ADP7112ACBZ-3.3-R7 datasheet(17/21 Pages) AD

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Data Sheet

ADP7112

Rev. D | Page 17 of 21

THERMAL CONSIDERATIONS

In applications with a low input-to-output voltage differential,

the ADP7112 does not dissipate much heat. However, in

applications with high ambient temperature and/or high input

voltage, the heat dissipated in the package can 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. 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 very 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 1.

To guarantee reliable operation, the junction temperature of

the ADP7112 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 solders the package GND pin

to the PCB.

Table 6 shows typical θJA values of the 6-ball WLCSP package

for various PCB copper sizes. The typical ΨJB value for the

6-ball WLCSP package is 58°C/W.

Table 6. Typical θJA Values

Copper Size (mm)

θJA (°C/W) for WLCSP

251

260

159

100

157

500

151

1 Device soldered to minimum size pin traces.

To calculate the junction temperature of the ADP7112, use

Equation 1.

TJ = TA + (PD × θJA)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

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

(10)

where:

VIN and VOUT are 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 the following:

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

(11)

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 49 to Figure 51 show junction temperature calculations

for different ambient temperatures, power dissipation, and

areas of PCB copper.

In the case where the board temperature is known, use the

thermal characterization parameter, ΨJB, to estimate the

junction temperature rise (see Figure 52). Calculate the

maximum junction temperature by using Equation 2.

TJ = TB + (PD × ΨJB)

The typical value of ΨJB is 58°C/W for the 6-ball WLCSP package.

145

135

125

115

105

0.1

0.2

0.3

0.4

0.5

TOTAL POWER DISSIPATION (W)

0.6

0.7

0.8

0.9

1.0

25mm2

100mm2

500mm2

TJ MAX

Figure 49. WLCSP, TA = 25°C

100

110

120

130

140

0.1

0.2

0.30.40.50.60.70.80.9

1.0

TOTAL POWER DISSIPATION (W)

25mm2

100mm2

500mm2

TJ MAX

Figure 50. WLCSP, TA = 50°C

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