AMC7638
APPLICATION INFORMATION
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
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
Thermal consideration
The AMC7638 series have internal power and thermal limiting circuitry designed to protect the device under
overload conditions. However maximum junction temperature ratings should not be exceeded under continuous normal
load conditions. The thermal protection circuit of AMC7638 series will prevent the device from damage due to
excessive power dissipation. When the device temperature rises to approximately 150
O
C, the regulator will be turned
off.
When power consumption is over about 775mW (SOT89 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
O
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.
PCB
θ
SA
(
O
C /W )
PCB heat sink size (mm
2
)
59
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
©
2007 ADDtek Corp.
6
DD054_N
--
MAY 2007
ADDTEK [ ADDTEK CORP ]
