PAM3103ADJUBADJ datasheet(7/10 Pages) PAM

7 / 10 page

Thermal considerations

Thermal protection limits power dissipation in the

PAM3103. When the junction temperature

exceeds 150°C, the OTP (Over Temperature

Protection) starts the thermal shutdown and turns

the pass transistor off. The pass transistor

resumes operation after the junction temperature

drops below 120°C.

For

c ontinuous

operation,

the

junction

temperature should be maintained below 125°C.

The power dissipation is defined as below:

P = (V -V

)*I +V *I

The maximum power dissipation depends on the

thermal resistance of IC package, PCB layout,

the rate of surrounding airflow and temperature

difference between junction and ambient. The

maximum power dissipation can be calculated by

the following formula:

Where T

is the maximum allowable junction

temperature 125°C, T is the ambient temperature

and

is the thermal resistance from the junction

to the ambient.

For example, as

is 250°C/W for the SOT-23

package based on the standard JEDEC 51-3 for

a single-layer thermal test board, the maximum

power dissipation at T =25°C can be calculated

by following formula:

= (125°C-25°C)/250=0.4W

It is also useful to calculate the junction

temperature of the PAM3103 under a set of

specific conditions. Suppose the input voltage

V =3.3V, the output current I =300mA and the

case temperature T =40°C measured by a

thermal couple during operation, the power

dissipation is defined as:

P =(3.3V-2.8V)*300mA+3.3V*70uA

150mW

And t he junction temperature T

can b e

calculated as follows:

T= T

T = 40°C+0.15W*250°C/W

=40°C+37.5°C

=77.5°C<T

= 125°C

For this application, T is lower than the absolute

maximum operating junction temperature

125°C, so it is safe to use the PAM3103 in this

configuration.

OUT

GND

J(MAX)

D(MAX)

J(MAX)

= (T

-T )/θ

+P *θ

D(MAX)

J(MAX)

≌

09/2008 Rev 1.2

Power Analog Microelectronics Inc

www.poweranalog.com

PAM3103

300mA Adjustable High PSRR CMOS Linear Regulator

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