MCP1727
The maximum power dissipation capability for a
package can be calculated given the junction-to-
ambient thermal resistance and the maximum ambient
temperature for the application. Equation 5-4 can be
used to determine the package maximum internal
power dissipation.
5.3
Typical Application
Internal power dissipation, junction temperature rise,
junction temperature and maximum power dissipation
is calculated in the following example. The power
dissipation as a result of ground current is small
enough to be neglected.
EQUATION 5-4:
5.3.1
POWER DISSIPATION EXAMPLE
(TJ(MAX) – TA(MAX)
)
PD(MAX) = ---------------------------------------------------
RθJA
Package
Package Type = 3x3DFN
PD(MAX) = Maximum device power dissipation
Input Voltage
TJ(MAX) = maximum continuous junction
temperature
V
IN = 3.3V ± 5%
LDO Output Voltage and Current
TA(MAX) = maximum ambient temperature
V
OUT = 2.5V
OUT = 1.5A
RθJA = Thermal resistance from junction to
I
ambient
Maximum Ambient Temperature
A(MAX) = 60°C
Internal Power Dissipation
T
EQUATION 5-5:
TJ(RISE) = PD(MAX) × RθJA
PLDO(MAX) = (VIN(MAX) – VOUT(MIN)) x IOUT(MAX)
PLDO = ((3.3V x 1.05) – (2.5V x 0.975))
x 1.5A
TJ(RISE) = Rise in device junction temperature
over the ambient temperature
PLDO = 1.54 Watts
PD(MAX) = Maximum device power dissipation
RθJA = Thermal resistance from junction to
5.3.1.1
Device Junction Temperature Rise
ambient
The internal junction temperature rise is a function of
internal power dissipation and the thermal resistance
from junction-to-ambient for the application. The
thermal resistance from junction-to-ambient (RθJA) is
derived from an EIA/JEDEC standard for measuring
thermal resistance for small surface-mount packages.
The EIA/JEDEC specification is JESD51-7 “High
Effective Thermal Conductivity Test Board for Leaded
Surface-Mount Packages”. The standard describes the
test method and board specifications for measuring the
thermal resistance from junction to ambient. The actual
thermal resistance for a particular application can vary
depending on many factors such as copper area and
thickness. Refer to AN792, “A Method to Determine
How Much Power a SOT23 Can Dissipate in an
Application” (DS00792), for more information regarding
this subject.
EQUATION 5-6:
TJ = TJ(RISE) + TA
TJ = Junction temperature
TJ(RISE) = Rise in device junction temperature
over the ambient temperature
TA = Ambient temperature
T
J(RISE) = PTOTAL x RθJA
TJRISE = 1.54 W x 41.0° C/W
JRISE = 63.14°C
T
© 2007 Microchip Technology Inc.
DS21999B-page 21
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