BQ34210-Q1 Datasheet, PDF (13/19 Pages) Texas Instruments – Automotive 1-Series Cell System-Side CEDV Fuel Gauge for Rarely Discharged Batteries

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bq34210-Q1
SLUSCG1 – AUGUST 2017
Typical Applications (continued)
8.2.2 Detailed Design Procedure
8.2.2.1 REGIN Voltage Sense Input
A ceramic capacitor at the input to the REGIN pin is used to bypass AC voltage ripple to ground, greatly reducing
its influence on battery voltage measurements.
8.2.2.2 Integrated LDO Capacitor
The fuel gauge has an integrated LDO with an output on the REG18 pin of approximately 1.8 V. A capacitor of at
least a 2.2-μF value should be connected between the REG18 pin and VSS. The capacitor must be placed close
to the fuel gauge and have short traces to both the REG18 pin and VSS. This regulator must not be used to
provide power for other devices in the system.
8.2.2.3 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, 50-ppm drift
sense resistor with a 1-W power rating. The power rating must be consistent with the maximum current and
sense resistor value. The bq34210-Q1 device supports sense resistors from 5 mΩ to 20 mΩ.
8.2.3 External Thermistor Support
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 Semitec 103AT 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, which can be
modified in RAM to ensure highest accuracy temperature measurement performance. For more details, see the
Temperature Measurement section of the bq34210-Q1 TRM (SLUUBE8).
8.2.4 Learning Load Enable (LEN) from Host
The learning load helps to determine the status of the battery (EOS). The host must control the load during the
learning phase and put the bq34210-Q1 gauge into the learning phase. The resistance is set by selecting the
learning current. With 220 mA and the charge voltage of 4.2 V, use Ohm's law to calculate the resistance
(19.09 Ω).
8.2.5 I²C
If the external pullup resistors on the SCL and SDA lines will be disconnected from the host during low-power
operation, it is recommended to use external 1-MΩ pulldown resistors to VSS to avoid floating inputs to the I²C
engine.
The value of the SCL and SDA pullup resistors should take into consideration the pullup voltage and the bus
capacitance along with the communication speed. Many communication errors are a result of improper sizing of
the resistors. Rounding of the clock and data signals indicated improper RC configurations. The maximum pullup
resistance (R_PUmax) can be estimated by this equation:
R_PUmax = t_r / (0.4873 × C_BUS)
Where t_r is the rise time and C_BUS is the total bus capacitance.
Assuming a bus capacitance of 10 pF, Table 1 shows some recommended values.
Table 1. Recommended Values for SCL and SDA Pullup Resistors
VPU
R_PU
1.8 V
Range
400 Ω ≤ R_PU ≤ 37.6 kΩ
Typical
10 kΩ
3.3 V
Range
900 Ω ≤ R_PU ≤ 29.2 kΩ
Typical
5.1 kΩ
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