BQ27520-G3 Datasheet, PDF (27/47 Pages) Texas Instruments – System-Side Impedance Track™ Fuel Gauge With Integrated LDO

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bq27520-G3
SLUSAP0 – DECEMBER 2011
Table 5-2. Constant-Power Model Used When Load Mode = 1
LoadSelect Value
0
1(default)
2
3
4
5
6
Power Model Used
Average discharge power from previous cycle: There is an internal register that records the average discharge power
through each entire discharge cycle. The previous average is stored in this register.
Present average discharge power: This is the average discharge power from the beginning of this discharge cycle
until present time.
Average current × voltage: based off the AverageCurrent( ) and Voltage( ).
Current × voltage: based off of a low-pass-filtered version of AverageCurrent( ) (τ=14 s) and Voltage( )
Design energy / 5: C Rate based off of Design Energy /5 or a C/5 rate in mA.
AtRate (10 mW): Use whatever value is in AtRate( ).
User_Rate-10mW: Use the value in User_Rate-10mW. This mode provides a completely user-configurable method.
5.2.3 Reserve Cap-mAh
Reserve Cap-mAh determines how much actual remaining capacity exists after reaching 0
RemainingCapacity( ), before Terminate Voltage is reached. A no-load rate of compensation is applied
to this reserve.
5.2.4 Reserve Cap-mWh
Reserve Cap-mWh determines how much actual remaining capacity exists after reaching 0
AvailableEnergy( ), before Terminate Voltage is reached. A no-load rate of compensation is applied to
this reserve capacity.
5.2.5 Dsg Current Threshold
This register is used as a threshold by many functions in the bq27520-G3 to determine if significant
discharge current is flowing into or out of the cell. The default for this register is in Table 4-7, which should
be sufficient for most applications. This threshold should be set low enough to be below any normal
application load current but high enough to prevent noise or drift from affecting the measurement.
5.2.6 Chg Current Threshold
This register is used as a threshold by many functions in the bq27520-G3 to determine if significant
charge current is flowing into or out of the cell. The default for this register is in Table 4-7, which should be
sufficient for most applications. This threshold should be set low enough to be below any normal charge
current but high enough to prevent noise or drift from affecting the measurement.
5.2.7 Quit Current, DSG Relax Time, CHG Relax Time, and Quit Relax Time
The Quit Current is used as part of the Impedance Track™ algorithm to determine when the bq27520-G3
enters relaxation mode from a current-flowing mode in either the charge direction or the discharge
direction. The value of Quit Current is set to a default value in Table 4-7 and should be above the standby
current of the system.
Either of the following criteria must be met to enter relaxation mode:
• | AverageCurrent( ) | < | Quit Current | for Dsg Relax Time
• | AverageCurrent( ) | < | Quit Current | for Chg Relax Time
After about 5 minutes in relaxation mode, the bq27520-G3 attempts to take accurate OCV readings. An
additional requirement of dV/dt < 1 μV/s is required for the to perform Qmax updates. These updates are
used in the Impedance Track™ algorithms. It is critical that the battery voltage be relaxed during OCV
readings to and that the current is not be higher than C/20 when attempting to go into relaxation mode.
Quit Relax Time specifies the minimum time required for AverageCurrent( ) to remain above the Dsg or
Chg Current Thresholdbefore exiting relaxation mode.
Copyright © 2011, Texas Instruments Incorporated
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FUNCTIONAL DESCRIPTION
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