ADP151ACBZ-2.75-R7 datasheet(15/24 Pages) AD

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Data Sheet

ADP151

Rev. E | 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

causes 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 a high

ambient temperature and a high supply voltage to output voltage

differential, the heat dissipated in the package 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 6 shows typical θJA values of the 5-lead TSOT, 6-lead

LFCSP, and 4-ball WLCSP packages for various PCB copper sizes.

Table 7 shows the typical ΨJB values of the 5-lead TSOT, 6-lead

LFCSP, and 4-ball WLCSP.

Table 6. Typical θJA Values

Copper Size (mm2)

θJA (°C/W)

TSOT

WLCSP

LFCSP

170

260

231.2

152

159

161.8

100

146

157

150.1

300

134

153

111.5

500

131

151

91.8

Device soldered to minimum size pin traces.

Table 7. Typical ΨJB Values

Model

ΨJB (°C/W)

TSOT

WLCSP

LFCSP

28.3

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 39

through Figure 59 show junction temperature calculations for

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

differentials, and areas of PCB copper.

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