The performance of the cell voltage values measured by the 14-bit ADC has a factory-calibrated accuracy, as follows:

• +/– 10 mV TYP, +/– 40 mV MIN and MAX from 3.6 to 4.3 V,
• +/– 15 mV TYP, +/– 40 mV MIN and MAX from 3.2 to 4.6 V, and
• +/– 50 mV MIN and MAX from 2.0 to 5.0 V

While this is suitable for the majority of pack protection and basic monitoring applications the BQ769x0 AFE family is intended to support, certain systems may require a higher accuracy performance.

To achieve this, use an available ADC channel and general purpose output terminal on the host microcontroller paired with the BQ769x0. A simple external circuit consisting of two precision resistors and a small-signal FET is activated by the host microcontroller to determine the total stack voltage, V_STACK. This is then compared against the sum of the individual cell voltages as measured by the internal ADC of the BQ769x0. The resulting transfer function coefficient, GAIN₂, is simply applied to each cell voltage ADC value for improved accuracy.

Figure 8-1 External Real-Time Calibration Circuit to Host Microcontroller

The process is as follows:

1. Periodically measure V_STACK.
   1. V_STACK = V_AD × (R1 + R2) / R1
2. Read out all V_CELL ADC readings from the BQ769x0 and apply the standard GAIN and OFFSET values stored in the BQ769x0.
   1. V(1) = GAIN x ADC₁ + OFFSET, V(2) = GAIN x ADC₂ + OFFSET, and so on
3. Sum up all V_CELL values, V_SUM.
   1. V_SUM = V(1) + V(2) + V(3) …
4. Calculate GAIN₂.
   1. GAIN₂ = V_STACK / V_SUM

As a general recommendation, a new GAIN₂ function should be generated when the cell voltages increase or decrease by more than 100 mV. With GAIN₂, each cell voltage calculation becomes:

For systems that do not require this additional in-use calibration function, GAIN₂ is simply “1”.