BQ27531-G1_16 Datasheet, PDF (24/34 Pages) Texas Instruments – Battery Management Unit Impedance Track Fuel Gauge

bq27531-G1
SLUSBE7C – MARCH 2013 – REVISED JANUARY 2016
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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 must 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, TI recommends selecting 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, because the TS input to system may be accessible in systems that
use removable battery packs. The capacitor must 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.
9.2.2.6 REGIN Power Supply Input Filtering
A ceramic capacitor is placed at the input to the fuel gauge internal LDO to increase power supply rejection
(PSR) and improve effective line regulation. It ensures that voltage ripple is rejected to ground instead of
coupling into the internal supply rails of the fuel gauge.
9.2.2.7 V_CC LDO Output Filtering
A ceramic capacitor is also needed at the output of the internal LDO to provide a current reservoir for fuel gauge
load peaks during high peripheral utilization. It acts to stabilize the regulator output and reduce core voltage
ripple inside of the fuel gauge.
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