BQ27500-V120 Datasheet, PDF (8/36 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge

bq27500-V120
System-Side Impedance Track™ Fuel Gauge
SLUS880 – OCTOBER 2008
4 GENERAL DESCRIPTION
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The bq27500 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 state-of-charge (SOC), time-to-empty (TTE) and time-to-full (TTF).
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 bq27500 control and
status registers, as well as its data flash locations. Commands are sent from system to gauge using the
bq27500 I²C serial communications engine, and can be executed during application development, pack
manufacture, or end-equipment operation.
Cell information is stored in the bq27500 in non-volatile flash memory. Many of these data flash locations
are accessible during application development. They cannot be accessed directly during end-equipment
operation. Access to these locations is achieved by either use of the bq27500 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 bq27500 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. This
data space is accessed through a data flash interface. For specifics on accessing the data flash, see
Section 4.3, Manufacturer Information Blocks.
The key to the high-accuracy fuel gauging prediction of the bq27500 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 bq27500 measures charge/discharge activity by monitoring the voltage across a small-value series
sense resistor (5 mΩ to 20 mΩ, typ.) located between the system Vss and the battery PACK– terminal.
When a cell is attached to the bq27500, cell impedance is computed, based on cell current, cell
open-circuit voltage (OCV), and cell voltage under loading conditions.
The bq27500 external temperature sensing is optimized with the use of a high accuracy negative
temperature coefficient (NTC) thermistor with R₂₅ = 10.0K ± 1% and B25/85 = 3435K ± 1% (such as
Semitec NTC 103AT). The bq27500 can also be configured to use its internal temperature sensor. When
an external themistor is used, a 18.2k pull up resistor between BT/TOUT and TS pins is also required. The
bq27500 uses temperature to monitor the battery-pack environment, which is used for fuel gauging and
cell protection functionality.
To minimize power consumption, the bq27500 has different power modes: NORMAL, SLEEP+, SLEEP,
HIBERNATE, and BAT INSERT CHECK. The bq27500 passes automatically between these modes,
depending upon the occurrence of specific events, though a system processor can initiate some of these
modes directly. More details can be found in Section 5.7, Power Modes.
NOTE
FORMATTING CONVENTIONS IN THIS DOCUMENT:
Commands: italics with parentheses and no breaking spaces, e.g., RemainingCapacity( ).
Data flash: italics, bold, and breaking spaces, e.g., Design Capacity
Register bits and flags: brackets and italics, e.g., [TDA]
Data flash bits: brackets, italics and bold, e.g., [LED1]
Modes and states: ALL CAPITALS, e.g., UNSEALED mode.
8
GENERAL DESCRIPTION
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