Data Sheet

ADM7150

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADM7150 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADM7150 is designed to current-limit when the

output load reaches 1.2 A (typical). When the output load

exceeds 1.2 A, 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 155°C (typical). Under

extreme conditions (that is, high ambient temperature and/or

high 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 output current is

restored to its operating value.

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

At first, the ADM7150 current limits, so that only 1.2 A 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 temperature cools and

drops below 140°C, the output turns on and conducts 1.2 A into

the short, again causing the junction temperature to rise above

155°C. This thermal oscillation between 140°C and 155°C

causes a current oscillation between 1.2 A 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 150°C.

THERMAL CONSIDERATIONS

In applications with low input to output voltage differential, the

ADM7150 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 large enough that it

causes the junction temperature of the die to exceed the maximum

junction temperature of 150°C.

When the junction temperature exceeds 155°C, the converter

enters thermal shutdown. It recovers only after the junction

temperature decreases below 140°C to prevent any permanent

damage. Therefore, thermal analysis for the chosen application

is 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

ADM7150 must not exceed 150°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

dependent on the package assembly compounds that are used

and the amount of copper used to solder the package GND pin

and exposed pad to the PCB.

LFCSP packages for various PCB copper sizes.

8-lead LFCSP.

8-Lead LFCSP

8-Lead SOIC

251

165.1

165

100

125.8

126.4

500

68.1

69.8

1000

56.4

57.8

6400

42.1

43.6

1

Device soldered to minimum size pin traces.

Package

8-Lead LFCSP

15.1

8-Lead SOIC

17.9

The junction temperature of the ADM7150 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, there

exists a minimum copper size requirement for the PCB to ensure

that the junction temperature does not rise above 150°C.

The heat dissipation from the package can be improved by

increasing the amount of copper attached to the pins and

exposed pad of the ADM7150. Adding thermal planes under

the package also improves thermal performance. However, as

listed in Table 6, a point of diminishing returns is eventually

reached, beyond which an increase in the copper area does not

yield significant reduction in the junction to ambient thermal

resistance.

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