BQ27210 Datasheet, PDF (10/30 Pages) Texas Instruments – Li-Ion AND Li-Pol BATTERY GAS GAUGE IC FOR PORTABLE APPLICATIONS (bqJUNIOR)

bq27010, bq27210
SLUS707B – APRIL 2006 – REVISED JANUARY 2007
www.ti.com
FUNCTIONAL DESCRIPTION (continued)
Register Initialization and Data Integrity Checks
Checksums of critical internal register data are continually checked during operation of the bqJUNIOR. A failed
checksum comparison will force a hardware reset. After a reset, the bqJUNIOR examines a checksum
containing LMD, CYCL, CYCT, and other critical data. If the checksum is incorrect, NAC, CYCL, and CYCT are
cleared, LMD and other parameters are initialized from EEPROM, the CI bit in FLAGS is set, and the INIT bit in
MODE is set. This process constitutes a full reset. If the checksum is correct, NAC, LMD, CYCL, and CYCT are
retained. In addition, a second checksum containing all user-updateable EEPROM coefficients is checked. If the
second checksum is incorrect, the values are all initialized from the EEPROM, the CI bit in FLAGS is set, and
the INIT bit in MODE is set. The INIT bit is used to inform the host that it may need to reinitialize these values if
the default values are not satisfactory. If the second checksum is correct, the INIT bit in MODE will not be set,
the CI bit in FLAGS will remain unchanged, and the register data that may have been updated by the host will
be retained. This process constitutes a partial reset. It is the host responsibility to clear the INIT bit in MODE
after any updates by the host are completed. See CTRL description for more on host updates. Other register
values not specifically retained are reinitialized on all resets.
GPIO
The GPIO pin can be used as an input or an output. The initial state can be established by programming bit 7 in
the PKCFG EEPROM location. The input/output state can be changed at any time by changing the value in bit 7
of MODE. If unused, the GPIO pin should be tied to V_SS.
Layout Considerations
The auto-calibrating DSCC approach effectively cancels the internal offset voltage within the bqJUNIOR, but any
external offset caused by PCB layout must be programmed in the EEPROM to be cancelled. The magnitude and
variability of the external offset makes it critical to pay special attention to the PCB layout. To obtain optimal
performance, the decoupling capacitor from V_CC to V_SS and the filter capacitors from SRP and SRN to V_SS
should be placed as closely as possible to the bqJUNIOR, with short trace runs to both signal and V_SS pins. All
low-current V_SS connections should be kept separate from the high-current discharge path from the battery and
should tie into the high-current trace at a point directly next to the sense resistor. This should be a trace
connection to the edge or inside of the sense resistor connection, so that no part of the V_SS interconnections
carry any load current and no portion of the high-current PCB trace is included in the effective sense resistor
(i.e. Kelvin connection).
Gas Gauge Operation
Figure 4 illustrates an operational overview of the gas gauge function.
The bqJUNIOR reports uncompensated nominal available capacity (NAC), uncompensated last measured
discharge (LMD), compensated available capacity (CAC), and full charge compensated available capacity
(FCAC). The compensated CAC and FCAC values are reduced from their respective uncompensated NAC and
LMD values by an amount that varies with discharge rate, temperature, and cycle count (age). CAC and FCAC
will equal their respective uncompensated NAC and LMD values at light loads and when charging.
The bqJUNIOR learns the capacity of the battery during actual use conditions and will update LMD when a valid
learning cycle occurs. The bqJUNIOR learns the capacity of the battery by measuring the capacity removed
from the battery when it is discharged from full (NAC = LMD) to the first end-of-discharge (EDV1) voltage
threshold without any learning cycle disqualifying event. The EDV1 threshold should be programmed to a value
determined from battery characterization that affords 6.25% of design capacity (DC) remaining before the battery
reaches zero capacity. The measured discharge capacity plus 6.25% of DC is the measured full capacity during
actual use (compensated) conditions. LMD will be updated with the measured discharge plus 6.25% of DC plus
the capacity compensation value due to discharge rate, temperature, and age. Thus the compensated FCAC
value, with the capacity compensation subtracted from it, will match the measured discharge capacity plus the
6.25% of DC unmeasured discharge capacity. The final end-of-discharge (EDVF) threshold is a fixed value and
should be set at a voltage that the system sees as the zero-capacity battery voltage. The bqJUNIOR does not
learn the capacity between the EDV1 and EDVF thresholds, so the EDV1 threshold should be based on actual
battery characterization. The EDV1 threshold can be compensated with discharge rate and temperature to
maintain the 6.25% remaining capacity with different use profiles.
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