ADP7185ACPZN2.5-R7 datasheet(17/19 Pages) AD

17 / 19 page

Data Sheet

ADP7185

Rev. 0 | Page 17 of 19

THERMAL CONSIDERATIONS

In applications with a low input to output voltage differential,

the ADP7185 does not dissipate much heat. However, in

applications with high ambient temperature and/or high input

voltage, the heat dissipated in the package may become large

enough to cause the junction temperature of the die to exceed

the maximum junction temperature of 125°C.

When the junction temperature exceeds 150°C, the converter

enters thermal shutdown. The converter recovers only after the

junction temperature decreases below 135°C to prevent any

permanent damage. Therefore, thermal analysis for the chosen

application is important to guarantee reliable performance over all

conditions. The junction temperature of the die is the sum of the

ambient temperature of the environment and the temperature

rise of the package due to the power dissipation, as shown in

Equation 5.

To guarantee reliable operation, the junction temperature of the

ADP7185 must not exceed 125°C. To ensure that the junction

temperature stays below this maximum value, the user must be

aware of the parameters that contribute to junction temperature

changes. These parameters include ambient temperature, power

dissipation in the power device, and thermal resistances between

the junction and ambient air (θJA). The θJA number is dependent

on the package assembly compounds that are used, and the amount

of copper used to solder the package VIN pins to the PCB.

Table 7 shows the typical θJA values for the 8-lead LFCSP

package and for various PCB copper sizes.

Table 7. Typical θJA Values

Copper Size (mm2)

θJA (°C/W), 8-Lead LFCSP

146.6

100

105.4

500

75.38

1000

65.16

6400

53.5

Calculate the junction temperatures of the ADP7185 by

TJ = TA + (PD × θJA)

(5)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

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

(6)

where:

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

ILOAD is the load current.

IGND is the ground current.

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation

simplifies to

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

(7)

As shown in Equation 7, 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 50 to Figure 52 show the junction temperature calculations

for the different ambient temperatures, power dissipation, and

areas of the PCB copper.

140

100

120

02.5

2.0

1.5

1.0

0.5

POWER DISSIPATION (W)

TJ MAX

6400mm2

1000mm2

500mm2

100mm2

25mm2

Figure 50. Junction Temperature vs. Total Power Dissipation, TA = −25°C

02.5

2.0

1.5

1.0

0.5

140

100

120

POWER DISSIPATION (W)

TJ MAX

6400mm2

1000mm2

500mm2

100mm2

25mm2

Figure 51. Junction Temperature vs. Total Power Dissipation, TA = −50°C

02.5

2.0

1.5

1.0

0.5

140

100

120

POWER DISSIPATION (W)

TJ MAX

6400mm2

1000mm2

500mm2

100mm2

25mm2

Figure 52. Junction Temperature vs. Total Power Dissipation, TA = −85°C

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