ADP151ACBZ-2.8-R7 datasheet(15/24 Pages) AD

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ADP151

Rev. 0 | Page 15 of 24

CURRENT LIMIT AND THERMAL OVERLOAD

PROTECTION

The ADP151 is protected against damage due to excessive

power dissipation by current and thermal overload protection

circuits. The ADP151 is designed to current limit when the

output load reaches 300 mA (typical). When the output load

exceeds 300 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

power dissipation) when the junction temperature starts to rise

above 150°C, the output is turned off, reducing the output

current to 0. 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 ground

occurs. At first, the ADP151 current limits, so that only 300 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 0. As the junction temperature

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

conducts 300 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

300 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 that junction temperatures do not exceed 125°C.

THERMAL CONSIDERATIONS

In most applications, the ADP151 does not dissipate much heat

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

ambient temperature, high supply voltage to 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 ADP151 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 5 shows typical θJA values of the 5-lead TSOT package for

various PCB copper sizes. Table 6 shows the typical ΨJB values of

the 5-lead TSOT and 4-ball WLCSP.

Table 5. Typical θJA Values

Copper Size (mm2)

θJA (°C/W)

TSOT

WLCSP

170

260

152

159

100

146

157

300

134

153

500

131

151

Device soldered to minimum size pin traces.

Table 6. Typical ΨJB Values

Model

ΨJB (°C/W)

TSOT

WLCSP

The junction temperature of the ADP151 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 that the junction temperature does not rise above 125°C.

Figure 37 to Figure 50 show junction temperature calculations

for different ambient temperatures, load currents, VIN-to-VOUT

differentials, and areas of PCB copper.

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