Data Sheet

ADP7142

Rev. H | Page 17 of 23

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP7142 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP7142 is designed to current limit when the

output load reaches 360 mA (typical). When the output load

exceeds 360 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/or

high 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, the output is turned on again, and output current

is restored to its operating value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADP7142 current limits, so that only 360 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 zero. As the junction temperature cools and

drops below 135°C, the output turns on and conducts 360 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 360 mA and 0 mA that

continues as long as the short remains at the output.

Current and thermal limit protections protect the device

against accidental overload conditions. For reliable operation,

device power dissipation must be externally limited so that the

junction temperature does not exceed 125°C.

THERMAL CONSIDERATIONS

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

the ADP7142 does not dissipate much heat. However, in appli-

cations 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 150°C, the converter

enters thermal shutdown. It recovers only after the junction

temperature has decreased below 135°C to prevent any permanent

damage. Therefore, thermal analysis for the chosen application

is very important to guarantee reliable performance over all

conditions. The junction temperature of the die is the sum of

the ambient temperature of the environment and the temperature

rise of the package due to the power dissipation, as shown in

Equation 2.

To guarantee reliable operation, the junction temperature of

the ADP7142 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 ambient

temperature, power dissipation in the power device, and

compounds that are used and the amount of copper used to

solder the package GND pins to the PCB.

and 5-Lead TSOT packages for various PCB copper sizes. Table 7

and 5-lead TSOT.

LFCSP

SOIC

TSOT

182.8

50

181.4

152

100

142.6

145.4

146

500

83.9

89.3

131

1000

71.7

77.5

6400

57.4

63.2

Model

6-Lead LFCSP

24

8-Lead SOIC

38.8

5-Lead TSOT

43

To calculate the junction temperature of the ADP7142, use

Equation 1:

where:

(10)

where:

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation

simplifies to the following:

(11)

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 52

to Figure 60 show junction temperature calculations for different

ambient temperatures, power dissipation, and areas of PCB

copper.