ADP7158ARDZ-3.3-R7 datasheet(19/23 Pages) AD

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ADP7158

Data Sheet

Rev. A | Page 18 of 22

Table 7 shows the typical θJA values of the 8-lead SOIC and

10-lead LFCSP packages for various PCB copper sizes. Table 8

shows the typical ΨJB values of the 8-lead SOIC and 10-lead

LFCSP.

Table 7. Typical θJA Values

θJA (°C/W)

Copper Size (mm2)

10-Lead LFCSP

8-Lead SOIC

251

130.2

123.8

100

93.0

90.4

500

65.8

66.0

1000

55.6

56.6

6400

44.1

45.5

1 Device soldered to minimum size pin traces.

Table 8. Typical ΨJB Values

Package

ΨJB (°C/W)

10-Lead LFCSP

29.1

8-Lead SOIC

30.1

Calculate the junction temperature (TJ) of the ADP7158 from

the following equation:

TJ = TA + (PD × θJA)

(2)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

PD = ((VIN − VOUT) × ILOAD) + (VIN × IGND)

(3)

where:

VIN and VOUT are the input and output voltages, respectively.

ILOAD is the load current.

IGND is the ground current.

Power dissipation caused by ground current is quite small and

can be ignored. Therefore, the junction temperature equation

simplifies to the following:

TJ = TA + (((VIN − VOUT) × ILOAD) × θJA)

(4)

As shown in Equation 4, for a given ambient temperature, input

to output voltage differential, and continuous load current, a

minimum copper size requirement exists for the PCB to ensure

that the junction temperature does not rise above 125°C.

The heat dissipation from the package can be improved by increas-

ing the amount of copper attached to the pins and exposed pad

of the ADP7158. Adding thermal planes underneath the package

also improves thermal performance. However, as shown in Table 7,

a point of diminishing returns is eventually reached, beyond

which an increase in the copper area does not yield significant

reduction in the junction to ambient thermal resistance.

Figure 54 to Figure 59 show junction temperature calculations

for various ambient temperatures, power dissipation, and areas

of PCB copper.

100

120

140

0 0.20.40.6 0.81.01.21.41.6 1.82.02.22.42.6 2.83.0

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAX

Figure 54. Junction Temperature vs. Total Power Dissipation for

the 10-Lead LFCSP, TA = 25°C

100

120

140

0.2 0.40.60.81.01.21.4 1.61.82.02.22.42.6

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAX

Figure 55. Junction Temperature vs. Total Power Dissipation for

the 10-Lead LFCSP, TA = 50°C

0.10.20.30.40.50.60.7

0.9

0.8

1.0

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAX

100

105

110

115

120

125

130

Figure 56. Junction Temperature vs. Total Power Dissipation for

the 10-Lead LFCSP, TA = 85°C

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