BQ24450 Datasheet, PDF (15/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
www.ti.com ..................................................................................................................................................................................................... SLUS929 – APRIL 2009
External
Supply
R_ISNS
Q_EXT
D_EXT
3
2
4
ISNSP ISNSM IFB
5 _IN
1 _ISNS
8 _BSTOP
bq24450
9 STAT2
GND COMP
6
14
16
DRVC
PRE-CHG ¹¹
R_T R_A
CE ¹²
VFB ¹³
R_B
R_D
STAT1 10
PGOOD ⁷
R_C
DRVE
15
C_COMP
R_P
SLA
Battery
Figure 9.
The first step is to decide on the value of the current in the voltage divider resistor string in FLOAT mode. This
should be substantially higher than the input bias current in the CE and VFB pins and the leakage current in the
STAT1 pin, but low enough such that the voltage on the PGOOD pin does not introduce errors. A value of 50µA
is suitable.
In FLOAT mode, STAT1 is OFF, so there is no current in R_D. The voltage on the VFB pin is 2.3V.
R_C = 2.30V ÷ 50µA = 46kΩ. The closest 1% value is 45.9kΩ.
V_FLOAT = V_REF × _A + R_B + R_C) ÷ R_C → R_A + R_B = 2×R_C = 91.8kΩ.
V_BOOST = V_REF × _A + R_B + R_C//R_D) ÷ R_C//R_D → R_D = 469.2kΩ. Pick the closest 1% value of 464kΩ.
V_TH = V_REF × _A + R_B + R_C//R_D) ÷ _B + R_C//R_D) → R_B = 36.8559kΩ. The closest 1% value is 36.5kΩ.
R_A = 91.8kΩ – R_B = 46.5kΩ. The closest standard value is 46.4kΩ.
I_PRE = (V_IN – V_BAT) ÷ RT → RT = 500Ω. Select 499Ω. At V_IN(MAX), I_PRE = 16mA, which is safe.
I_MAX-CHG = V_ILIM ÷ R_ISNS → R_ISNS = 250mV ÷ 600mA = 0.417Ω. The closest 1% value is 0.422Ω.
For Q_EXT, the BD242 is suitable, and a 1N4001 will do for D_EXT
R_P = (V_IN(MIN) – 2.0V) ÷ I_MAX-CHG × h_FE(MIN) = 7 ÷ 0.6 x 25 = 291.6Ω. Pick 294Ω from the standard values.
P_D = (V_IN(MAX) – 0.7V) ÷ h_FE × I_MAX-CHG – (I_MAX-CHG)² ÷ (h_FE)² × R_D = 126mW under worst case conditions.
Choose C_COMP = 0.1µF.
REFERENCES
1. Yuasa Battery Co., NP Valve Regulated Lead Acid Battery Manual
2. Panasonic, Methods of charging the Valve-Regulated Lead-Acid Battery
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