Datasheet

The resistor values are calculated from the following:
Equation 1
RB1
RB2
NV
V
FLT
=
∗
−
()
.22
1
Equation 2
RB1
RB2
RB1
RB3
(
N
)
BLK
+=
∗
−
V
22
1
.
Equation 3
I
V
R
MAX
SNS
=
0 250.
where:
n
N = Number of cells
n
V
FLT
= Desired float voltage
n
V
BLK
= Desired bulk charging voltage
n
I
MAX
= Desired maximum charge current
These parameters are typically specified by the battery
manufacturer. The total resistance presented across the
battery pack by RB1 + RB2 should be between 150k
Ω
and 1M
Ω
. The minimum value ensures that the divider
network does not drain the battery excessively when the
power source is disconnected. Exceeding the maximum
value increases the noise susceptibility of the BAT pin.
An empirical procedure for setting the values in the re-
sistor network is as follows:
1. Set RB2 to 49.9 k
Ω
. (for 3 to 18 series cells)
2. Determine RB1 from equation 1 given V
FLT
3. Determine RB3 from equation 2 given V
BLK
4. Calculate R
SNS
from equation 3 given I
MAX
Battery Insertion and Removal
The bq2031 uses V
BAT
to detect the presence or absence
of a battery. The bq2031 determines that a battery is
present when V
BAT
is between the High-Voltage Cutoff
(V
HCO
= 0.6
*
V
CC
) and the Low-Voltage Cutoff (V
LCO
=
0.8V). When V
BAT
is outside this range, the bq2031 de
-
termines that no battery is present and transitions to
the Fault state. Transitions into and out of the range
between V
LCO
and V
HCO
are treated as battery inser
-
tions and removals, respectively. Besides being used to
detect battery insertion, the V
HCO
limit implicitly serves
as an over-voltage charge termination, because exceed
-
ing this limit causes the bq2031 to believe that the bat
-
tery has been removed.
The user must include a pull-up resistor from the posi
-
tive terminal of the battery stack to VDC (and a diode to
prevent battery discharge through the power supply
when the supply is turned off) in order to detect battery
removal during periods of voltage regulation. Voltage
regulation occurs in pre-charge qualification test 1 prior
to all of the fast charge algorithms, and in phase 2 of the
Two-Step Voltage fast charge algorithm.
Temperature Monitoring
The bq2031 monitors temperature by examining the
voltage presented between the TS and SNS pins (V
TEMP
)
by a resistor network that includes a Negative Tempera
-
ture Coefficient (NTC) thermistor. Resistance variations
around that value are interpreted as being proportional
to the battery temperature (see Figure 7).
The temperature thresholds used by the bq2031 and
their corresponding TS pin voltage are:
n
TCO—Temperature cutoff—Higher limit of the tem
-
perature range in which charging is allowed. V
TCO
=
0.4
*
V
CC
n
HTF—High-temperature fault—Threshold to
which temperature must drop after temperature
cutoff is exceeded before charging can begin again.
V
HTF
= 0.44
*
V
CC
7
bq2031
FG203104.eps
V
CC
V
LTF
= 0.6V
V
HTF
= 0.44V
V
TCO
= 0.4V
HotterV
SS
TCO
HTF
LTF
Colder
Voltage
Temperature
Figure 7. Voltage Equivalent
of Temperature Thresholds