6.5.3 Calculating SOA Adjustments

• SOA adjustment for input voltage ≈ 0.14°C (Figure 6)
• SOA adjustment for output voltage ≈ 0.17°C (Figure 7)
• SOA adjustment for switching frequency ≈ 1.32°C (Figure 5)
• SOA adjustment for output inductor ≈ 0.18°C (Figure 8)
• Final calculated SOA adjustment = 0.14 + 0.17 + 1.32 + 0.18 ≈ 1.81°C

In the design example above, the estimated power loss of the CSD87335Q3D would increase to 2.14 W. In addition, the maximum allowable board and/or ambient temperature would have to decrease by 1.81°C. Figure 32 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 1.81°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 32. Power Block SOA