AN018 Datasheet, PDF (1/6 Pages) Analog Intergrations Corporation – High Power

AN018
An Analysis for Power Dissipation of LDO Application
with High Power
When an LDO is operating, its on-chip devices
dissipate power as a heat way. Heat flow from a
higher to a lower temperature region. For LDO
regulator, most of the dissipated power comes
from the power or pass transistor of IC. Therefore,
the current passes through the power transistor
and the voltage across the transistor, which
determine the amount of heat generated. The
quantity that resists or impedes this flow of heat
energy is called thermal resistance.
Introduction:
The paper provides user to know the thermal
dissipation of LDO regulators, how to select a heat
sink and prevent thermal problem from exceeding
maximum allowable junction temperature. For LDO
regulators, the thermal resistance data of different
packages are compliant with different applications
and user’s requirements, as well as determine the
thermal limits of different packages as in the spec.
Thermal resistance, such as Rθ_JA and Rθ_JC, are
applied to determine thermal dissipation performance
for LDO regulator. A low thermal resistance
represents better performance than a high thermal
resistance. A system that has a lower thermal
resistance can either dissipate more heat for a given
temperature difference or dissipate a given amount of
heat with a smaller temperature difference.
May 2001
Thermal topology of LDO:
As shown in figure 1, the thermal resistance is the
resistance from the package to heat dissipation and is
the reciprocal of thermal conductivity of the package.
The source of heat is the chip in the package. All
materials in the heat flow path between the IC and
the outside environment have thermal resistance and
the temperature of the chip rises above ambient
environment. Therefore, the thermal conductivity of
the silicon chip, molding compound, copper of
leadframe, and the thickness and thermal conductivity
of die adhesion material all affect the rate at which
the heat is dissipated. As figure1, the thermal
resistance Rθ_JP and Rθ_PA can be neglected because
most of the heat flows through the case and into the
surrounding air. Same as an electrical resistance,
summing these series resistance determines the
value of Rθ_JA (the chip junction to ambient thermal
resistance).
Rθ _JA = Rθ _JC + Rθ_CS + Rθ_SA
Rθ _JA = Rθ _JC + Rθ_CA
for heat sink of PCB issue
Rθ_JC = thermal resistance from junction to case
Rθ_CS = thermal resistance from case to heat sink
Rθ_SA = thermal resistance from heat sink to ambient
Rθ_CA = thermal resistance from case to ambient
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