CSD87381P

SLPS405F – MARCH 2013 – REVISED MARCH 2015

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Application Information (continued)

6.1.4 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 outlines the steps the user should take to predict product performance for any set of system

conditions.

6.1.4.1 Design Example

Operating Conditions:

•

Output Current = 8 A

•

Input Voltage = 4 V

•

Output Voltage = 1 V

•

Switching Frequency = 800 kHz

•

Inductor = 0.2 µH

6.1.4.2 Calculating Power Loss

•

Power Loss at 8 A = 1.44 W (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 = 1.44 W × 1.06 × 0.97 × 1.11 × 1.13

≈ 1.86 W

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

addition, the maximum allowable board or ambient temperature, or both, 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 or ambient temperature.

3. Adjust the SOA board or ambient temperature by subtracting the temperature adjustment value.

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