ADP7118ACPZN2.5-R7 datasheet(17/24 Pages) AD

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Data Sheet

ADP7118

Rev. F | Page 17 of 24

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP7118 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP7118 is designed to current limit when the

output load reaches 360 mA (typical). When the output load

exceeds 360 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 the operating value.

Consider the case where a hard short from VOUT to ground

occurs. At first, the ADP7118 current limits, so that only 360 mA

is conducted into the short. If self heating of the junction is

great enough to cause the 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

360 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 360 mA and 0 mA that continues as long as the short

remains at the output.

Current and thermal limit protections 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 a low input-to-output voltage differential,

the ADP7118 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 to cause 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 ADP7118 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, 6-lead LFCSP,

and 5-lead TSOT packages for various PCB copper sizes. Table 7

shows the typical ΨJB values of the 8-lead SOIC, 6-lead LFCSP,

and 5-lead TSOT.

Table 6. Typical θJA Values

Copper Size (mm2)

θJA (°C/W)

LFCSP

SOIC

TSOT

251

182.8

N/A2

181.4

152

100

142.6

145.4

146

500

83.9

89.3

131

1000

71.7

77.5

N/A2

6400

57.4

63.2

N/A2

Device soldered to minimum size pin traces.

N/A means not applicable.

Table 7. Typical ΨJB Values

Model

ΨJB (°C/W)

6-Lead LFCSP

8-Lead SOIC

38.8

5-Lead TSOT

To calculate the junction temperature of the ADP7118, use

Equation 1.

TJ = TA + (PD × θJA)

where:

TA is the ambient temperature.

PD is the power dissipation in the die, given by

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

(10)

where:

VIN and VOUT are input and output voltages, respectively.

ILOAD is the load current.

IGND is the ground current.

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}

(11)

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 52 to Figure 60 show junction temperature calculations

for different ambient temperatures, power dissipation, and

areas of PCB copper.

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