Datasheet

V
REF
250mV
25mV
+
+
0.90V
REF
0.95V
REF
V
REF
Voltage
Reference
V
IN
V
REF
V
VFB
ChargingState Logic
1
2
3
4
5
6
8
9
10
11
12
13
14
15
16
7
Q1
Q2
Q3
Q4
Q5
Q
EXT
External
Supply
R
ISNS
VRLA
Battery
R
A
R
B
R
C
Q6
Q7
Q8
Q9
COMP
VFB
STAT1
BSTOP
IN
DRVE
DRVC
IFBISNSM
ISNSP
ISNS
GND
R
D
CE
PRE-CHG
R
T
I
CHG
V
BAT
bq24450
SLUS929C –APRIL 2009– REVISED FEBRUARY 2012 www.ti.com
An Improved Dual-Level Float-Cum-Boost Charger with Pre-Charge
The problem with the charger circuit shown in Figure 4 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 6, 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. In the following equation, V
PRE
is the voltage drop across the internal transistor, Q5, and the internal diode.
I
PRE
= (V
IN
– V
PRE
– V
BAT
) ÷ R
T
Once the battery voltage rises above a safe threshold V
TH
at (2) in Figure 7, 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
× (R
A
+ R
B
+ R
C
//R
D
) ÷ (R
B
+ R
C
//R
D
)
Figure 6.
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