AMC1117
Application Note:
Maximum Power Calculation:
T
J(MAX)
– T
A(MAX)
θ
JA
O
T
J
( C): Maximum recommended junction temperature
T
A
(
O
C): Ambient temperature of the application
θ
JA
(
O
C/W): Junction-to-junction temperature thermal resistance of the package, and other heat dissipating
materials.
The maximum power dissipation of a single-output regulator
:
P
D(MAX)
=
[(V
IN(MAX)
- V
OUT(NOM)
)]
×
I
OUT(NOM)
+ V
IN(MAX)
×
I
Q
P
D(MAX)
=
Where:
V
OUT(NOM)
= the nominal output voltage
I
OUT(NOM)
= the nominal output current, and
I
Q
= the quiescent current the regulator consumes at I
OUT(MAX)
V
IN(MAX)
= the maximum input voltage
θ
JA
= (150
O
C – T
A
)/ P
D
Then
Thermal consideration:
When power consumption is over about 404 mW (for SOT-223 package, 687mW for TO-252 package, at T
A
=70
O
C),
additional heat sink is required to control the junction temperature below 125
O
C.
The junction temperature is: T
J
= P
D
(θ
JT
+
θ
CS
+
θ
SA
) + T
A
P
D
: Dissipated power.
θ
JT
: Thermal resistance from the junction to the mounting tab of the package.
θ
CS
: Thermal resistance through the interface between the IC and the surface on which it is mounted. (Typically,
θ
CS
< 1.0°C/W)
θ
SA
: Thermal resistance from the mounting surface to ambient (thermal resistance of the heat sink).
If PC Board copper is going to be used as a heat sink, below table can be used to determine the appropriate size of
copper foil required. For multi-layered PCB, these layers can also be used as a heat sink. They can be connected with
several through hole vias.
59
PCB
θ
SA
(
O
C /W)
2
PCB heat sink size (mm ) 500
45
1000
38
1500
33
2000
27
3000
24
4000
21
5000
Recommended figure of PCB area used as a heat sink.
through hole vias
(Top View)
(Bottom View)
Copyright
©
2006 ADDtek Corp.
7
DD004_K --
JULY 2006
ADDTEK [ ADDTEK CORP ]
