Datasheet


SLUS491 – JULY 2001
6
www.ti.com
APPLICATION INFORMATION
NC
1
VCC
2
VSS
3
HDQ
4
NC
8
SR1
7
SR2
6
RBI
5
U1
bq26231
V
CC
R6
100 Ω
D2
5.6 V
R5
100 Ω
BAT+
HDQ
C2
0.01 µF
R3
100 kΩ
C1
0.01 µF
R2
100 kΩ
R1
0.02 Ω
BAT–
PACK–
RBI
C3
0.1 µF
C4
0.1 µF
R4
1 MΩ
D1
1N914
Figure 2. Typical Application
functional description
The bq26231 measures the voltage drop across a low-value series current sense resistor between the SR1 and
SR2 pins using a voltage-to-frequency converter. This information is placed into various internal counter and
timer registers. Using information from the bq26231, the system host can determine the battery state-of-charge,
estimate self-discharge, and calculate the average charge and discharge currents. During pack storage
periods, the use of an internal temperature sensor doubles the self-discharge count rate every 10° above 25°C.
A register is available to store the calculated offset, allowing current calibration. The offset cancellation register
is written by the bq26231 during pack assembly and is available to the host system to adjust the current
measurements. By adding or subtracting the offset value stored in the OFR, the true charge and discharge
counts can be calculated to a high degree of certainty.
A typical application diagram is shown in Figure 2 and operation states are shown in Table 1.
Table 1. bq26231 Operational States
HDQ PIN DCR/CCR/SCR WOE OPERATING STATE
HDQ high Yes V
(SRO)
 > V
(WOE)
Normal
HDQ high Yes V
(SRO)
 < V
(WOE)
Normal
HDQ low No V
(SRO)
 < V
(WOE)
Sleep
NOTE: V
(SRO)
is the voltage difference between SR1 and SR2 plus the offset voltage,
V
(OS)
.
RBI input
The RBI input pin is used with a storage capacitor or external supply to provide backup potential to the internal
registers when V
CC
drops below 2.4 V. The maximum discharge current is 100 nA in this mode. The bq26231
outputs V
CC
on RBI when the supply is above 2.4 V; therefore, a diode is required to isolate an external supply.
(See the application diagram.)