BQ27520-G4 Datasheet, PDF (24/34 Pages) Texas Instruments – System-Side Impedance Track Fuel Gauge

bq27520-G4
SLUSB20B – NOVEMBER 2012 – REVISED DECEMBER 2015
www.ti.com
NAME
CC Gain
CC Delta
CC Offset
Board Offset
Pack V Offset
Table 4. Key Data Flash Parameters for Configuration (continued)
DEFAULT
10
10
–1418
0
0
UNIT
mΩ
mΩ
Counts
Counts
mV
RECOMMENDED SETTING
Calibrate this parameter using TI-supplied bqStudio software and calibration
procedure in the TRM. Determines conversion of coulomb counter measured
sense resistor voltage to current.
Calibrate this parameter using TI-supplied bqStudio software and calibration
procedure in the TRM. Determines conversion of coulomb counter measured
sense resistor voltage to passed charge.
Calibrate this parameter using TI-supplied bqStudio software and calibration
procedure in the TRM. Determines native offset of coulomb counter hardware
that should be removed from conversions.
Calibrate this parameter using TI-supplied bqStudio software and calibration
procedure in the TRM. Determines native offset of the printed circuit board
parasitics that should be removed from conversions.
Calibrate this parameter using TI-supplied bqStudio software and calibration
procedure in the TRM. Determines voltage offset between cell tab and ADC
input node to incorporate back into or remove from measurement, depending
on polarity.
9.2.2 Detailed Design Procedure
9.2.2.1 BAT Voltage Sense Input
A ceramic capacitor at the input to the BAT pin is used to bypass AC voltage ripple to ground, greatly reducing
its influence on battery voltage measurements. It proves most effective in applications with load profiles that
exhibit high-frequency current pulses (that is, cell phones) but is recommended for use in all applications to
reduce noise on this sensitive high-impedance measurement node.
9.2.2.2 SRP and SRN Current Sense Inputs
The filter network at the input to the coulomb counter is intended to improve differential mode rejection of voltage
measured across the sense resistor. These components should be placed as close as possible to the coulomb
counter inputs and the routing of the differential traces length-matched to best minimize impedance mismatch-
induced measurement errors.
9.2.2.3 Sense Resistor Selection
Any variation encountered in the resistance present between the SRP and SRN pins of the fuel gauge will affect
the resulting differential voltage, and derived current, it senses. As such, it is recommended to select a sense
resistor with minimal tolerance and temperature coefficient of resistance (TCR) characteristics. The standard
recommendation based on best compromise between performance and price is a 1% tolerance, 100 ppm drift
sense resistor with a 1-W power rating.
9.2.2.4 TS Temperature Sense Input
Similar to the BAT pin, a ceramic decoupling capacitor for the TS pin is used to bypass AC voltage ripple away
from the high-impedance ADC input, minimizing measurement error. Another helpful advantage is that the
capacitor provides additional ESD protection since the TS input to system may be accessible in systems that use
removable battery packs. It should be placed as close as possible to the respective input pin for optimal filtering
performance.
9.2.2.5 Thermistor Selection
The fuel gauge temperature sensing circuitry is designed to work with a negative temperature coefficient-type
(NTC) thermistor with a characteristic 10-kΩ resistance at room temperature (25°C). The default curve-fitting
coefficients configured in the fuel gauge specifically assume a 103AT-2 type thermistor profile and so that is the
default recommendation for thermistor selection purposes. Moving to a separate thermistor resistance profile (for
example, JT-2 or others) requires an update to the default thermistor coefficients in data flash to ensure highest
accuracy temperature measurement performance.
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