AMC1117
Application Note:
Maximum Power Calculation:
T
J(MAX) – TA(MAX)
PD(MAX)
=
θJA
TJ(OC): Maximum recommended junction temperature
TA(OC): Ambient temperature of the application
O
Junction-to-junction temperature thermal resistance of the package, and other heat dissipating
materials.
θJA( C/W):
The maximum power dissipation of a single-output regulator :
PD(MAX) = [(VIN(MAX) - VOUT(NOM))] × IOUT(NOM) + VIN(MAX) × IQ
Where:
Then
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
θJA = (150OC – TA)/ PD
Thermal consideration:
When power consumption is over about 404 mW (for SOT-223 package, 687mW for TO-252 package, at TA=70OC),
additional heat sink is required to control the junction temperature below 125OC.
The junction temperature is: TJ = P D (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 < 1.0°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 (OC /W)
59
45
1000
38
1500
33
2000
27
3000
24
4000
21
5000
PCB heat sink size (mm2) 500
Recommended figure of PCB area used as a heat sink.
through hole vias
(Bottom View)
(Top View)
Copyright © 2006 ADDtek Corp.
7
DD004_K -- JULY 2006
ADDTEK [ ADDTEK CORP ]
