CSD87330Q3D_15 Datasheet, PDF (12/23 Pages) Texas Instruments – Synchronous Buck NexFET™ Power Block

CSD87330Q3D
SLPS284C – AUGUST 2011 – REVISED APRIL 2013
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Calculating Power Loss and SOA
The user can estimate product loss and SOA boundaries by arithmetic means (see Design Example). Though
the Power Loss and SOA curves in this data sheet are taken for a specific set of test conditions, the following
procedure will outline the steps the user should take to predict product performance for any set of system
conditions.
Design Example
Operating Conditions:
• Output Current = 15A
• Input Voltage = 12V
• Output Voltage = 1.2V
• Switching Frequency = 1000kHz
• Inductor = 0.4µH
Calculating Power Loss
• Power Loss at 15A = 2.2W (Figure 1)
• Normalized Power Loss for input voltage ≈ 1.0 (Figure 7)
• Normalized Power Loss for output voltage ≈ 0.98 (Figure 8)
• Normalized Power Loss for switching frequency ≈ 1.17 (Figure 6)
• Normalized Power Loss for output inductor ≈ 1.06 (Figure 9)
• Final calculated Power Loss = 2.2W × 1.0 × 0.98 × 1.17 × 1.06 ≈ 2.67W
Calculating SOA Adjustments
• SOA adjustment for input voltage ≈ 0ºC (Figure 7)
• SOA adjustment for output voltage ≈ –0.29ºC (Figure 8)
• SOA adjustment for switching frequency ≈ 4.1ºC (Figure 6)
• SOA adjustment for output inductor ≈ 1.5ºC (Figure 9)
• Final calculated SOA adjustment = 0 + (–0.29) + 4.1 + 1.5 ≈ 5.3ºC
In the design example above, the estimated power loss of the CSD87330Q3D would increase to 2.67W. In
addition, the maximum allowable board and/or ambient temperature would have to decrease by 5.3ºC. Figure 33
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 5.3ºC. In the event the adjustment value is a negative number, subtracting the negative number
would yield an increase in allowable board/ambient temperature.
25
20
1
15
10 _{VGS = 5V}
2
V_IN = 12V
5
V_OUT = 1.3V
f_SW = 500kHz
L_OUT = 1mH
3
0
0
20
40
60
80 100 120 140
Board Temperature (°C)
G028
Figure 33. Power Block SOA
12
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