AMC7638
APPLICATION INFORMATION
The maximum power dissipation of a single-output regulator
PD(MAX) = [(VIN(MAX) - VOUT(NOM))] × IOUT(NOM) + VIN(MAX) × IQ
VOUT(NOM) = the nominal output voltage
I
OUT(NOM) = the nominal output current, and
IQ = the quiescent current the regulator consumes at IOUT(MAX)
VIN(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 150OC, the regulator will be turned
off.
O
When power consumption is over about 775mW (SOT89 package, at TA=70 C), additional heat sink is required
to control the junction temperature below 125 OC.
The junction temperature is: TJ = PD (θJT + θCS + θSA ) + TA
P D:Dissipated power.
θJT:Thermal resistance from the junction to the mounting tab of the package.
θCS:Thermal resistance through the interface between the IC and the surface on which it is mounted.
(typically, θCS < 1.0OC /W)
θSA: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 θSA (OC /W )
59
45
38
33
27
24
21
PCB heat sink size (mm2)
500
1000
1500
2000
3000
4000
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