BQ7693003DBT datasheet(22/63 Pages) TI1

22 / 63 page

Gen. purpose output

A/D input

Host microcontroller

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, VSTACK.

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 7-2. External Real-Time Calibration Circuit to Host Microcontroller

The process is as follows:

1. Periodically measure VSTACK.

(a) VSTACK = VAD × (R1 + R2) / R1

2. Read out all VCELL 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 VCELL values, VSUM.

(a) VSUM = V(1) + V(2) + V(3) …

4. Calculate GAIN2.

(a) GAIN2 = VSTACK / VSUM

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”.

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Product Folder Links: bq76920 bq76930 bq76940

BQ7693003DBT Datasheet(PDF) 22 Page - Texas Instruments

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