BQ76920 Datasheet, PDF (22/63 Pages) Texas Instruments – 15-Series Cell Battery Monitor Family

bq76920, bq76930, bq76940
SLUSBK2F – OCTOBER 2013 – REVISED NOVEMBER 2015
www.ti.com
7.3.1.1.2.1 Optional Real-time Calibration Using the Host Microcontroller
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.
Host microcontroller
A/D input
Gen. purpose output
Figure 7-2. External Real-Time Calibration Circuit to Host Microcontroller
The process is as follows:
1. Periodically measure V_STACK.
(a) 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.
(a) V(1) = GAIN x ADC₁ + OFFSET, V(2) = GAIN x ADC₂ + OFFSET, and so on
3. Sum up all V_CELL values, V_SUM.
(a) V_SUM = V(1) + V(2) + V(3) …
4. Calculate GAIN₂.
(a) 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:
V(cell) = GAIN₂ × (GAIN x ADC(cell) + OFFSET)
(2)
For systems that do not require this additional in-use calibration function, GAIN₂ is simply “1”.
22
Detailed Description
Copyright © 2013–2015, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: bq76920 bq76930 bq76940