A705
APPLICATION INFORMATION
The Maximum Power Dissipation on Regulator:
PD(MAX) = VOUT(MAX) × IOUT(NOM) + VIN(MAX) × IQ
VOUT(MAX) = the maximum voltage on output pin;
IOUT(NOM) = the nominal output current;
IQ = the quiescent current the regulator consumes at IOUT(MAX)
VIN(MAX) = the maximum input voltage.
;
Thermal Consideration:
The A705 has 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 A705 prevents the device from damage due to excessive power dissipation.
When the device temperature rises to approximately 150°C, the regulator will be turned off. When power consumption
is over about 700mW (SOT-89 package, at TA=70°C) or 1000mW (TO-252 package, at TA=70°C), additional heat sink
is required to control the junction temperature below 120°C.
The junction temperature is:
TJ = PD (θJT +θCS +θSA ) + TA
PD : 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)
PCB heat sink size (mm2)
59
45
38
33
27
24
21
500
1000
1500
2000
3000
4000
5000
Copyright © 2007 ADDtek Corp.
4
DD077_A -- AUGUST 2007
ADDTEK [ ADDTEK CORP ]
