BQ28550-R1 Datasheet, PDF (10/42 Pages) Texas Instruments – Single Cell Li-Ion Battery Gas Gauge

Not Recommended for New Designs
bq28550-R1
SLUSAS4A – OCTOBER 2012 – REVISED SEPTEMBER 2014
7 Detailed Description
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7.1 Overview
The bq28550-R1 gas gauge accurately predicts the battery capacity and other operational characteristics of a
single Li-Ion based rechargeable cell, while it also provides a state-of-the-art protection function against short
circuit, overcurrent, and overvoltage. It can be integrated by a system processor to provide cell information, such
as state-of-charge (SOC), Remaining Capacity, and Full Charge Capacity (FCC).
NOTE
Formatting conventions in this document:
Commands: Italics with parentheses and no breaking spaces; for example,
RemainingCapacity()
Data Flash: Italics, bold, and breaking spaces; for example, Design Capacity
Register Bits and Flags: Brackets only; for example, [TDA]
Data Flash Bits: Italic and bold; for example, [NR]
Modes and States: All capitals; for example, SEALED mode
7.2 Data Acquisition
7.2.1 Cell Voltage
The bq28550-R1 gas gauge samples the single cell voltage from the BAT input terminal. The cell voltage is
sampled and updated every 1 s in normal mode. The VSS ground connection of the bq28550-R1 gas gauge
should be connected to the negative terminal of the sense resistor. This will prevent any error in short circuit and
overcurrent measurements across the external CHG and DSG FETs.
7.2.2 Charge Measurement
The device samples the charge into and out of the single cell using a low value sense resistor. The resistor
(typically 5 mΩ to 20 mΩ) is connected between SRP and SRN to form a differential input to an integrating ADC
(Coulomb counter). Charge activity is detected when V_SR = V_(SRP) – V_(SRN) is positive, and discharge activity is
detected when V_SR = V_(SRP) – V_(SRN) is negative. This data is integrated over a period of time, using an internal
counter, and updates Remaining Capacity with charge and discharge amount every 1 s in normal mode.
7.2.3 Current Measurement
The device has a FIFO buffer, which uses the last four Coulomb counter readings to calculate the current. The
current is updated every 1 s in normal mode.
7.2.4 Temperature Measurement and the TS Input
The bq28550-R1 gas gauge measures external temperature via the TS pin in order to supply battery temperature
status information to the gas gauging algorithm and charger-control sections of the gauge. Alternatively, the
gauge can also measure internal temperature via its on-chip temperature sensor. Refer to the Pack
Configuration[TEMPS] control bit.
Regardless of which sensor is used for measurement, a system processor can request the current battery
temperature by calling the Temperature() function (see Data Commands for more information). The temperature
information is updated every 1 s in normal mode.
The bq28550-R1 external temperature sensing is optimized with the use of a high accuracy negative temperature
coefficient (NTC) thermistor with R25 = 10 KΩ ± 1% and B25/85 = 3435 KΩ ± 1% (such as Semitec 103AT for
measurement). The shows additional circuit information for connecting this thermistor to the bq28550-R1 gas
gauge.
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