SLUSB85E May   2013  – January 2016


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Comparison Table
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1  Absolute Maximum Ratings
    2. 7.2  ESD Ratings
    3. 7.3  Recommended Operating Conditions
    4. 7.4  Thermal Information
    5. 7.5  Supply Current
    6. 7.6  Digital Input and Output DC Characteristics
    7. 7.7  LDO Regulator, Wake-Up, and Auto-Shutdown DC Characteristics
    8. 7.8  ADC (Temperature and Cell Measurement) Characteristics
    9. 7.9  Integrating ADC (Coulomb Counter) Characteristics
    10. 7.10 Integrated Sense Resistor Characteristics, -40°C to 85°C
    11. 7.11 Integrated Sense Resistor Characteristics, -40°C to 70°C
    12. 7.12 I2C-Compatible Interface Communication Timing Characteristics
    13. 7.13 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
    4. 8.4 Device Functional Modes
    5. 8.5 Programming
      1. 8.5.1 Standard Data Commands
      2. 8.5.2 Control(): 0x00 and 0x01
      3. 8.5.3 Extended Data Commands
      4. 8.5.4 Communications
        1. I2C Interface
        2. I2C Time Out
        3. I2C Command Waiting Time
        4. I2C Clock Stretching
  9. Applications and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. BAT Voltage Sense Input
        2. Integrated LDO Capacitor
        3. Sense Resistor Selection
      3. 9.2.3 Application Curves
  10. 10Power Supply Recommendation
    1. 10.1 Power Supply Decoupling
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • YZF|9
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8 Detailed Description

8.1 Overview

The 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 state-of-charge (SOC).


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

8.2 Functional Block Diagram

bq27421-G1 typ_app_bq27421.gif

8.3 Feature Description

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 control and status registers, as well as its data 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.

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 high-accuracy across a wide variety of operating conditions and over the lifetime of the battery.

The fuel gauge measures the charging and discharging of the battery by monitoring the voltage across a small-value sense resistor. When a cell is attached to the fuel gauge, cell impedance is computed, based on cell current, cell open-circuit voltage (OCV), and cell voltage under loading conditions.

The fuel gauge uses an integrated temperature sensor for estimating cell temperature. Alternatively, the host processor can provide temperature data for the fuel gauge.

The bq27421-G1 Technical Reference Manual (SLUUAC5) provides more details.

8.4 Device Functional Modes

To minimize power consumption, the fuel gauge has several power modes: INITIALIZATION, NORMAL, SLEEP, HIBERNATE, and SHUTDOWN. The fuel gauge passes automatically between these modes, depending upon the occurrence of specific events, though a system processor can initiate some of these modes directly. See the bq27421-G1 Technical Reference Manual (SLUUAC5) for more details.

8.5 Programming

8.5.1 Standard Data Commands

The fuel gauge uses a series of 2-byte standard commands to enable system reading and writing of battery information. Each standard command has an associated command-code pair, as indicated in Table 1. Because each command consists of two bytes of data, two consecutive I2C transmissions must be executed both to initiate the command function, and to read or write the corresponding two bytes of data. See the bq27421-G1 Technical Reference Manual (SLUUAC5) for more details.

Table 1. Standard Commands

Control() CNTL 0x00 and 0x01 NA RW
Temperature() TEMP 0x02 and 0x03 0.1°K RW
Voltage() VOLT 0x04 and 0x05 mV R
Flags() FLAGS 0x06 and 0x07 NA R
NominalAvailableCapacity() 0x08 and 0x09 mAh R
FullAvailableCapacity() 0x0A and 0x0B mAh R
RemainingCapacity() RM 0x0C and 0x0D mAh R
FullChargeCapacity() FCC 0x0E and 0x0F mAh R
AverageCurrent() 0x10 and 0x11 mA R
StandbyCurrent() 0x12 and 0x13 mA R
MaxLoadCurrent() 0x14 and 0x15 mA R
AveragePower() 0x18 and 0x19 mW R
StateOfCharge() SOC 0x1C and 0x1D % R
InternalTemperature() 0x1E and 0x1F 0.1°K R
StateOfHealth() SOH 0x20 and 0x21 num/% R
RemainingCapacityUnfiltered() 0x28 and 0x29 mAh R
RemainingCapacityFiltered() 0x2A and 0x2B mAh R
FullChargeCapacityUnfiltered() 0x2C and 0x2D mAh R
FullChargeCapacityFiltered() 0x2E and 0x2F mAh R
StateOfChargeUnfiltered() 0x30 and 0x31 % R

8.5.2 Control(): 0x00 and 0x01

Issuing a Control() command requires a subsequent 2-byte subcommand. These additional bytes specify the particular control function desired. The Control() command allows the system to control specific features of the fuel gauge during normal operation and additional features when the device is in different access modes, as described in Table 2. See the bq27421-G1 Technical Reference Manual (SLUUAC5) for more details.

