Datasheet

Fast charge terminates when the charging current drops
below a minimum current threshold programmed by the
value of I
TERM
(see Table 2) and remains below that
level for 120
±
40ms.
Figure 3 shows the bq2054 configured for display mode 2
and I
MIN
=I
MAX
/10.
Voltage and Current Monitoring
The bq2054 monitors battery pack voltage at the BAT
pin. The user must implement a voltage divider be-
tween the positive and negative terminals of the battery
pack to present a scaled battery pack voltage to the BAT
pin. The bq2054 also uses the voltage across a sense re-
sistor (R
SNS
) between the negative terminal of the bat-
tery pack and ground to monitor the current into the
pack. See Figure 4 for the configuration of this network.
The resistor values are calculated from the following:
Equation 1
RB1
RB2
NV
V
REG
=−
*
.205
1
where:
n
N = Number of cells in series
n
V
REG
= Desired fast-charging voltage per cell
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.
The current sense resistor, R
SNS
(see Figure 5), deter
-
mines the fast charge current. The value of R
SNS
is
given by the following:
Equation 2
I
V
R
MAX
SNS
=
0 250.
where:
n
I
MAX
= Desired maximum charge current
Hold-Off Period
Both V
HCO
and I
MIN
terminations are ignored during
the first 1.33
±
0.19 seconds of both the Charge Qualifi-
cation and Fast Charge phases. This condition prevents
premature termination due to voltage spikes that may
occur when charge is first applied.
6
bq2054
Table 2. I
MIN
Termination Thresholds
I
TERM
I
MIN
0I
MAX
/10
1I
MAX
/20
Float I
MAX
/30