TPS68625 datasheet(13/17 Pages) TI

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0.1

100

Copper Heatsink Area − cm2

TA = 55°C

No Air Flow

150 LFM

250 LFM

100

120

140

160

180

PCB Copper Area − in2

No Air Flow

0.1

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 − Ambient Temperature − °C

4 in2 PCB Area

0.5 in2 PCB Area

TPS78601, TPS78618

TPS78625, TPS78628

TPS78630, TPS78633

SLVS389D – SEPTEMBER 2002 – REVISED OCTOBER 2004

From

the

data

Figure

and

rearranging

dissipate 800 mW. The operating environment used

Equation 6, the maximum power dissipation for a

to construct Figure 30 consisted of a board with 1 oz.

different ground plane area and a specific ambient

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

temperature can be computed.

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

through thermal vias to the 1 oz. ground plane.

Figure 29. Maximum Power Dissipation vs Copper

Heatsink Area

Figure 30. SOT223 Thermal Resistance vs PCB

Area

From

the

data

Figure

and

rearranging

Equation 6, the maximum power dissipation for a

The SOT223 package provides an effective means of

different ground plane area and a specific ambient

managing power dissipation in surface mount appli-

temperature can be computed (see Figure 31).

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 TPS78625 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 shown in Equation 10:

Substituting TJmax for TJ into Equation 6 gives

Equation 11:

(11)

From Figure 30, RΘJA vs PCB Copper Area, the

Figure 31. SOT223 Power Dissipation

ground plane needs to be 0.55 in

2 for the part to

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