BQ2085 Datasheet, PDF (8/56 Pages) Texas Instruments – SBS-COMPLIANT GAS GAUGE IC FOR USE WITH THE bq29311

bq2085
SLUS541 – OCTOBER 2002
Measurements
The bq2085 uses an integrating sigma-delta analog-to-digital converter (ADC) for current measurement and
a second sigma delta ADC for battery voltage and temperature measurement. Voltage, current, and
temperature measurements are made every second.
Charge and Discharge Counting
The integrating ADC measures the charge and discharge flow of the battery by monitoring a small-value sense
resistor between the SR1 and SR2 pins as shown in the schematic. The integrating ADC measures bipolar
signals from –0.3 to 1.0 V. The bq2085 detects charge activity when V_SR = V_(SR1)–V_(SR2) is positive and
discharge activity when V_SR = V_(SR1)–V_(SR2) is negative. The bq2085 continuously integrates the signal over
time, using an internal counter. The fundamental rate of the counter is 2.6 nVh. The bq2085 updates Remaining
Capacity() with the charge or discharge accumulated in this internal counter once every second.
Offset Calibration
The bq2085 provides an autocalibration feature to cancel the voltage offset error across SR₁ and SR₂ for
maximum charge measurement accuracy. The bq2085 performs autocalibration when the SMBus lines stay low
for a minimum of 20 s. The bq2085 is capable of automatic offset calibration down to 1µV.
Digital Filter
The bq2085 does not measure charge or discharge counts below the digital filter threshold. The digital filter
threshold is programmed in the Digital Filter DF 0x2b and should be set sufficiently high to prevent false signal
detection with no charge or discharge flowing through the sense resistor.
Voltage
While monitoring SR1 and SR2 for charge and discharge currents, the bq2085 monitors the individual series
cell voltages through the bq29311. The bq2085 configures the bq29311 to present the selected cell to the
VCELL pin of the bq29311 which should be connected to VIN of the bq2085. The internal ADC of the bq2085
then measures the voltage and scales it appropriately. The bq2085 then reports the Voltage( ) and the individual
cell voltages in VCELL1, VCELL2, VCELL3, and VCELL4 located in 0x3c–0x3f.
Current
The bq2085 uses the SR1 and SR2 inputs to measure and calculate the battery charge and discharge current
as represented in the data register Current( ).
Temperature
The TS input of the bq2085 in conjunction with an NTC thermistor measures the battery temperature as shown
in the schematic. The bq2085 reports temperature in Temperature( ).
The bq2085 can also be configured to use its internal temperature sensor by setting the IT bit in Misc
Configuration DF 0x2a. Data flash locations DF 0xa4 through DF 0xad also have to be changed to prescribed
values if the internal temperature sensor option is selected.
Gas Gauge Operation
Table 1. Data Flash Settings for Internal or External Temperature Sensor
LABEL
LOCATION
Dec (Hex)
INTERNAL TEMP SENSOR SETTING
Dec (Hex)
EXTERNAL TEMP SENSOR SETTING
Dec (Hex)
Misc. Configuration
42 (0x2a)
Bit 7 = 1
Bit 7 = 0
TS Const1
164/5 (0xa4/5)
0 (a3)
37251 (0x9183)
TS Const2
166/7 (0xa6/7)
0 (a2)
20848 (0x5170)
TS Const3
168/9 (0xa8/9)
–11136 (a1)
57999 (0xe28f)
TS Const4
170/1 (0xaa/b)
5754 (a0)
4012 (0x0fac)
TS Const5
172/3 (0xac/d)
13348 (3424)
5190 (0x1446)
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