Data Sheet

ADP1763

APPLICATIONS INFORMATION

analog.com

Rev. E | 17 of 22

Use Equation 5 to calculate the output of the two paralleled

ADP1763 LDOs.

VOUT=2×AD× RADJ×IADJ

(5)

where:

AD is the gain factor with a typical value of 3.0 between the VADJ

pin and the VOUT pin.

IADJ is the 50.0 µA constant current out of the VADJ pin.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP1763 must not exceed 125°C. To ensure that the junction

temperature stays below this maximum value, the user needs to

be aware of the parameters that contribute to junction temperature

changes. These parameters include ambient temperature, power

dissipation in the power device, and thermal resistance between the

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

package assembly compounds used and the amount of copper to

which the GND pin and the exposed pad (EPAD) of the package

are soldered on the PCB. Table 6 shows typical θJA values for the

16-lead LFCSP for various PCB copper sizes. Table 7 shows typical

ΨJB values for the 16-lead LFCSP.

Table 6. Typical θJA Values

Copper Size (mm2)

θJA (°C/W), LFCSP

25

138.1

100

102.9

500

76.9

1000

67.3

6400

56

Table 7. Typical ΨJB Values

Copper Size (mm2)

ΨJB (°C/W) at 1 W

100

33.3

500

28.9

1000

28.5

To calculate the junction temperature of the ADP1763, use the

following equation:

TJ=TA+ PD×θJA

(6)

where:

TA is the ambient 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.

As shown in Equation 6, for a given ambient temperature, and

computed power dissipation, a minimum copper size requirement

exists for the PCB to ensure that the junction temperature does not

rise above 125°C.

Figure 35 through Figure 40 show junction temperature calculations

for different ambient temperatures, load currents, VIN to VOUT differ-

entials, and areas of PCB copper.

Figure 35. 6400 mm2 of PCB Copper, TA = 25°C

Figure 36. 500 mm2 of PCB Copper, TA = 25°C

Figure 37. 25 mm2 of PCB Copper, TA = 25°C