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ADP121

Rev. 0 | Page 14 of 20

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP121 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP121 is designed to current limit when the

output load reaches 225 mA (typical). When the output load

exceeds 225 mA, the output voltage is reduced to maintain a

constant current limit.

Thermal overload protection is built-in, which limits the

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

extreme conditions (that is, high ambient temperature and

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 nominal value.

Consider the case where a hard short from VOUT to GND

occurs. At first, the ADP121 current limits, so that only

225 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 tempera-

ture cools and drops below 135°C, the output turns on and

conducts 225 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 225 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 junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

In most applications, the ADP121 does not dissipate a lot of

heat due to high efficiency. However, in applications with a high

ambient temperature and high supply voltage to an output voltage

differential, the heat dissipated in the package is large enough

that it can 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

ADP121 must not exceed 125°C. To ensure that 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 used

and the amount of copper to which the GND pins of the package

are soldered on the PCB. Table 5 shows typical θJA values for

various PCB copper sizes and Table 6 shows the typical ΨJB values

for the ADP121.

Table 5. Typical θJA Values

Copper Size (mm2)

TSOT (°C/W)

WLCSP (°C/W)

170

260

152

159

100

146

157

300

134

153

500

131

151

1 Device soldered to minimum size pin traces.

Table 6. Typical ΨJB Values

TSOT (°C/W)

WLCSP (°C/W)

42.8

58.4

The junction temperature of the ADP121 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

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 34 to Figure 47 show junction temperature

calculations for different ambient temperatures, load currents,

VIN-to-VOUT differentials, and areas of PCB copper.

In cases where the board temperature is known, the thermal

characterization parameter, ΨJB, can be used to estimate the

junction temperature rise. TJ is calculated from TB and PD using

the formula

TJ = TB + (PD × ΨJB)

(5)

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