TIDUEO0C July 2019 – March 2021
The TIDA-01546 software implements the logic for the system monitoring and battery gauge functions under the assumption that the load profile is a square wave. This square wave alternates between above the window comparator high and below the window comparator low thresholds of the ADS7142, both of which are user programmable.
There is only one set of high- and low-threshold registers in the ADS7142 device, which means that the user-defined threshold values represent different current values, depending on which current range (or resistor) is active. In other words, the ADS7142 threshold settings are used to detect the rising and falling edges of the load profile and respectively switch the 0.1-Ω resistor on and off.
Using the 10.47 µA and 475.86 mA from the list in Section 188.8.131.52, calculate and program the threshold current values as appropriate. The conversion output by the ADS7142 device at 3.3 V is equal to 0xFFF (this is a 12-bit SAR ADC). The default firmware settings for the high- and low-threshold are 0x998 and 0x028 respectively, and correspond to the following:
Table 2-1 shows the conversion of these threshold values into a current value.
|LOAD PROFILE||THRESHOLD (ADS |
|THRESHOLD CURRENT (A)||SHUNT RESISTANCE |
(Ω) IN USE
|Low to High transition||HTRH = 0x998||0.6 × 10.47 µA = 6.28 µA||6800|
|High to Low transition||LTHR = 0x028||0.01 × 476 mA = 4.76 mA||0.1 + 0.045|
The stand-by current of the board must be below 6.28 µA, and the active current must stay above
4.76 mA up to 476 mA; otherwise, the 0.1-Ω resistor switches on and off unnecessarily. Modify these settings by changing the values in the high- and low-threshold registers of the ADS7142 device.
Due to the asymmetric threshold current values when switching from the low-current to the high-current range and back, carefully select the high and low thresholds (including the optional ADS7142 window comparator hysteresis settings) to avoid erroneous changes from one current range to the other.