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100

120

140

160

180

PCB Copper Area − in2

No Air Flow

0.1

100

Copper Heatsink Area − cm2

TA = 55°C

No Air Flow

150 LFM

250 LFM

SOT223 Power Dissipation

Dmax +

(3.3 * 2.5) V x 1 A + 800 mW

(10)

RθJAmax + (125 * 55)°C 800 mW + 87.5°C W

100

150

125

TA = 25°C

TA (°C)

4 in2 PCB Area

0.5 in2 PCB Area

TPS79601, TPS79618, TPS79625

TPS79628, TPS79630, TPS79633

SLVS351D – SEPTEMBER 2002 – REVISED OCTOBER 2004

Figure 28. SOT223 Thermal Resistance vs PCB

Figure 27. Maximum Power Dissipation vs Copper

Area

Heatsink Area

From

the

data

Figure

and

rearranging

Equation 6, the maximum power dissipation for a

different ground plane area and a specific ambient

The SOT223 package provides an effective means of

temperature can be computed (see Figure 29).

managing power dissipation in surface mount appli-

cations. The SOT223 package dimensions are pro-

vided in the Mechanical Data section at the end of

the data sheet. The addition of a copper plane

directly underneath the SOT223 package enhances

the thermal performance of the package.

To illustrate, the TPS72525 in a SOT223 package

was chosen. For this example, the average input

voltage is 3.3 V, the output voltage is 2.5 V, the

average output current is 1 A, the ambient tempera-

ture 55

°C, no air flow is present, and the operating

environment is the same as documented below.

Neglecting the quiescent current, the maximum aver-

age power is calculated as Equation 10:

Substituting TJmax for TJ into Equation 6 gives

Equation 11:

(11)

From Figure 28, RΘJA vs PCB Copper Area, the

Figure 29. SOT223 Power Dissipation

ground plane needs to be 0.55 in

2 for the part to

dissipate 800 mW. The operating environment used

to construct Figure 28 consisted of a board with 1 oz.

copper planes. The package is soldered to a 1 oz.

copper pad on the top of the board. The pad is tied

through thermal vias to the 1 oz. ground plane.

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