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Developing a cell-monitoring system

Posted: 12 Dec 2013     Print Version  Bookmark and Share

Keywords:cell measurement system  cell-monitoring  analogue front-ends  analogue-to-digital converter  AFE 

By using the external reference, initial accuracy improves and temperature drift is lower. The IN/OUT ratio of the voltage-divider drops with a higher voltage reference, so the inaccuracies associated with that multiplying factor drop as well. Ultimately, a designer must decide if the additional cost of the external reference is sufficiently offset by the increased accuracy of this system.

The important parameters
This microcontroller must integrate at least one 12bit (minimum) ADC capable of providing the desired accuracy. Also important for accuracy is the selection of R1 and R2 in the voltage-divider.5

Selection of the external reference depends both on the budget and the capabilities of the microcontroller. Many microcontrollers operate from a 3.3V supply and can only handle a 3.3V reference. For maximum accuracy, select a reference voltage as close to the full-scale input as possible and make sure that it operates within the capabilities of the microcontroller. This step is quite important. The objective is to lower the IN/OUT ratio as much as possible and, therefore, decrease the amount by which the output must be multiplied to regain the original scale. For example, the IN/OUT ratio for the internal voltage reference of the Freescale K10P64M72SF1 microcontroller is 4V/1.195V = 3.35. However, using a 3.3V external reference produces an IN/OUT ratio of 4V/3.3V = 1.21.

Finally, the reference's initial accuracy and temperature coefficient parameters are important, although they do not influence accuracy as much as the reference voltage.

The component selection
Once again, the Freescale Semiconductor K10P64M72SF1 microcontroller is used. The MAX6034B voltage reference was selected for its price point as well as its 13mV initial accuracy and 75ppm/°C (max) temperature drift. It also has a 3.3V option, which the selected microcontroller will accept and which greatly improves accuracy compared to a 1.195V reference used in the cost-optimised example above (figure 5).

Selecting R1 = 1MΩ and R2 = 213kΩ with 0.1% tolerance, the maximum error introduced by the voltage-divider is 3.55mV.

The cost-optimised, accuracy-enhanced architecture using the Freescale Semiconductor K10P64M72SF1 and the MAX6034B voltage reference achieves six-sigma error as low as 31.154mV and three-sigma error as low as 17.632mV. Compared to the cost-optimised architecture discussed above, this example design offers an 87.6% decrease in six-sigma error with only a minor increase in the cost of the system.

Conclusion
Four architectures have been outlined to give system designers flexibility in implementing battery-management systems. In all cases the MAX14921 (or MAX14920) was the optimal AFE capable of delivering high-performance battery monitoring and cell-balancing capabilities for multiple design constraints. The selection of ADC, reference, and microcontroller will depend on the accuracy-cost trade-off.

References
1. If the MAX6194 reference is used in the design, then Cell B4 in the "Total Error Calculations" worksheet in the Error Measurements spreadsheet must be changed to 4.5V to account for the higher reference voltage. A complete schematic, layout, and Bill of Materials for this architecture can be found in the data sheet for the MAX14921EVKIT.
2. The MAX14920 will monitor up to 12 cells and provide the same high accuracy.
3. See the Typical Operating Characteristics in the MAX14920/MAX14921 data sheet.
4. If a low-cost 3.3V voltage reference with higher accuracy than the MAX6034B is needed for an application, the MAX6034A voltage reference is recommended.
5. Please refer to the Cost-Optimised Architecture section for analysis of the impact of the voltage-divider on accuracy.

About the author
Jeremy Georges is with Maxim Integrated.

To download the PDF version of this article, click here.


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