BQ27000 Datasheet, PDF (8/29 Pages) Texas Instruments – SINGLE CELL Li-Ion AND Li-Pol BATTERY GAS GAUGE IC FOR PORTABLE APPLICATIONS (bqJUNIOR)

bq27000, bq27200
SLUS556B – SEPTEMBER 2004 – REVISED NOVEMBER 2004
www.ti.com
FUNCTIONAL DESCRIPTION (continued)
Figure 3 shows a typical application circuit. Differential sense of the voltage across the current sense resistor,
R_S, improves device performance, leading to an improvement in reported time-to-empty accuracy. An internal, 3
µA pull-down on the HDQ or SDA and SCL lines ensures that the device detects a logic 0 on the communication
lines and allows the device to automatically enter the low-power sleep mode when the system power is switched
off or the pack is removed. A 100 kΩ pullup to V_CC can be added to the communication lines if this feature needs
to be disabled. The bqJUNIOR can operate directly from a single Li-Ion or Li-Pol cell.
PACK+
SCL
SDA
ESD Protection
R5
100 Ω
R6
100 Ω
D1
5.6 V
R7
100 Ω
R3
1 kΩ
1 RBI
PGM TP
PGM 10
C4
0.1 µF
bq27200DRK
2 VCC
GPIO 9
C3
0.1 µF
3 VSS
SRP 8
R8
100 Ω
D2
5.6 V
4 SCL
SRN 7
5 SDA
BAT 6
11
R4
10 kΩ
C3
0.1 µF
R1
1 kΩ
C1
0.1 µF
R2
1 kΩ
C2
0.1 µF
+
Li-Ion
or
Li-Pol
R_S
0.02 Ω
PACK−
ESD Protection
Li-Ion Protector
Figure 3. Typical Application Circuit (bq27000)
UDG−03041
Measurements
As shown in the functional block diagram, the bqJUNIOR uses a dedicated fully differential Delta-Sigma Coulomb
Counter (DSCC) for charge and discharge current and coulometric measurements and an analog-to-digital
converter (ADC) for battery voltage and temperature measurements. Both DSCC and ADC are automatically
compensated for offset. No user calibration or compensation is required. An EEPROM offset value can be
programmed to compensate for contributions to the DSCC offset due to the PCB layout.
Charge and Discharge Coulometric and Current Measurements
The bqJUNIOR uses a DSCC to perform a continuous integration of the voltage waveform across a small value
sense resistor in the negative lead of the battery, as shown in Figure 3. The integration of the voltage across the
sense resistor is the charge added or removed from the battery. Because the DSCC does a direct integration of
the waveform, the shape of the current waveform through the sense resistor does not have any effect on the
coulometric measurement accuracy. The low-pass filter that feeds the sense resistor voltage to the bqJUNIOR
SRP and SRN inputs filters out system noise and does not affect the coulometric measurement accuracy,
because the low-pass filter does not change the integrated value of the waveform. The bqJUNIOR also uses the
DSCC to measure current. The reported current is determined by the average voltage across the sense resistor
over a 5.12 second interval.
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