BQ27541 Datasheet, PDF (25/38 Pages) Texas Instruments – Single Cell Li-Ion Battery Fuel Gauge for Battery Pack Integration

bq27541
www.ti.com ........................................................................................................................................................................................... SLUS861 – DECEMBER 2008
High Byte
Low Byte
bit7
RESCAP
–
Table 10. Pack Configuration Bit Definition
bit6
bit5
bit4
bit3
bit2
–
–
–
–
IWAKE
–
SLEEP
RMFCC
SE_PU
SE_POL
bit1
RSNS1
–
bit0
RSNS0
TEMPS
RESCAP = No-load rate of compensation is applied to the reserve capacity calculation. True when set. Default is 0.
IWAKE/RSNS1/RSNS0 = These bits configure the current wake function (see Table 11). Default is 0/0/1.
SLEEP = The fuel gauge can enter sleep, if operating conditions allow. True when set. Default is 1.
RMFCC = RM is updated with the value from FCC, on valid charge termination. True when set. Default is 1.
SE_PU = Pull-up enable for SE pin. True when set (push-pull). Default is 0.
SE_POL = Polarity bit for SE pin. SE is active low when clear. Default is 1 (makes SE high when gauge is ready for
shutdown).
TEMPS = Selects external thermistor for Temperature( ) measurements. True when set. Default is 1.
TEMPERATURE MEASUREMENT AND THE TS INPUT
The bq27541 measures battery temperature via the TS input, in order to supply battery temperature status
information to the fuel gauging algorithm and charger-control sections of the gauge. Alternatively, the gauge can
also measure internal temperature via its on-chip temperature sensor, but only if the [TEMPS] bit of Pack
Configuration register is cleared.
Regardless of which sensor is used for measurement, a system processor can request the current battery
temperature by calling the Temperature( ) function (see Section Standard Data Commands, for specific
information).
The thermistor circuit requires the use of an external 10Kohm thermistor with negative temperature coefficients,
such as Semetic 103AT-type thermistor that connects between the Vcc and TS pins. Additional circuit
information for connecting this thermistor to the bq27541 is shown in Section 4, Reference Schematic.
OVERTEMPERATURE INDICATION
2.3.1 Overtemperature: Charge
If during charging, Temperature( ) reaches the threshold of OT Chg for a period of OT Chg Time and
AverageCurrent( ) > Chg Current Threshold, then the [OTC] bit of Flags( ) is set. When Temperature( ) falls to
OT Chg Recovery, the [OTC] of Flags( ) is reset.
If OT Chg Time = 0, the feature is completely disabled.
Overtemperature: Discharge
If during discharging, Temperature( ) reaches the threshold of OT Dsg for a period of OT Dsg Time, and
AverageCurrent( ) ≤ -Dsg Current Threshold, then the [OTD] bit of Flags( ) is set. When Temperature( ) falls to
OT Dsg Recovery, the [OTD] bit of Flags( ) is reset.
If OT Dsg Time = 0, the feature is completely disabled.
CHARGE-TERMINATION/-INHIBIT INDICATORS
Detection Charge Termination
For proper bq27541 operation, the cell charging voltage must be specified by the user. The default value for this
variable is in the data flash Charging Voltage.
The bq27541 detects charge termination when (1) during 2 consecutive periods of Current Taper Window, the
AverageCurrent( ) is < Taper Current, (2) during the same periods, the accumulated change in capacity >
0.25mAh / Current Taper Window, and (3) Voltage( ) > Charging Voltage – Taper Voltage. When this occurs,
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