ADP172

Rev. B | Page 14 of 20

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP172 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP172 is designed to limit the current when the

output load reaches 450 mA (typical). When the output load

exceeds 450 mA, the output voltage is reduced to maintain a

constant current limit.

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 dissip-

ation), when the junction temperature starts to rise above 150°C,

the output is turned off, reducing the output current to 0. When

the junction temperature drops below 135°C, the output is turned

on again, and output current is restored to its nominal value.

Consider the case where a hard short from VOUT to GND occurs.

At first, the ADP172 limits the current so that only 450 mA is

conducted into the short. If self-heating of the junction is great

enough to cause its temperature to rise above 150°C, thermal

shutdown activates, turning off the output and reducing the

output current to 0. As the junction temperature cools and

drops below 135°C, the output turns on and conducts 450 mA

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 450 mA and 0 mA, which

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.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP172 must not exceed 125°C. To ensure that the junction temp-

erature 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

on the package assembly compounds used and the amount of

copper to which the GND pin of the package is soldered on the

for various PCB copper sizes.

01

260

50

159

100

157

300

153

500

151

The junction temperature of the ADP172 can be calculated

from the following equation:

(2)

where:

(3)

where:

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation

simplifies to the following:

(4)

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 30 to Figure 35 show junction temperature calculations

differentials, and areas of PCB copper.

0

20

40

60

80

100

120

140

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5