ADM7154ARDZ-1.2-R7 datasheet(19/23 Pages) AD

19 / 23 page

Data Sheet

ADM7154

Rev. B | Page 19 of 23

The junction temperature of the ADM7154 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 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,

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 ADM7154. 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 55 to Figure 60 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 55. 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 56. 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 57. Junction Temperature vs. Total Power Dissipation for

the 8-Lead LFCSP, TA = 85°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 58. Junction Temperature vs. Total Power Dissipation for

the 8-Lead SOIC, TA = 25°C

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