Datasheet

1
2
3
4
5
6
8
9
11
10
12
16
7
Q
EXT
D
EXT
External
Supply
R
ISNS
SLA
Battery
R
A
R
B
R
P
R
D
R
C
R
T
ISNSP
C
COMP
ISNSM
IFB
DRVC
PRE-CHG
CE
VFB
STAT1
PGOOD
GND
DRVE
STAT2
ISNS
BSTOP
IN
bq24450
COMP
1514
13
bq24450
www.ti.com SLUS929C –APRIL 2009– REVISED FEBRUARY 2012
Figure 10.
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 (V
REF
) is 2.3V.
R
C
= 2.30V ÷ 50μA = 46kΩ. The closest 1% value is 46.4kΩ.
V
FLOAT
= V
REF
× (R
A
+ R
B
+ R
C
) ÷ R
C
→ R
A
+ R
B
= 2 × R
C
= 92.8kΩ.
V
BOOST
= V
REF
× (R
A
+ R
B
+ R
C
//R
D
) ÷ R
C
//R
D
→ R
D
= 474.3kΩ. Pick the closest 1% value of 475kΩ.
V
TH
= V
REF
× (R
A
+ R
B
+ R
C
//R
D
) ÷ (R
B
+ R
C
//R
D
) → R
B
= 16.9kΩ.
R
A
= 92.8kΩ – R
B
= 75.9kΩ. The closest standard value is 75kΩ.
I
PRE
= (V
IN
– V
PRE
– V
DEXT
– V
BAT
) ÷ R
T
. Select R
T
= 634Ω.
For example: I
PRE
= (13 – 2 – 0.7 – 4) / 634 = 10mA
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
P
= 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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