BQ2085 Datasheet, PDF (7/56 Pages) Texas Instruments – SBS-COMPLIANT GAS GAUGE IC FOR USE WITH THE bq29311

SMBus TIMING DIAGRAMS
SCLK
TLOW
TR
bq2085
SLUS541 – OCTOBER 2002
TF
THD:STA
SDATA
P
THD:STA
THD:DAT
THIGH
TSU:STA
TSU:DAT
TSU:STO
TBUF
S
S
P
Start
TLOW:SEXT
SCLKACK^†
SCLKACK^†
TLOW:MEXT
TLOW:MEXT
Stop
TLOW:MEXT
SCLK
SDATA
† _{SCLKACK is the acknowledge-related clock pulse generated by the master.}
Figure 2. SMBus Timing Diagram
FUNCTIONAL DESCRIPTION
Internal Oscillator Function
The internal oscillator performance is additionally dependent on the tolerance of the 113k resistor connected
between RSOC (pin 33) and VSSA (pin 34). It is recommended that this resistor be as close to the bq2085 as
possible and that it have a specification of ±0.1% tolerance and ±50 ppm temperature drift or better. The layout
of the PCBA is also an additional contributing factor to performance degradation.
The average temperature drift error of the oscillator function over a learning charge or discharge cycle
introduces an equal capacity prediction error in a learned full charge capacity (FCC).
General Operation
The bq2085 determines battery capacity by monitoring the amount of charge input or removed from a
rechargeable battery. In addition to measuring charge and discharge, the bq2085 measures battery voltage,
temperature, and current, estimates battery self-discharge, and monitors the battery for low-voltage thresholds.
The bq2085 measures charge and discharge activity by monitoring the voltage across a small-value series
sense resistor between the battery’s negative terminal and the negative terminal of the battery pack. The
available battery charge is determined by monitoring this voltage and correcting the measurement for
environmental and operating conditions.
The bq2085 interfaces with the bq29311 to perform battery protection, cell balancing, and voltage translation
functions.
The bq2085 can accept any NTC thermistor (default is Semitec 103AT) for temperature measurement or can
be configured to use its internal temperature sensor. The bq2085 uses temperature to monitor the battery pack
and to compensate the self-discharge estimate.
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