ADM7151ARDZ-04-R7 datasheet(20/24 Pages) AD

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ADM7151

Data Sheet

Rev. B | Page 20 of 24

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

LFCSP packages for various PCB copper sizes.

Table 9 shows the typical ΨJB values of the 8-lead SOIC and

8-lead LFCSP.

Table 8. Typical θJA Values

θJA (°C/W)

Copper Size (mm2)

8-Lead LFCSP

8-Lead SOIC

251

165.1

165

100

125.8

126.4

500

68.1

69.8

1000

56.4

57.8

6400

42.1

43.6

Device soldered to minimum size pin traces.

Table 9. Typical ΨJB Values

Package

ΨJB (°C/W)

8-Lead LFCSP

15.1

8-Lead SOIC

17.9

The junction temperature of the ADM7151 is calculated 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 thinput and output voltages, respectively.

ILOAD is the load current.

IGND is the groune d current.

Power dissipation due to 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, there

exists a minimum copper size requirement for the PCB to ensure

that the junction temperature does not rise above 150°C.

The heat dissipation from the package can be improved by

increasing the amount of copper attached to the pins and exposed

pad of the ADM7151. Adding thermal planes under the package

also improves thermal performance. However, as listed in Table 8, 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 61 to Figure 66 show junction temperature calculations for

different ambient temperatures, power dissipation, and areas of

PCB copper.

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAX

105

115

125

135

145

155

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Figure 61. Junction Temperature vs. Total Power Dissipation for

the 8-Lead LFCSP, TA = 25°C

TOTAL POWER DISSIPATION (W)

1.8

2.0

2.2

2.4

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

100

110

120

140

160

130

150

6400mm2

500mm2

25mm2

TJ MAX

Figure 62. Junction Temperature vs. Total Power Dissipation for

the 8-Lead LFCSP, TA = 50°C

TOTAL POWER DISSIPATION (W)

1.5

0.8 0.9 1.0 1.1 1.2 1.3 1.4

0.7

0.6

0.5

0.4

0.3

0.2

0.1

105

115

125

135

155

145

6400mm2

500mm2

25mm2

TJ MAX

Figure 63. Junction Temperature vs. Total Power Dissipation for

the 8-Lead LFCSP, TA = 85°C

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