ADP124/ADP125

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CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP124/ADP125 are protected from damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP124/ADP125 are designed to limit the current

when the output load reaches 750 mA (typical). When the output

load exceeds 750 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 con-

ditions (that is, high ambient temperature and power dissipation),

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

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

junction temperature cools to less than 135°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 VOUT to GND occurs.

At first, the ADP124/ADP125 limit the current so that only 750 mA

is conducted into the short. If self-heating causes the junction

temperature to rise above 150°C, thermal shutdown activates,

turning off the output and reducing the output current to zero.

When the junction temperature cools to less than 135°C, the

output turns on and conducts 750 mA into the short, again

causing the junction temperature to rise above 150°C. This

thermal oscillation between 135°C and 150°C results in a current

oscillation between 750 mA and 0 mA that continues as long as

the short remains at the output.

Current and thermal limit protections are intended to protect the

device from damage due to accidental overload conditions. For

reliable operation, the device power dissipation must be externally

limited so that the junction temperature does not exceed 125°C.

THERMAL CONSIDERATIONS

To guarantee reliable operation, the junction temperature of the

ADP124/ADP125 must not exceed 125°C. To ensure that the

junction temperature is less than 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

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

8-lead MSOP package for various PCB copper sizes.

40

102.8

100

75.5

500

42.5

1000

34.7

6400

26.1

The junction temperature of the ADP124/ADP125 can be

calculated from the following equation:

(2)

where:

(3)

where:

The power dissipation due to ground current is quite small and

can be ignored. Therefore, the junction temperature equation

can be simplified as follows:

(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 34

through Figure 40 show junction temperature calculations for

differentials, and areas of PCB copper.

In cases where the board temperature is known, the thermal

(5)