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ADP120

Rev. A | Page 14 of 20

CURRENT-LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP120 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP120 is designed to current limit when the

output load reaches 150 mA (typical). When the output load

exceeds 150 mA, the output voltage reduces to maintain a

constant current limit.

Thermal overload protection is built-in, limiting the junction

temperature to a maximum of 150°C (typical). Under extreme

conditions (that is, high ambient temperature and power dissi-

pation) when the junction temperature starts to rise above 150°C,

the output turns off, reducing the output current to zero. When

the junction temperature drops below 135°C, the output turns

on again thereby restoring output current to its nominal value.

Consider the case where a hard short from VOUT to GND

occurs. At first, the ADP120 current limits, conducting only

150 mA 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 150 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 150 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

to prevent junction temperatures from exceeding 125°C.

THERMAL CONSIDERATIONS

In most applications, the ADP120 does not dissipate much heat

due to its high efficiency. However, in applications with high

ambient temperature and high supply voltage-to-output voltage

differential, the heat dissipated in the package is 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 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 ADP120 must not exceed 125°C. To ensure the junction

temperature stays below this maximum value, the user needs to

be aware of the parameters that contribute to junction

temperature changes. These parameters include ambient temper-

ature, 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

5-lead TSOT and 4-ball WLCSP packages for various PCB

copper sizes. Table 7 shows the typical ΨJB value of the 5-lead

TSOT and 4-ball WLCSP.

Table 6. Typical θJA Values

Copper Size (mm2)

θJA (°C/W)

TSOT

WLCSP

170

260

152

159

100

146

157

300

134

153

500

131

151

1 Device soldered to minimum size pin traces.

Table 7. Typical ΨJB Values

ΨJB (°C/W)

TSOT

WLCSP

42.8

58.4

The junction temperature of the ADP120 can be 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

the junction temperature does not rise above 125°C. The following

figures show junction temperature calculations for different

ambient temperatures, load currents, VIN-to-VOUT differentials,

and areas of PCB copper.

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