ADP7104ACPZ-3.3-R7 datasheet(20/28 Pages) AD

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ADP7104

Data Sheet

Rev. B | Page 20 of 28

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP7104 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP7104 is designed to current limit when the

output load reaches 600 mA (typical). When the output load

exceeds 600 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 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 output current is

restored to its operating value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADP7104 current limits, so that only 600 mA

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

600 mA 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 600 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 against accidental overload conditions. For reliable

operation, device power dissipation must be externally limited

so the junction temperature does not exceed 125°C.

THERMAL CONSIDERATIONS

In applications with low input-to-output voltage differential, the

ADP7104 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 ADP7104 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 pins to the PCB.

Table 6 shows typical θJA values of the 8-lead SOIC and 8-lead

LFCSP packages for various PCB copper sizes. Table 7 shows

the typical ΨJB values of the 8-lead SOIC and 8-lead LFCSP.

Table 6. Typical θJA Values

Copper Size (mm2)

θJA (°C/W)

LFCSP

SOIC

251

165.1

167.8

100

125.8

111

500

68.1

65.9

1000

56.4

56.1

6400

42.1

45.8

Device soldered to minimum size pin traces.

Table 7. Typical ΨJB Values

Model

ΨJB (°C/W)

LFCSP

15.1

SOIC

31.3

The junction temperature of the ADP7104 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 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,

there exists a minimum copper size requirement for the PCB

to ensure that the junction temperature does not rise above 125°C.

Figure 70 to Figure 77 show junction temperature calculations

for different ambient temperatures, power dissipation, and areas

of PCB copper.

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