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iP1201

Adjusting the Power Loss and SOA curves for different operating conditions

To make adjustments to the power loss curves in Fig. 2, multiply the normalized value obtained from the curves in Figs. 4,

5, 6 or 7 by the value indicated on the power loss curve in Fig. 2. Remember that the power loss in Fig 2. is the power loss

for 2 outputs operating with the same output voltage. If differing output voltages are used the initial power loss for each

channel needs to be divided by 2. Then if multiple adjustments are required, multiply all of the normalized values together,

then multiply that product by the value indicated on the power loss curve in Fig. 2. The resulting product is the final power

loss based on all factors. See example no. 1.

To make adjustments to the SOA curve in Fig. 3, determine your maximum PCB Temp & Case Temp at the maximum operating

intercept (see procedure no. 2 above) in Fig. 3. When multiple adjustments are required, add all of the temperatures

Note: First check Fig. 8, Fig. 9 or Fig. 10 for maximum current capability

Operating Conditions for the following examples:

Output1

Output Current = 10A

Input Voltage = 3.3V

Inductor = 1.75µH

Output Voltage = 1.5V

Sw Freq= 200kHz

Output2

Output Current = 13A

Input Voltage =3.3V

Inductor = 1.75µH

Output Voltage = 1.0V

Sw Freq= 200kHz

Example 1) Adjusting for Maximum Power Loss:

Output1 (Fig. 2)

Maximum power loss

(Fig. 4)

(Fig. 5)

(Fig. 6)

(Fig. 7)

Applying the Safe Operating Area (SOA) Curve

The SOA graph incorporates power loss and thermal resistance information in a way that allows one to solve for maximum

current capability in a simplified graphical manner. It incorporates the ability to solve thermal problems where heat is drawn

out through the printed circuit board and the top of the case.

Procedure

1) Draw a line from Case Temp axis at T

Temp axis at T

2) Draw a vertical line from the T

curve. (see AN-1047 for further explanation of T

3) Draw a horizontal line from the intersection of the vertical

line with the SOA curve to the Y axis. The point at which

the horizontal line meets the y-axis is the SOA current.

4) If no top sided heatsinking is available, assume T

CASE

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PCB Temperature (ºC)

Case Temperature (

ºC)

L = 1.8uH

Safe

Operating

Area

1

3

2