Datasheet

© 2007 Microchip Technology Inc. DS21936C-page 19
MCP1726
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. The following equation
can be used to determine the package maximum
internal power dissipation.
EQUATION 5-4:
EQUATION 5-5:
EQUATION 5-6:
5.3 Typical Application
Internal power dissipation, junction temperature rise,
junction temperature and maximum power dissipation
is calculated in the following example. The power dissi-
pation as a result of ground current is small enough to
be neglected.
5.3.1 POWER DISSIPATION EXAMPLE
Device Junction Temperature Rise
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 (Rθ
JA
) 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 Appli-
cation” (DS00792), for more information regarding this
subject.
P
DMAX()
T
JMAX()
T
AMAX()
–()
Rθ
JA
---------------------------------------------------=
P
D(MAX)
= Maximum device power dissipation
T
J(MAX)
= maximum continuous junction
temperature
T
A(MAX)
= maximum ambient temperature
Rθ
JA
= Thermal resistance from junction-to-
ambient
T
JRISE()
P
DMAX()
Rθ
JA
×=
T
J(RISE)
= Rise in device junction temperature
over the ambient temperature
P
D(MAX)
= Maximum device power dissipation
Rθ
JA
= Thermal resistance from junction-to-
ambient
T
J
T
JRISE()
T
A
+=
T
J
= Junction temperature
T
J(RISE)
= Rise in device junction temperature
over the ambient temperature
T
A
= Ambient temperature
Package
Package Type = 3x3DFN
Input Voltage
V
IN
= 3.3V ± 10%
LDO Output Voltage and Current
V
OUT
=2.5V
I
OUT
=1.0A
Maximum Ambient Temperature
T
A(MAX)
=70°C
Internal Power Dissipation
P
LDO(MAX)
=(V
IN(MAX)
– V
OUT(MIN)
) x I
OUT(MAX)
P
LDO
= (3.3V x 1.1) – (0.975 x 2.5V))
x 1.0A
P
LDO
= 1.192 Watts
T
J(RISE)
=P
TOTAL
x Rθ
JA
T
JRISE
= 1.192 W x 41.0° C/W
T
JRISE
=48.8°C