BQ27520-G4 Datasheet, PDF (12/34 Pages) Texas Instruments – System-Side Impedance Track Fuel Gauge

bq27520-G4
SLUSB20B – NOVEMBER 2012 – REVISED DECEMBER 2015
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8.3 Feature Description
The bq27520-G4 measures the voltage, temperature, and current to determine battery capacity and state-of-
charge (SOC) based on the patented Impedance Track™ algorithm (Refer to Application Report, Theory and
Implementation of Impedance Track Battery Fuel-Gauging Algorithm, SLUA450 for more information). The
bq27520-G4 monitors charge and discharge activity by sensing the voltage across a small-value resistor (5 mΩ
to 20 mΩ typical) between the SRP and SRN pins and in series with the battery. By integrating charge passing
through the battery, the battery’s SOC is adjusted during battery charge or discharge. Battery capacity is found
by comparing states of charge before and after applying the load with the amount of charge passed. When a
system load is applied, the impedance of the battery is measured by comparing the open circuit voltage (OCV)
obtained from a predefined function for present SOC with the measured voltage under load. Measurements of
OCV and charge integration determine chemical state-of-charge and chemical capacity (Qmax). The initial Qmax
values are taken from a cell manufacturers' data sheet multiplied by the number of parallel cells. It is also used
for the value in Design Capacity. The bq27520-G4 acquires and updates the battery-impedance profile during
normal battery usage. It uses this profile, along with SOC and the Qmax value, to determine
FullChargeCapacity( ) and StateOfCharge( ), specifically for the present load and temperature.
FullChargeCapacity( ) is reported as capacity available from a fully charged battery under the present load and
temperature until Voltage( ) reaches the Terminate Voltage. NominalAvailableCapacity( ) and
FullAvailableCapacity( ) are the uncompensated (no or light load) versions of RemainingCapacity( ) and
FullChargeCapacity( ), respectively. The bq27520-G4 has two Flags( ) bits and two pins to warn the host if the
battery’s SOC has fallen to critical levels. If RemainingCapacity( ) falls below the first capacity threshold specified
by SOC1 Set Threshold, the Flags ( ) [SOC1] bit is set and is cleared if RemainingCapacity( ) rises above the
SOC1 Clear Threshold. If enabled via OpConfig C [BATLSPUEN], the BAT_LOW pin reflects the status of the
[SOC1] flag bit. If enabled by OpConfig B [BL_INT], the SOC_INT will toggle upon a state change of the [SOC1]
flag bit. As Voltage( ) falls below the SysDown Set Volt Threshold, the Flags( ) [SYSDOWN] bit is set and
SOC_INT will toggle once to provide a final warning to shut down the system. As Voltage( ) rises above
SysDown Clear Voltage the [SYSDOWN] bit is cleared and SOC_INT will toggle once to signal the status
change. Additional details are found in the bq27520-G4 Technical Reference Manual, SLUUA35.
8.4 Device Functional Modes
8.4.1 Power Modes
The fuel gauge has different power modes:
• BAT INSERT CHECK: The BAT INSERT CHECK mode is a powered-up, but low-power halted, state where
the fuel gauge resides when no battery is inserted into the system.
• NORMAL: In NORMAL mode, the fuel gauge is fully powered and can execute any allowable task.
• SLEEP: In SLEEP mode, the fuel gauge turns off the high-frequency oscillator and operates in a reduced-
power state, periodically taking measurements and performing calculations.
• SLEEP+: In SLEEP+ mode, both low-frequency and high-frequency oscillators are active. Although the
SLEEP+ mode has higher current consumption than the SLEEP mode, it is also a reduced power mode.
• HIBERNATE: In HIBERNATE mode, the fuel gauge is in a low power state, but can wake up by
communication or certain I/O activity.
The relationship between these modes is shown in Figure 6 and Figure 7.
8.4.1.1 BAT INSERT CHECK Mode
This mode is a halted-CPU state that occurs when an adapter, or other power source, is present to power the
fuel gauge (and system), but no battery has been detected. When battery insertion is detected, a series of
initialization activities begin, which include: OCV measurement, setting the Flags() [BAT_DET] bit, and selecting
the appropriate battery profiles.
Some commands, issued by a system processor, can be processed while the fuel gauge is halted in this mode.
The gauge wakes up to process the command, then returns to the halted state awaiting battery insertion.
8.4.1.2 NORMAL Mode
The fuel gauge is in NORMAL mode when not in any other power mode. During this mode, AverageCurrent() ,
Voltage() , and Temperature() measurements are taken, and the interface data set is updated. Decisions to
change states are also made. This mode is exited by activating a different power mode.
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