bq20z80-V101
www.ti.com
SLUS625D–SEPTEMBER 2004–REVISED OCTOBER 2005
FUNCTIONAL DESCRIPTION
Within the functional description of the bq20z80 System Management Bus (SMBus), commands are prefixed with
SBS. and data flash variables are prefixed with DF: and are both italicized in full. For example:
SBS.RemainingCapacity( ) and DF:Fast Charge Current.
OSCILLATOR FUNCTION
The oscillator of the bq20z80 can be set up for internal or external operation. On power up, the bq20z80
automatically attempts to start the internal oscillator. If a 100-kΩ resistor is not connected to ROSC (pin 33), then
it attempts to start the oscillator using an external 32.768-kHz crystal.
NOTE:
Install either the 100-kΩ ROSC resistor or the 12-pF, 32.768-kHz crystal. Do not
install both.
The performance of the internal oscillator depends on the tolerance of the 100-kΩ resistor between RSOC (pin
33) and VSSA (pin 34). Choose a resistor with a tolerance of ±0.1%, and 50-ppm or better temperature drift.
Place this resistor as close as possible to the bq20z80. If a 12-pF crystal is used, place it as close as possible to
the XCK1 (pin 34) and XCK2 (pin 33) pins. If not properly implemented, the PCB layout in this area can degrade
oscillator performance.
SYSTEM PRESENT OPERATION
The bq20z80 periodically (1 s) pulls the PU output high. Connect this pin to the PRES pin of the bq20z80 via a
resistor of approximately 5 kΩ. The bq20z80 measures the PRES input during the PU-active period to determine
its state.
The bq20z80 detects that the battery is present in the system via a low state on the PRES input. When this
occurs, the bq20z80 enters normal operating mode and sets the PRES bit in SBS.OperationStatus( ). When the
pack is removed from the system and the PRES input is high, the bq20z80 enters the battery-removed state,
disabling the charge and discharge FETs, and enabling the 0-V/precharging FET. If DF:Operation Cfg B [NR] is
set, the PRES input is ignored and can be left floating.
NAME
CLASS / SUBCLASS
FORMAT
VALID RANGE SIZE (BYTES)
0x0000 to 0xffff
UNITS
DEFAULT VALUE
Operation Cfg B
Configuration / Registers (64)
Hex
2
N/A
0x6140
GENERAL OPERATION
The bq20z80 determines battery capacity by monitoring individual cell voltages and the amount of charge input
or removed from a rechargeable battery. In addition, the bq20z80 measures individual cell voltages, pack
voltage, temperature, and current using features of the bq29312A analog front end (AFE) device and calculates
individual cell impedances using collected data.
The bq20z80 measures charge/discharge activity by monitoring the voltage across a small-value series sense
resistor (5 mΩ to 20 mΩ typ.) between the cell stack negative terminal and the negative terminal of the battery
pack. When an applications load is applied, impedance of each cell is measured by comparing its Open Circuit
Voltage (OCV) obtained for the present state of charge using a predefined OCV(SOC) function with measured
voltage under load.
The bq20z80 interfaces with the bq29312A to perform battery protection, cell balancing, and voltage translation
functions. The bq20z80 can accept inputs of up to two identical NTC thermistors (default is Semitec 103AT) for
temperature measurement, or can also be configured to use its internal temperature sensor. The bq20z80 uses
temperature to monitor the battery-pack environment.
BATTERY PARAMETER MEASUREMENTS
The bq20z80 uses an integrating delta-sigma analog-to-digital converter (ADC) for current measurement, and a
second delta-sigma ADC for individual cell and battery voltage, and temperature measurement. The individual
cell and pack voltages, SBS.CellVoltagex( ), SBS.Voltage( ), SBS.Current( ), SBS.AverageCurrent( ), and
SBS.Temperature( ) are updated at 1-second intervals during normal operation.
7
TI [ TEXAS INSTRUMENTS ]
