BQ27000 Datasheet, PDF (9/29 Pages) Texas Instruments – SINGLE CELL Li-Ion AND Li-Pol BATTERY GAS GAUGE IC FOR PORTABLE APPLICATIONS (bqJUNIOR)

bq27000, bq27200
www.ti.com
SLUS556B – SEPTEMBER 2004 – REVISED NOVEMBER 2004
FUNCTIONAL DESCRIPTION (continued)
Offset Calibration
The offset voltage of the DSCC measurement must be very low to be able to measure small signal levels
accurately. The bqJUNIOR provides an auto-calibration feature to cancel the internal voltage offset error across
SRP and SRN for maximum charge measurement accuracy. NO CALIBRATION IS REQUIRED. External
voltage offset error caused by the PCB layout cannot be automatically calibrated out by the gauge, but the
external offset can be determined using a built-in user offset measurement command and can be programmed
into the EEPROM for inclusion in the offset compensation performed by the gauge. See the Layout
Considerations section for details on minimizing PCB induced offset across the SRP and SRN pins.
The bqJUNIOR auto-calibration of the DSCC offset is performed from time-to-time as operating conditions
change, to keep the measurement error small. A Calibration-In-Progress (CALIP) flag is set in FLAGS to indicate
when the operation occurs. Capacity, voltage, and temperature is updated during the 5.12 second offset
calibration time, but other parameters are not updated until the calibration has completed. When there is a full
reset, the gauge makes an initial quick offset calibration and delays the 5.12 second full offset calibration for at
least 40 seconds. This is done to prevent the full 5.12 second calibration operation from interfering with module
test functions that need to be performed immediately after power application during manufacturing test. The
quick offset calibration after a full reset is a 1.28 second offset measurment used as a delay, followed by a 1.28
second offset measurement that is used as the initial offset value. The 1.28 second delay allows V_CC to settle
before the initial offset measurement. If manufacturing test does not need the additional V_CC settling time or can
use a slightly worse initial offset measurement, the tester may write bit 0 of CTRL (address 0x00) to 1 during the
first 1.28 seconds after the reset and the first offset sample is used, cutting the initial quick offset calibration time
in half.
Digital Magnitude Filter
The Digital Magnitude Filter (DMF) threshold can be set in EEPROM to indicate a threshold below which any
charge or discharge accumulation is ignored. This allows setting a threshold above the maximum DSCC offset
expected from the IC and PCB combination, so that when no charge or discharge current is present, the
measured capacity change by the bqJUNIOR is zero. Note that even a small offset can add up to a large error
over a long period. In addition to setting the threshold above the largest offset expected, the DMF should be set
below the minimum signal level to be measured. The sense resistor value should be large enough to allow the
minimum current level to provide a signal level substantially higher than the maximum offset voltage. Conversely,
the sense resistor must be small enough to meet the system requirement for insertion loss as well as keep the
maximum voltage across the sense resistor below the 100 mV maximum that the DSCC can accurately measure.
The DMF threshold is programmed in EEPROM in increments of 4.9 µV. Programming a zero in the DMF value
will disable the DMF function and all non-zero DSCC measurements are counted.
Voltage
The bqJUNIOR monitors the battery voltage through the BAT pin and reports an offset corrected value through
the internal registers. The bqJUNIOR also monitors the voltage for the end-of-discharge (EDV) thresholds. The
EDV threshold levels are used to determine when the battery has been discharged to 6.25% or empty and
synchronizes the reported capacity to these levels when the programmed EDV thresholds are detected.
Temperature
The bqJUNIOR uses an integrated temperature sensor to monitor the pack temperature and is reported through
the internal registers. The temperature measurement is used to adjust compensated available capacity and
self-discharge capacity loss.
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