Data Sheet

ADP7105

Rev. 0 | Page 21 of 28

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP7105 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP7105 is designed to limit the current when the

output load reaches 775 mA (typical). When the output load

exceeds 775 mA, the output voltage is reduced to maintain a

constant current limit. As the output voltage drops, the current

is folded back to approximately 50 mA to minimize heat

generation inside the LDO regulator.

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 falls 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 ADP7105 limits the current so that only

775 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 is activated, turning off the output and

reducing the output current to zero. As the junction temperature

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

775 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 775 mA and

0 mA that continues as long as the short remains at the output.

Current-limit and thermal limit protections are intended to

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 ADP7105 does not dissipate much heat. However, in

applications with high ambient temperature and/or high input

voltage, the heat dissipated in the package may become

significant enough that it causes the junction temperature of the

die to exceed the maximum junction temperature of 125°C.

When the junction temperature exceeds 150°C, the regulator

enters thermal shutdown. It recovers only after the junction

temperature decreases 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 tempera-

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

in Equation 2.

To guarantee reliable operation, the junction temperature of the

ADP7105 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 thermal resistances between

the package assembly compounds that are used and the amount of

copper used to solder the package GND pins to the PCB.

LFCSP packages for various PCB copper sizes. Table 7 shows

with PCB area.

LFCSP

SOIC

251

165.1

167.8

100

125.8

111

500

68.1

65.9

1000

56.4

56.1

6400

42.1

45.8

1

Device soldered to minimum size pin traces.

Model

8-Lead LFCSP1

15.1

8-Lead SOIC

31.3

1

is being used as a heat sink via the exposed pad, whereas the value in Table 4 is

per the JEDEC standard.

The junction temperature of the ADP7105 is 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, a

minimum copper size requirement for the PCB exists to ensure

that the junction temperature does not rise above 125°C. Figure 71

to Figure 76 show junction temperature calculations for

different ambient temperatures, power dissipation, and areas of

PCB copper.