PAM3101
300mA High PSRR Low Dropout CMOS Linear Regulator
Thermal considerations
Thermal protection limits power dissipation in the
PA M 3 1 0 1 . W h e n t h e j u n c t i o n t e m p e r a t u r e
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 continuous operation, the junction
temperature should be maintained below 125°C.
The power dissipation is defined as:
P
D
= (V
IN
-V
OUT
)*I
O
+V
IN
*I
GND
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:
P
D(MAX)
= (T
J(MAX)
-T
A
)/θ
JA
Where T
J(MAX)
is the maximum allowable junction
temperature 125°C, T
A
is the ambient temperature
and θ
JA
is the thermal resistance from the junction
to the ambient.
For example, as θ
JA
is 250°C/W for the SOT-23
package and 180°C/W for the SOT-89 package
based on the standard JEDEC 51-3 for a single-
layer thermal test board, the maximum power
dissipation at T
A
=25°C can be calculated by
following formula:
P
D(MAX)
= (125°C-25°C)/250=0.4W
P
D(MAX)
= (125°C-25°C)/180=0.55W
SOT-23
SOT-89
It is also useful to calculate the junction
temperature of the PAM3101 under a set of
specific conditions. For example, suppose the
input voltage V
I N
=3.3V, the output current
I
O
=300mA and the case temperature T
A
=40°C
measured by a thermalcouple during operation,
the power dissipation for the Vo=2.8V version of
the PAM3101 can be calculated as:
P
D
=(3.3V-2.8V)*300mA+3.3V*70 μ A
≌
150mW
And the junction temperature, T
J
, can be
calculated as follows:
T
J
= T
A
+P
D
*θ
JA
T
J
= 40°C+0.15W*250°C/W
=40°C+37.5°C
=77.5°C<T
J(MAX)
= 125°C
For this operating condition, T
J
is lower than the
absolute maximum operating junction
temperature, 125°C, so it is safe to use the
PAM3101 in this configuration.
Power Analog Microelectronics
,
Inc
www.poweranalog.com
09/2008 Rev 1.4
13
PAM [ POWER ANALOG MICOELECTRONICS ]
