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Data Sheet

ADP151

APPLICATIONS INFORMATION

analog.com

Rev. J | 15 of 23

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 the 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-limit and thermal limit protections 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 temper-

ature of the die to exceed the maximum junction temperature of

125°C.

When the junction temperature exceeds 150°C, the converter

enters thermal shutdown. The converter recovers only after the

junction temperature has decreased below 135°C to prevent any

permanent damage. Therefore, thermal analysis for the chosen

application is 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

To guarantee reliable operation, the junction temperature of the

ADP151 must not exceed 125°C. To ensure that the junction tem-

perature 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, the 6-lead

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

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

LFCSP, and the 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

1 Device soldered to minimum size pin traces.

Table 7. Typical ΨJB Values

Model

ΨJB (°C/W)

5-Lead TSOT

4-Ball WLCSP

6-Lead LFCSP

28.3

To calculate the junction temperature of the ADP151, use 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:

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 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 differ-

entials, and areas of PCB copper.

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