CSD87381P

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SLPS405B – MARCH 2013 – REVISED MAY 2013

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 = 8A

•

Input Voltage = 4V

•

Output Voltage = 1V

•

Switching Frequency = 800kHz

•

Inductor = 0.2µH

Calculating Power Loss

•

Power Loss at 8A = 1.44W (Figure 1)

•

Normalized Power Loss for input voltage

≈ 1.06 (Figure 6)

•

Normalized Power Loss for output voltage

≈ 0.97 (Figure 7)

•

Normalized Power Loss for switching frequency

≈ 1.11(Figure 5)

•

Normalized Power Loss for output inductor

≈ 1.13 (Figure 8)

•

Final calculated Power Loss = 1.44W x 1.06 x 0.97 x 1.11 x 1.13

≈ 1.86W

Calculating SOA Adjustments

•

SOA adjustment for input voltage

≈ 0.7ºC (Figure 6)

•

SOA adjustment for output voltage

≈ -0.3ºC (Figure 7)

•

SOA adjustment for switching frequency

≈ 1.03ºC (Figure 5)

•

SOA adjustment for output inductor

≈ 1.5ºC (Figure 8)

•

Final calculated SOA adjustment = 0.7 + (-0.3) + 1.3 + 1.5

≈ 2.2ºC

In the design example above, the estimated power loss of the CSD87381P would increase to 1.86W. In addition,

the maximum allowable board and/or ambient temperature would have to decrease by 2.2ºC. Figure 28

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 2.2ºC. In the event the adjustment value is a negative number, subtracting the negative number

would yield an increase in allowable board/ambient temperature.

Figure 28. Power Block SOA

Copyright © 2013, Texas Instruments Incorporated

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