Table 2. Control() Subcommands

CONTROL_STATUS 0x0000 Yes Reports the status of device
DEVICE_TYPE 0x0001 Yes Reports the device type (0x0421)
FW_VERSION 0x0002 Yes Reports the firmware version of the device
DM_CODE 0x0004 Yes Reports the Data Memory Code number stored in NVM
PREV_MACWRITE 0x0007 Yes Returns previous MAC command code
CHEM_ID 0x0008 Yes Reports the chemical identifier of the battery profile used by the fuel gauge
BAT_INSERT 0x000C Yes Forces the Flags() [BAT_DET] bit set when the OpConfig [BIE] bit is 0
BAT_REMOVE 0x000D Yes Forces the Flags() [BAT_DET] bit clear when the OpConfig [BIE] bit is 0
SET_CFGUPDATE 0x0013 No Force CONTROL_STATUS [CFGUPMODE] to 1 and gauge enters CONFIG UPDATE mode
SHUTDOWN_ENABLE 0x001B No Enables device SHUTDOWN mode
SHUTDOWN 0x001C No Commands the device to enter SHUTDOWN mode
SEALED 0x0020 No Places the device in SEALED ACCESS mode
TOGGLE_GPOUT 0x0023 Yes Commands the device to toggle the GPOUT pin for 1 ms
RESET 0x0041 No Performs a full device reset
SOFT_RESET 0x0042 No Gauge exits CONFIG UPDATE mode
EXIT_CFGUPDATE 0x0043 No Exits CONFIG UPDATE mode without an OCV measurement and without resimulating to update StateOfCharge()
EXIT_RESIM 0x0044 No Exits CONFIG UPDATE mode without an OCV measurement and resimulates with the updated configuration data to update StateOfCharge()

8.5.3 Extended Data Commands

Extended data commands offer additional functionality beyond the standard set of commands. They are used in the same manner; however, unlike standard commands, extended commands are not limited to 2-byte words. The number of command bytes for a given extended command ranges in size from single to multiple bytes, as specified in Table 3.

Table 3. Extended Commands

Name Command Code Unit SEALED
Access(1) (2)
Access(1) (2)
OpConfig() 0x3A and 0x3B NA R R
DesignCapacity() 0x3C and 0x3D mAh R R
DataClass() (2) 0x3E NA NA RW
DataBlock() (2) 0x3F NA RW RW
BlockData() 0x40 through 0x5F NA R RW
BlockDataCheckSum() 0x60 NA RW RW
BlockDataControl() 0x61 NA NA RW
Reserved 0x62 through 0x7F NA R R
(1) SEALED and UNSEALED states are entered via commands to Control() 0x00 and 0x01
(2) In SEALED mode, data cannot be accessed through commands 0x3E and 0x3F.

8.5.4 Communications I2C Interface

The fuel gauge supports the standard I2C read, incremental read, quick read, one-byte write, and incremental write functions. The 7-bit device address (ADDR) is the most significant 7 bits of the hex address and is fixed as 1010101. The first 8 bits of the I2C protocol are, therefore, 0xAA or 0xAB for write or read, respectively.

bq27421-G1 i2c_packet_format.gif Figure 5. I2C Format

The quick read returns data at the address indicated by the address pointer. The address pointer, a register internal to the I2C communication engine, increments whenever data is acknowledged by the fuel gauge or the I2C master. “Quick writes” function in the same manner and are a convenient means of sending multiple bytes to consecutive command locations (such as two-byte commands that require two bytes of data).

The following command sequences are not supported:

bq27421-G1 i2c_invalid_write.gif Figure 6. Attempt To Write a Read-only Address (Nack After Data Sent By Master)
bq27421-G1 i2c_invalid_read.gif Figure 7. Attempt To Read an Address Above 0x6B (Nack Command) I2C Time Out

The I2C engine releases both SDA and SCL if the I2C bus is held low for 2 seconds. If the fuel gauge is holding the lines, releasing them frees them for the master to drive the lines. I2C Command Waiting Time

To ensure proper operation at 400 kHz, a t(BUF) ≥ 66 μs bus-free waiting time must be inserted between all packets addressed to the fuel gauge. In addition, if the SCL clock frequency (fSCL) is > 100 kHz, use individual 1-byte write commands for proper data flow control. The following diagram shows the standard waiting time required between issuing the control subcommand the reading the status result. For read-write standard command, a minimum of 2 seconds is required to get the result updated. For read-only standard commands, there is no waiting time required, but the host must not issue any standard command more than two times per second. Otherwise, the gauge could result in a reset issue due to the expiration of the watchdog timer.

bq27421-G1 i2c_comm_wait.gif Figure 8. I2C Command Wait Time I2C Clock Stretching

A clock stretch can occur during all modes of fuel gauge operation. In SLEEP and HIBERNATE modes, a short ≤ 100-µs clock stretch occurs on all I2C traffic as the device must wake-up to process the packet. In the other modes (INITIALIZATION, NORMAL) a ≤ 4-ms clock stretching period may occur within packets addressed for the fuel gauge as the I2C interface performs normal data flow control.