BQ28400PWR Datasheet, PDF (14/24 Pages) Texas Instruments – Tablet PC and Netbook 2-Series Cell Li-Ion Battery Gas Gauge and Protection

bq28400
SLUSA61A – OCTOBER 2010 – REVISED DECEMBER 2010
www.ti.com
or SMBus activity detected.
• In Shutdown Mode, the bq28400 is completely disabled by turning off all FETs and powering down the
bq28400.
CONFIGURATION
Oscillator Function
The bq28400 fully integrates the system oscillator; therefore, no external components are required for this
feature.
System Present Operation
The device checks the PRES pin periodically. If the PRES pin input is pulled to ground by the external system,
the bq28400 detects this event as the presence of the system.
2-Series Cell Configuration
The bq28400 supports 2-series cell battery pack configurations.
Cell Balancing Configuration
If cell balancing is required, the bq28400 cell balance control enables a weak, internal pull-down for each VCx
pin. The purpose of this weak pull-down is to enable an external FET for current bypass. Series resistors placed
between the input VCx pins and the positive battery cell terminals control the VGS of the external FET.
Alternatively, CB_EN output can be used with the bq29200 device to control the auto cell-balancing feature for
the system (see Figure 5). Further details are provided in the APPLICATION INFORMATION section of this
document.
BATTERY PARAMETER MEASUREMENTS
The bq28400 uses an integrating delta-sigma analog-to-digital converter (ADC) for current measurement, and a
second delta-sigma ADC for individual cell voltage, battery voltage, and temperature measurements. The
individual cell voltages, Voltage, Current, AverageCurrent, and Temperature are updated in 1-second intervals
during normal operation.
Charge and Discharge Counting
The integrating ADC measures the charge and discharge flow of the battery by monitoring a small-value sense
resistor between the SRP and SRN pins. The bq28400 integrating ADC measures bipolar signals across the
SRP and SRN pins from –0.20 V to 0.25 V induced by current through the sense resistor (typically 5 mΩ to 20
mΩ). Charge activity is detected when V_SR = V_SRP – V_SRN is positive and discharge activity when V_SR = V_SRP –
V_SRN is negative. The bq28400 continuously integrates the signal over time, using an internal counter and
updates RemainingCapacity with the charge or discharge amount every second.
Voltage
While monitoring the SRP and SRN pins for charge and discharge currents, the bq28400 monitors the individual
series cell voltages. The internal bq28400 ADC then measures the voltage, scales, applies offsets, and calibrates
it appropriately.
NOTE
For accurate differential voltage sensing, the VSS ground should be connected directly to
the most negative terminal of the battery stack, not to the positive side of the sense
resistor. This minimizes the voltage drop across the PCB trace.
Voltage Calibration and Accuracy
The bq28400 is calibrated for voltage prior to shipping from TI. The bq28400 voltage measurement signal chain
(ADC, high voltage translation, circuit interconnect) is calibrated for each cell. The external filter resistors,
connected from each cell to the VCx input of the bq28400, are required to be 1 kΩ. If different voltage accuracy
is desired, customer voltage calibration is required.
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