ADM7171ACPZ-R7 datasheet(21/24 Pages) AD

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ADM7171

Data Sheet

Rev. C | Page 20 of 23

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 ADM7171 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

ADM7171 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 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 ADM7171 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

Device soldered to minimum size pin traces.

The junction temperature of the ADM7171 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 63

to Figure 65 show junction temperature calculations for differ-

ent ambient temperatures, power dissipation, and areas of PCB

copper.

Figure 63. LFCSP, TA = 25°C

TOTAL POWER DISSIPATION (W)

6400mm2

500mm2

25mm2

TJ MAX

105

115

125

135

145

155

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

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