ADP1764ACPZ-1.1-R7 datasheet(17/20 Pages) AD

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Data Sheet

ADP1764

Rev. A | Page 17 of 20

THERMAL CONSIDERATIONS

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

the ADP1764 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 152°C, the regulator

enters thermal shutdown. The regulator recovers only after the

junction temperature decreases below 136°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 board temperature and the temperature rise of the package

due to the power dissipation, as shown in Equation 6.

To guarantee reliable operation, the junction temperature of the

ADP1764 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 board temperature, power

dissipation in the power device, and thermal characterization

parameter between the junction and board (ΨJB). The ΨJB

parameter is dependent on the package assembly compounds and

the PCB copper area. Table 7 shows the typical ΨJB values for the

16-lead LFCSP package for various PCB copper areas.

Table 7. Typical Non-JEDEC ΨJB Values

PCB Copper Area (mm2)

ΨJB (°C/W) at 2W

71.05

100

18.9

500

13.45

1000

13.15

Calculate the junction temperatures of the ADP1764 by

TJ = TB + (PD × ΨJB)

(6)

where:

TB is the board temperature.

PD is the power dissipation in the die, given by

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

(7)

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 = TB + (((VIN − VOUT) × ILOAD) × ΨJB)

(8)

As shown in Equation 8, for a given board temperature, input-

to-output voltage differential, and continuous load current,

a minimum copper area requirement exists for the PCB to ensure

that the junction temperature does not rise above 125°C.

Figure 50 to Figure 55 show the junction temperature calculations

for the different board temperatures, power dissipation, and

areas of the PCB copper.

140

1.4

VIN – VOUT (V)

100

120

0.2

0.4

0.6

0.8

1.0

1.2

0.1A

1.0A

2.0A

3.0A

4.0A

TJ MAX

Figure 50. 1000 mm2 of PCB Copper, TB = 25°C

140

1.4

VIN – VOUT (V)

100

120

0.2

0.4

0.6

0.8

1.0

1.2

0.1A

1.0A

2.0A

3.0A

4.0A

TJ MAX

Figure 51. 500 mm2 of PCB Copper, TB = 25°C

140

1.4

VIN – VOUT (V)

100

120

0.2

0.4

0.6

0.8

1.0

1.2

0.1A

1.0A

2.0A

3.0A

4.0A

TJ MAX

Figure 52. 100 mm2 of PCB Copper, TB = 25°C

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