ADP322ACPZ-155-R7 datasheet(21/24 Pages) AD

21 / 24 page

Data Sheet

ADP322/ADP323

Rev. E | Page 21 of 24

The junction temperature of the ADP322/ADP323 can be

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 = Σ[(VIN − VOUT) × ILOAD] + Σ(VIN × IGND)

(3)

where:

ILOAD is the load current.

IGND is the ground current.

VIN and VOUT are input and output voltages, respectively.

Power dissipation due to ground current is quite small and

can be ignored. Therefore, the junction temperature equation

simplifies to

TJ = TA + {Σ[(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 125°C. Figure 58

to Figure 61 show junction temperature calculations for different

ambient temperatures, total power dissipation, and areas of

PCB copper.

In cases where the board temperature is known, the thermal

characterization parameter, ΨJB, can be used to estimate the

junction temperature rise. TJ is calculated from TB and PD using

the formula

TJ = TB + (PD × ΨJB)

(5)

The typical ΨJB value for the 16-lead, 3 mm × 3 mm LFCSP is

25.2°C/W.

100

120

140

0.2

0.4

0.6

0.8

1.0

1.2

TOTAL POWER DISSIPATION (W)

1000mm2

500mm2

100mm2

50mm2

JEDEC

TJ MAX

Figure 58. Junction Temperature vs. Total Power Dissipation, TA = 25°C

100

120

140

0.2

0.4

0.6

0.8

1.0

1.2

TOTAL POWER DISSIPATION (W)

1000mm2

500mm2

100mm2

50mm2

JEDEC

TJ MAX

Figure 59. Junction Temperature vs. Total Power Dissipation, TA = 50°C

100

120

140

0.2

0.4

0.6

0.8

1.0

1.2

TOTAL POWER DISSIPATION (W)

1000mm2

500mm2

100mm2

50mm2

JEDEC

TJ MAX

Figure 60. Junction Temperature vs. Total Power Dissipation, TA = 85°C

100

120

140

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

TOTAL POWER DISSIPATION (W)

TB = 25°C

TB = 50°C

TB = 85°C

TJ MAX

Figure 61. Junction Temperature vs. Total Power Dissipation and

Board Temperature

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