6.2.2.2 Calculating SOA Adjustments

• SOA adjustment for input voltage ≈ 1.37°C (Figure 5)
• SOA adjustment for output voltage ≈ 1.48°C (Figure 6)
• SOA adjustment for switching frequency ≈ 2.34°C (Figure 4)
• SOA adjustment for output inductor ≈ 0.03°C (Figure 7)
• Final calculated SOA adjustment = 1.37 + 1.48 + 2.34 + 0.03 ≈ 5.2°C

In the previous design example, the estimated power loss of the CSD58915Q5D would increase to 8.5 W. In addition, the maximum allowable board and/or ambient temperature would have to decrease by 5.2°C. Figure 31 graphically shows how the SOA curve would be adjusted accordingly.

1. Start by drawing a horizontal line from the application current to the SOA curve.
2. Draw a vertical line from the SOA curve intercept down to the board/ambient temperature.
3. Adjust the SOA board/ambient temperature by subtracting the temperature adjustment value.

In the design example, the SOA temperature adjustment yields a reduction in allowable board/ambient temperature of 5.2°C. In the event the adjustment value is a negative number, subtracting the negative number would yield an increase in allowable board/ambient temperature.

Figure 31. Power Block SOA