ADP1707ARDZ-0.9-R7 datasheet(13/20 Pages) AD

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ADP1706/ADP1707/ADP1708

Rev. 0 | Page 13 of 20

Thermal overload protection is included, which limits the

junction temperature to a maximum of 150°C (typical). Under

extreme conditions (that is, high ambient temperature and

power dissipation) when the junction temperature starts to

rise above 150°C, the output is turned off, reducing the output

current to zero. When the junction temperature drops below

135°C (typical), the output is turned on again and output

current is restored to its nominal value.

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 device 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 contrib-

ute 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 5 shows typical θJA values

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

Table 5. Typical θJA Values

Copper Size (mm2)

θJA (°C/W), SOIC

θJA (°C/W), LFCSP

57.6

65.9

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 temperatures, load currents, VIN to VOUT

differentials, and areas of PCB copper.

140

0.5

5.0

VIN – VOUT (V)

120

100

1.01.5

2.02.53.03.5

4.04.5

1mA

10mA

100mA

300mA

500mA

750mA

(LOAD CURRENT)

MAX TJ (DO NOT OPERATE ABOVE THIS POINT)

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

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