ADP7157ACPZ-04-R7 datasheet(19/24 Pages) AD

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ADP7157

Data Sheet

Rev. A | Page 18 of 23

Table 7 shows 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

The junction temperature of the ADP7157 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 equation:

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 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 ADP7157. 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.

100

120

140

0.20.40.60.81.0 1.21.41.61.82.02.22.42.6 2.83.0

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAXIMUM

Figure 55. Junction Temperature vs. Total Power Dissipation for

the 10-Lead LFCSP, TA = 25°C

100

120

140

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

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAXIMUM

Figure 56. Junction Temperature vs. Total Power Dissipation for

the 10-Lead LFCSP, TA = 50°C

100

105

110

115

120

125

130

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAXIMUM

Figure 57. Junction Temperature vs. Total Power Dissipation for

the 10-Lead LFCSP, TA = 85°C

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