BQ24450 Datasheet, PDF (13/20 Pages) Texas Instruments – INTEGRATED CHARGE CONTROLLER FOR LEAD-ACID BATTERIES

bq24450
www.ti.com ..................................................................................................................................................................................................... SLUS929 – APRIL 2009
Selecting the External Pass Transistor
All the examples so far have used a PNP transistor for the external pass element. But the driver transistor in the
bq24450 can be configured to drive many different types of pass transistors. This section will look at some of the
different configurations that are possible. In all configurations, though, these factors hold:
1. The external pass device must have sufficient voltage rating for the application, and must have the current
and power handling capabilities to charge at the desired rate at the maximum input to output differential in
the application.
2. The device must have enough current gain at the required charging current to keep the drive current below
25mA.
The choice of the pass device and the configuration of the internal driver transistor have an effect on the
following:
1. The minimum and maximum practical charging current.
2. The open-loop gains of the current and voltage loops, and hence the value of the compensation capacitor at
the COMP pin. In battery charging applications, dynamic response is not a requirement, and the values of
C_COMP given below should give stable operation under all conditions.
3. The IC's power dissipation and thus its self-heating. The IC typically has a thermal resistance of 100°C/W.
An external resistance R_P can be added to share some of the power dissipation and reduce the IC's
self-heating.
4. The minimum differential voltage ΔV (from the input to the battery) required to operate.
The next section addresses a few topologies, and gives values for the charge current range, the minimum input
to output differential ΔV, power dissipation P_D in the IC, R_P and C_COMP for each of the topologies. (In the
expressions below, h_FE is the current gain of the external transistor).
Common-Emitter PNP
I_MAX-CHG range:
Minimum ΔV:
25mA to 1000mA
0.5V
R_P = (V_IN(MIN) – 2.0V) ÷ I_MAX-CHG × h_FE(MIN)
P_D = (V_IN(MAX) – 0.7V) ÷ h_FE × I_MAX-CHG – (I_MAX-CHG)² ÷ (h_FE)² × R_P
C_COMP = 0.1 µF
V_IN
Q_EXT
DRVC
16
V_OUT
Q1
DRVE
15
R_P
PNP in a Quasi-Darlington With Internal Driver
I_MAX-CHG range:
Minimum ΔV:
25mA to 1000mA
2V
R_P = (V_IN(MIN) – V_OUT(MAX) – 1.2 V) ÷ I_MAX-CHG × h_FE(MIN)
P_D = (V_IN(MAX) – V_OUT – 0.7V) ÷ h_FE × I_MAX-CHG – (I_MAX-CHG)² ÷ (h_FE)² × R_P
C_COMP = 0.01µF to 0.047µF
V_IN
Q_EXT
V_OUT
R_P
DRVC
16
Q1
15
DRVE
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s) :bq24450
Submit Documentation Feedback
13