ZHCSMK4A september 2022 – july 2023 ADS131B26-Q1
PRODUCTION DATA
The 800-V battery-pack voltage is divided down to the voltage range of ADC3A using a high-voltage resistor divider consisting of R23, R24, R25, R26, and R27. Gain = 1 is used for ADC3A in this case to allow differential voltage measurements of VIN3A = VVPA – VVNA = ±1.25 V. The battery-pack voltage measurement is a unipolar, single-ended measurement with VNA connected to HV_BAT– = AGNDA. Thus, only the voltage range from 0 V to 1.25 V of ADC3A is used. Equation 23 calculates the resistor divider ratio.
The leakage current drawn by the resistor
divider should be less than 100 μA in this example to avoid unnecessarily draining the
battery. The resistance of the divider must therefore be larger than RTOTAL ≥
VBAT_MAX / ILEAKAGE = 800 V / 100 μA = 8
MΩ. The resistor values are chosen as R23 = R24 = R25 =
R26 = 2 MΩ and R27 = 12 kΩ. Thus, the maximum voltage across
R27 is 1.2 V at VBAT_MAX = 800 V, leaving some headroom to the
maximum input voltage of 1.25 V of ADC3A.
The maximum resistance of a single resistor that can be used in an automotive circuit design is often limited to a certain value. Also, the maximum voltage a single resistor can withstand is limited. These reasons are why the high-side resistor of the divider is split into multiple resistors (R23, R24, R25, and R26). Another reason is that if a single resistor has a short-circuit fault, the remaining resistors still limit the current into the ADC3A analog input pins,VPA and VNA, to safe levels.