ADP7182ACPZ-R7 datasheet(25/31 Pages) AD

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Data Sheet

ADP7182

Rev. I | Page 25 of 31

Current-limit and thermal overload protections are intended to

protect the device against accidental overload conditions. For

reliable operation, device power dissipation must be externally

limited so that the junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

In most applications, the ADP7182 does not dissipate much heat

due to the high efficiency. However, in applications with high

ambient temperature, and high supply voltage to output voltage

differential, the heat dissipated in the package is large enough that

it can 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. It recovers only after the junction

temperature has decreased 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 3.

To guarantee reliable operation, the junction temperature of the

ADP7182 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 that solders the package VIN pins to the PCB.

Table 8 and Table 9 show typical θJA values of the 6- and 8-lead

and 5-lead TSOT packages for various PCB copper sizes.

Table 10 shows the typical ΨJB values of the 6- and 8-lead and

and 5-lead TSOT.

Table 8. Typical θJA Values of the LFCSP

θJA (°C/W)

Copper Size (mm2)

8-Lead LFCSP

6-Lead LFCSP

251

175

177.8

100

135.6

138.2

500

77.3

79.8

1000

65.2

67.8

6400

53.5

1 Device soldered to minimum size pin traces.

Table 9. Typical θJA Values of the 5-Lead TSOT

Copper Size (mm2)

θJA (°C/W)

170

152

100

146

300

134

500

131

1 Device soldered to minimum size pin traces.

Table 10. Typical ΨJB Values

Model

ΨJB (°C/W)

6-lead LFCSP

44.1

8-lead LFCSP

18.2

5-lead TSOT

The junction temperature of the ADP7182 can be calculated by

TJ = TA + (PD × θJA)

(3)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

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

(4)

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}

(5)

As shown in Equation 5, 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 89

to Figure 97 show junction temperature calculations for different

ambient temperatures, power dissipation, and areas of PCB copper.

Heat dissipation from the package can be improved by increasing

the amount of copper attached to the pins of the ADP7182.

Adding thermal planes under the package also improves thermal

performance. However, as listed in Table 8 and Table 9, a point

of diminishing returns is reached eventually, beyond which an

increase in the copper area does not yield significant reduction

in the junction-to-ambient thermal resistance.

140

120

100

01.2

1.0

0.8

0.6

0.4

0.2

TOTAL POWER DISSIPATION (W)

6400mm2

1000mm2

500mm2

100mm2

25mm2

JEDEC

TJ MAX

Figure 89. Junction Temperature vs. Total Power Dissipation for the

8-Lead LFCSP, TA = 25°C

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