3 Evaluating MOSFETs Using the Design Calculator

The BQ25756 Design Calculator can be used for all of the BQ2575X family of devices. This calculator can help with resistor programming, IC set-up, and the BOM selection process. The calculator can also take user inputs about the operating conditions, BOM, and system requirements to estimate the power loss and efficiency. Note that the calculator does not take into account power losses from other sources, like magnetic core losses, or copper trace losses. The efficiency calculator is best used to compare one FET to another FET.

This calculator also includes a thermistor qualification tab to estimate the minimum/typical/maximum temperature falling and rising thresholds for the various temperature charging windows. This tab can be used to select a thermistor for the application of interest, but this section focuses on the power loss/efficiency tab of the calculator.

To use this calculator, ignore the warning about circular references and fill in the yellow cells. The cells with red tabs contain additional information about the adjacent cell. Red cells highlight the cells that can have an issue. After filling out all of the yellow cells in the design calculator, the efficiency/power loss analyzer section can be used to plot the efficiency, sense resistor loss, and power losses of the two MOSFETs.

Figure 3-1 shows the input voltage max is outside of the operating range. As a result, the text is red. When the cursor hovers over the red tab, the description text appears.

Figure 3-1 BQ2575x Design Calculator Example