BQ24450 Datasheet, PDF (14/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
SLUS929 – APRIL 2009 ..................................................................................................................................................................................................... www.ti.com
External Quasi-Darlington
I_MAX-CHG range:
Minimum ΔV:
0.6A to 15A
1.2V
R_P = (V_IN(MIN) – 0.7 V) ÷ I_MAX-CHG × h_FE1(MIN)h_FE2(MIN)
P_D = (V_IN(MAX) – 0.7 V) ÷ (h_FE1 × h_FE2) × I_MAX-CHG – (I_MAX-CHG)² ै_FE1 × h_FE2)² × R_P
C_COMP = 0.22µF with 470Ω series resistor to GND
V_IN
Q2
Q1
DRVC
16
V_OUT
Q1
DRVE
15
R_P
NPN Emitter-Follower
I_MAX-CHG range:
Minimum ΔV:
25mA to 1000mA
2.7V
R_P = (V_IN(MIN) – V_OUT(MAX) – 1.2 V) ÷ I_MAX-CHG × h_FE(MIN)
P_D = (V_IN(MAX) – V_OUT – 0.7 V) ÷ h_FE × I_MAX-CHG – (I_MAX-CHG)² ै_FE)² × R_P
C_COMP = 0.01µF to 0.047µF
V_IN
R_P
DRVC
16
Q_EXT
V_OUT
Q1
15
DRVE
DESIGN EXAMPLE
This section covers the design of a dual-level charger for a 6V 4Ah sealed lead-acid battery. The application is a
system where the battery is used in standby mode, and the load on the battery when it powers the system is
250mA (0.06C).
The battery parameters are (see References 1 and 2)
Final discharge voltage
Float voltage
Voltage in boost mode
Charge rate
V_BAT(MIN)
Trickle charge rate
1.75V per cell
2.30V per cell
2.45V per cell
0.05C to 0.3C
5.25V
6.9V
7.35V
Use 0.15C = 600 mA
4V
10 mA
V_TH
V_FLOAT
V_BOOST
I_MAX-CHG
The charger is required to operate from a supply voltage of 9V to 13V. Therefore, the minimum input to output
differential is 1.65V. To block reverse current from the battery to the input supply use a blocking diode as in
Figure 7. This leaves only 0.65V as the differential across the external transistor, forcing the use of the
Common-Emitter PNP topology.
Figure 9 is the schematic for this charger (from Figure 7, with the pass transistor topology changed), with the
remaining task being the calculation of all the component values.
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