AMC DOC. #: AMC7580_E (LF)
AMC7580
Feb 2005
7A LOW DROPOUT REGULATOR
Maximum Power Calculation:
TJ(MAX) – TA(MAX)
PD(MAX)=
θ
JA
TJ(
OC): Maximum recommended junction temperature
TA(
OC): Ambient temperature of the application
θ
JA(
OC/W): Junction-to-junction temperature thermal resistance of the package, and other heat dissipating
materials.
The maximum power dissipation of a single-output regulator :
PD(MAX) = [(VIN(MAX) - VOUT(NOM))] × IOUT(NOM) + VIN(MAX) × IQ
Where: VOUT(NOM) = the nominal output voltage
IOUT(NOM) = the nominal output current, and
IQ = the quiescent current the regulator consumes at IOUT(MAX)
VIN(MAX) = the maximum input voltage
Then
θ
JA = (150
OC – TA) / PD
Thermal consideration:
When power consumption is over about 1.2W( at 70oC ambient temperature), additional heat sink is required to control the
junction temperature below 125OC.
The junction temperature is: Tj = P D (θJT + θCS + θSA ) + T A
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 < 1.0 °C / 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 (°C / 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
(Bottom View)
(Top View)
Copyright
© 2002, ADD Microtech Corp.
10
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