BQ28550-R1 Datasheet, PDF (11/42 Pages) Texas Instruments – Single Cell Li-Ion Battery Gas Gauge

Not Recommended for New Designs
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bq28550-R1
SLUSAS4A – OCTOBER 2012 – REVISED SEPTEMBER 2014
7.3 Over-Temperature Indication
7.3.1 Over-Temperature: 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 [OC] bit is set based on the charge fault configuration
setting of the CHG bit in the Control Status Register. When Temperature() falls to OT Chg Recovery, the [OC]
bit is reset.
If OT Chg Time = 0, the feature is completely disabled.
7.3.2 Over-Temperature: 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 [DSGOFFREQ] bit is set. When Temperature() falls to
OT Dsg Recovery, the [DSGOFFREQ] bit is reset.
If OT Dsg Time = 0, the feature is completely disabled.
7.4 Gas Gauging
Gas gauging information is accessed through a series of commands called Standard Commands. Further
capabilities are provided by the additional Extended Commands set. Both sets of commands, indicated by the
general format Command(), are used to read and write information contained within the bq28550-R1 device
control and status registers, as well as their data flash locations. Commands are sent from the system to the
gauge using the bq28550-R1 device’s serial interface and can be executed during application development, pack
manufacture, or end-equipment operation.
Cell information is stored in the bq28550-R1 non-volatile data flash memory. Many of these data flash locations
are accessible during application development. They cannot, generally, be accessed directly during end-
equipment operation. Access to these locations is achieved by using the bq28550-R1 device’s companion
evaluation software, through individual commands, or through a sequence of data-flash-access commands. To
access a desired data flash location, the correct data flash subclass and offset must be known. The bq28550-R1
device provides 96 bytes of user-programmable data flash memory, partitioned into three, 32-byte blocks:
Manufacturer Info Block A, Manufacturer Info Block B, and Manufacturer Info Block C. For specifics on
accessing the data flash, see Manufacturer Information Blocks.
The bq28550-R1 device’s gas gauging prediction uses a Compensated End of Discharge Voltage (CEDV)
method. This algorithm mathematically models the cell voltage as a function of the battery state-of-charge (SOC),
temperature, and current. The algorithm also models the battery impedance (Z) as a function of SOC and
temperature, with other parameters included in the calculation. The battery voltage model is used to calibrate full
charge capacity (FCC), and the compensated battery voltage can be used to indicate low battery voltage or
alarm function through firmware settings (Low Battery %, Fully Discharged).
The bq28550-R1 device measures discharge activity by monitoring the voltage across a small-value series sense
resistor (5 mΩ to 20 mΩ typ) located between the CELL– and the battery’s PACKN terminal. This information is
used to integrate the battery discharge capacity and to estimate state of charge Q. This is then calculated as a
percentage of maximum capacity Qmax to indicate remaining state-of-charge (RSOC). The maximum capacity
parameter is updated on every full discharge cycle. There are other factors to be considered in estimating RSOC,
such as battery impedance, temperature, and aging due to number of charge/discharge cycles. The equations
used to determine the battery capacity factor these variables into the calculation based on battery chemistry.
7.5 Protection
7.5.1 Overcharge Detector
When charging a battery, if the V_BAT voltage becomes greater than the overcharge detection voltage
(V_DET1 = 4.250 V typ) for a period up to the overcharge detection delay time (t_DET1 = 1.00 s typ), the bq28550-R1
device detects the overcharge state of the battery, and the COUT pin transitions to a low level. This prohibits
charging the battery by turning off the external charge control N-channel MOSFET.
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