshold V_TH at (2) in Figure 6, the enable comparator turns OFF
Q3 and Q4, thus turning OFF Q5 and enabling Q1. Q_EXT then provides I_MAX-CHG, and the circuit after this
performs as described before.
V_TH = V_REF × _A + R_B + R_C//R_D) ÷ _B + R_C//R_D)
External
Supply
R_ISNS
Q_EXT
ISNS
1
Q6
ISNSP
3
ISNSM IFB DRVC
2
4
16
DRVE
15
250mV
Q1
+
+
25mV
V_BAT ^ICHG
VRLA
Battery
R_A
R_T
COMP
14
V_REF
VFB
R_B
13
5
IN
Voltage
Reference
V_IN
V_REF
Q5
Q2 Q3
Q4
11 _{PRE-CHG RC}
12 ^CE
V_REF
8
BSTOP
0.95V_REF
0.90V_REF
V_VFB
7
Q7
STAT1
R_D
10
Q8
Charging State Logic
9
Q9
6
GND
Figure 5.
10
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