CSD97374Q4M_15 Datasheet, PDF (11/20 Pages) Texas Instruments – Synchronous Buck NexFET Power Stage

Not Recommended For New Designs
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CSD97374Q4M
SLPS382C – JANUARY 2013 – REVISED JULY 2013
Calculating Power Loss and SOA
The user can estimate product loss and SOA boundaries by arithmetic means (see the Design Example).
Though the Power Loss and SOA curves in this datasheet are taken for a specific set of test conditions, the
following procedure will outline the steps engineers should take to predict product performance for any set of
system conditions.
Design Example
Operating Conditions: Output Current (l_OUT) = 15A, Input Voltage (V_IN ) = 7V, Output Voltage (V_OUT) = 1.5V,
Switching Frequency (f_SW) = 800kHz, Output Inductor (L_OUT) = 0.2µH
Calculating Power Loss
• Typical Power Loss at 15A = 2.8W (Figure 3)
• Normalized Power Loss for switching frequency ≈ 1.02 (Figure 7)
• Normalized Power Loss for input voltage ≈ 1.07 (Figure 8)
• Normalized Power Loss for output voltage ≈ 0.94(Figure 9)
• Normalized Power Loss for output inductor ≈ 1.08 (Figure 10)
• Final calculated Power Loss = 2.8W × 1.02 × 1.07 × 0.94 × 1.08 ≈ 3.1W
Calculating SOA Adjustments
• SOA adjustment for switching frequency ≈ 0.3°C (Figure 7)
• SOA adjustment for input voltage ≈ 1.2°C (Figure 8)
• SOA adjustment for output voltage ≈ –1.1°C (Figure 9)
• SOA adjustment for output inductor ≈ 1.4°C (Figure 10)
• Final calculated SOA adjustment = 0.3 + 1.2 + (–1.1) + 1.4 ≈ 1.8°C
Figure 18. Power Stage CSD97374Q4M SOA
In the design example above, the estimated power loss of the CSD97374Q4M would increase to 3.1W. In
addition, the maximum allowable board and/or ambient temperature would have to decrease by 1.8°C. Figure 18
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 1.8°C. In the event the adjustment value is a negative number, subtracting the negative number
would yield an increase in allowable board/ambient temperature.
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