Data Sheet

ADP1706/ADP1707/ADP1708

APPLICATIONS INFORMATION

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Rev. B | 15 of 19

Consider the case where a hard short from OUT to ground occurs.

At first, the ADP1706/ADP1707/ADP1708 reach current limit so

that only 1.5 A is conducted into the short. If self-heating of the

junction becomes great enough to cause its temperature to rise

above 150°C, thermal shutdown activates, turning off the output

and reducing the output current to zero. As the junction temperature

cools and drops below 135°C, the output turns on and conducts

1.5 A into the short, again causing the junction temperature to rise

above 150°C. This thermal oscillation between 135°C and 150°C

causes a current oscillation between 1.5 A and 0 A that continues

as long as the short remains at the output.

Current and thermal limit protections are intended to protect the de-

vice against accidental overload conditions. For reliable operation,

device power dissipation should be externally limited so junction

temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP1706/ADP1707/ADP1708 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 pins of the package are

soldered on the PCB. Table 7 shows typical θJA values of the 8-lead

SOIC and 8-lead LFCSP for various PCB copper sizes.

Table 7. Typical θJA Values

Copper Size (mm2)

θJA (°C/W), SOIC

θJA (°C/W), LFCSP

01

57.6

65.9

50

53.1

62.3

100

52.3

61.2

300

51.3

59.7

500

51.3

59.4

1 Device soldered to minimum size pin traces.

The junction temperature of the ADP1706/ADP1707/ADP1708 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:

ILOAD is the load current.

IGND is the ground current.

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

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)

(5)

As shown in Equation 5, for a given ambient temperature, input-to-

output voltage differential, and continuous load current, a minimum

copper size requirement exists for the PCB to ensure the junction

temperature does not rise above 125°C. Figure 36 to Figure 41

show junction temperature calculations for different ambient tem-

peratures, load currents, VIN to VOUT differentials, and areas of PCB

copper.

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

Figure 37. 100 mm2 of PCB Copper, TA = 25°C, SOIC