BQ27520-G4 Datasheet, PDF (10/34 Pages) Texas Instruments – System-Side Impedance Track Fuel Gauge

bq27520-G4
SLUSB20B – NOVEMBER 2012 – REVISED DECEMBER 2015
8 Detailed Description
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8.1 Overview
The bq27520-G4 fuel gauge accurately predicts the battery capacity and other operational characteristics of a
single Li-based rechargeable cell. It can be interrogated by a system processor to provide cell information, such
as time-to-empty (TTE), state-of-charge (SOC), and SOC interrupt signal to the host.
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 device control and status registers, as
well as its data flash locations. Commands are sent from system to gauge using the I²C serial communications
engine, and can be executed during application development, system manufacture, or end-equipment operation.
Cell information is stored in the device in non-volatile 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 either use of the 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 key to the high-accuracy gas gauging prediction is Texas Instruments proprietary Impedance Track™
algorithm. This algorithm uses cell measurements, characteristics, and properties to create state-of-charge
predictions that can achieve less than 1% error across a wide variety of operating conditions and over the
lifetime of the battery.
The fuel gauge measures charge and discharge activity by monitoring the voltage across a small-value series
sense resistor (5 mΩ to 20 mΩ, typical) located between the system V_SS and the battery PACK– terminal. When
a cell is attached to the device, cell impedance is learned based on cell current, cell open-circuit voltage (OCV),
and cell voltage under loading conditions.
The external temperature sensing is optimized with the use of a high-accuracy negative temperature coefficient
(NTC) thermistor with R25 = 10.0 kΩ ±1%. B25/85 = 3435K ± 1% (such as Semitec NTC 103AT). Alternatively,
the fuel gauge can also be configured to use its internal temperature sensor or receive temperature data from the
host processor. When an external thermistor is used, a 18.2-kΩ pullup resistor between the BI/TOUT and TS
pins is also required. The fuel gauge uses temperature to monitor the battery-pack environment, which is used
for fuel gauging and cell protection functionality.
To minimize power consumption, the device has different power modes: NORMAL, SNOOZE, SLEEP,
HIBERNATE, and BAT INSERT CHECK. The fuel gauge automatically changes modes depending upon the
occurrence of specific events, though a system processor can initiate some of these modes directly.
For complete operational details, see bq27520-G4 Technical Reference Manual, SLUUA35.
NOTE
The following formatting conventions are used in this document:
Commands: italics with parentheses( ) and no breaking spaces, for example, Control( ).
Data Flash: italics, bold, and breaking spaces, for example, Design Capacity.
Register bits and flags: italics with brackets [ ], for example, [TDA]
Data flash bits: italics, bold, and brackets [ ], for example, [LED1]
Modes and states: ALL CAPITALS, for example, UNSEALED mode.
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