Pam3103, Application information – Diodes PAM3103 User Manual
Page 7
PAM3103
Document number: DSxxxxx Rev. 1 - 5
7 of 11
July 2013
© Diodes Incorporated
PAM3103
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Thermal Considerations
Thermal protection limits power dissipation in the PAM3103. When the junct ion 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 continuous operation, the junction temperature should be maintained below +125°C. The power dissipation is defined as below:
I
*
V
I
*
V
V
P
GND
IN
O
OUT
IN
D
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:
JA
A
)
MAX
(
J
)
MAX
(
D
/
T
T
P
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 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:
W
4
.
0
250
/
C
25
C
125
P
)
MAX
(
D
It is also useful to calculate the junction temperature of the PAM3103 under a set of specific conditions. Suppose the input voltage V
IN
= 3.3V,
the output current I
O
=300mA and the case temperature T
A
= +40°C measured by a thermal couple during operation, the power dissipation is
defined as:
mW
150
A
70
*
V
3
.
3
mA
300
*
V
8
.
2
V
3
.
3
P
D
And the junction temperature T
J
can be calculated as follows:
T
J
= TA + PD*
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 application, T
J
is lower than the absolute maximum operating junction temperature +125°C, so it is safe to use the PAM3103 in this
configuration.