ADM7172ACPZ-5.0-R7 datasheet(21/24 Pages) AD

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ADM7172

Data Sheet

Rev. C | Page 20 of 23

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADM7172 is protected against damage due to excessive

power dissipation by current-limit and thermal overload protection

circuits. The ADM7172 is designed to current limit when the

output load reaches 3 A (typical). When the output load exceeds

3 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 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 the output current is

restored to its operating value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADM7172 current limits, so that only 3 A 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 3 A 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 3 A and 0 mA that

continues for 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 low input-to-output voltage differential, the

ADM7172 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 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

ADM7172 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 junction and ambient air (θJA). The θJA number is dependent

on the package assembly compounds that are used and the

amount of copper used to solder the package GND pin to the PCB.

Table 7 shows typical θJA values of the 8-lead LFCSP package for

various PCB copper sizes. The typical value of ΨJB is 15.1°C/W for

the 8-lead LFCSP package.

Table 7. Typical θJA Values

Copper Size (mm2)

θJA (°C/W) of LFCSP

251

165.1

100

125.8

500

68.1

1000

56.4

6400

42.1

1 Device soldered to minimum size pin traces.

The junction temperature of the ADM7172 is calculated from

the following equation:

TJ = TA + (PD × θJA)

(2)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

PD = ((VIN − VOUT) × ILOAD) + (VIN × IGND)

(3)

where:

ILOAD is the load current.

IGND is the ground current.

VIN and VOUT are the input and output voltages, respectively.

Power dissipation due to ground current is quite small and can

be ignored. Therefore, the junction temperature equation

simplifies to the following:

TJ = TA + (((VIN − VOUT) × ILOAD) × θJA)

(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 exists for the PCB to ensure

that the junction temperature does not rise above 125°C. Figure 65

to Figure 67 show junction temperature calculations for different

ambient temperatures, power dissipation, and areas of PCB copper.

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