REV. 0

ADP3339

–7–

CALCULATING POWER DISSIPATION

Device power dissipation is calculated as follows:

PV

V

I

V

I

D

IN

OUT

LOAD

IN

GND

=−

(

dissipation is:

P

V

V

mA

V

mA

mW

D

=

()

+

()

=

3 3

2 5

1500

3 3

14

1246

.

– ..

So, for a maximum junction temperature of 125

°C and a

maximum ambient temperature of 85

°C, the required ther-

mal resistance from junction to ambient is:

θ

JA

CC

W

CW

=

°− ° =°

125

85

1 246

32 1

.

./

PRINTED CIRCUIT BOARD LAYOUT

CONSIDERATIONS

The SOT-223’s thermal resistance,

θ

sum of the junction-to-case and the case-to-ambient thermal

resistances. The junction-to-case thermal resistance,

determined by the package design and specified at 26.8

°C/W.

However, the case-to-ambient thermal resistance is determined

by the printed circuit board design.

As shown in Figures 4a–4c, the amount of copper to which the

ADP3339 is mounted affects the thermal performance. When

mounted to just the minimal pads of 2 oz. copper (Figure 4a),

the

ADP3339 (Figure 4b), reduces the

θ

°C/W. Increasing

the copper pad to 1 square inch (Figure 4c), reduces the

even further to 52.8

°C/W.

a.

b.

c.

Figure 4. PCB Layouts

Use the following general guidelines when designing printed

circuit boards:

1. Keep the output capacitor as close to the output and ground

pins as possible.

2. Keep the input capacitor as close to the input and ground

pins as possible.

3. PC board traces with larger cross sectional areas will remove

more heat from the ADP3339. For optimum heat transfer,

specify thick copper and use wide traces.

4. The thermal resistance can be decreased by adding a copper

pad under the ADP3339 as shown in Figure 4b.

5. If possible, utilize the adjacent area to add more copper

around the ADP3339. Connecting the copper area to the

output of the ADP3339, as shown in Figure 4c, is best but

will improve thermal performance even if it is connected to

other pins.

6. Use additional copper layers or planes to reduce the thermal

resistance. Again, connecting the other layers to the output

of the ADP3339 is best, but not necessary. When connecting

the output pad to other layers use multiple vias.