ADP320

Rev. 0 | Page 17 of 20

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP320 triple LDO is protected against damage due to

excessive power dissipation by current and thermal overload

protection circuits. The ADP320 triple LDO is designed to

current limit when the output load reaches 300 mA (typical).

When the output load exceeds 300 mA, the output voltage is

reduced to maintain a constant current limit.

Thermal overload protection is built-in, which limits the

junction temperature to a maximum of 155°C (typical). Under

extreme conditions (that is, high ambient temperature and

power dissipation) when the junction temperature starts to

rise above 155°C, the output is turned off, reducing the output

current to zero. When the junction temperature drops below

140°C, the output is turned on again and the output current

is restored to its nominal value.

Consider the case where a hard short from VOUTx to GND

occurs. At first, the ADP320 triple LDO current limits, so that

only 300 mA is conducted into the short. If self-heating of the

junction is great enough to cause its temperature to rise above

155°C, thermal shutdown activates turning off the output and

reducing the output current to zero. As the junction tempera-

ture cools and drops below 140°C, the output turns on and

conducts 300 mA into the short, again causing the junction

temperature to rise above 155°C. This thermal oscillation

between 140°C and 154°C causes a current oscillation between

0 mA and 300 mA 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 must be externally limited

so junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

In most applications, the ADP320 triple LDO does not dissipate

a lot of heat due to high efficiency. However, in applications

with a high ambient temperature and high supply voltage to out-

put voltage differential, the heat dissipated in the package is

large enough that it can cause the junction temperature of the

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

When the junction temperature exceeds 155°C, the converter

enters thermal shutdown. It recovers only after the junction

temperature has decreased below 140°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 ADP320 triple LDO 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 tem-

perature, power dissipation in the power device, and thermal

number is dependent on the package assembly compounds used

and the amount of copper to which the GND pins of the package

ADP320 triple LDO for various PCB copper sizes.

ADP320 Triple LDO (°C/W)

49.5

100

83.7

500

68.5

1000

64.7

The junction temperature of the ADP320 triple LDO 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

(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 the junction temperature does not rise

above 125°C. Figure 47 to Figure 50 show junction temperature

calculations for different ambient temperatures, total power

dissipation, and areas of PCB copper.

In cases where the board temperature is known, the thermal

the formula

(5)

25.2°C/W.