Datasheet
MCP1700
DS20001826C-page 14 2005-2013 Microchip Technology Inc.
EQUATION 6-5:
6.3 Voltage Regulator
Internal power dissipation, junction temperature rise,
junction temperature and maximum power dissipation
are calculated in the following example. The power
dissipation resulting from ground current is small
enough to be neglected.
6.3.1 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 (R
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 SOT-23 Can Dissipate in an
Application” (DS00792), for more information regarding
this subject.
Junction Temperature Estimate
To estimate the internal junction temperature, the
calculated temperature rise is added to the ambient or
offset temperature. For this example, the worst-case
junction temperature is estimated below.
Maximum Package Power Dissipation at +40°C
Ambient Temperature
Package
Package Type = SOT-23
Input Voltage
V
IN
= 2.3V to 3.2V
LDO Output Voltages and Currents
V
OUT
= 1.8V
I
OUT
=150mA
Maximum Ambient Temperature
T
A(MAX)
= +40°C
Internal Power Dissipation
Internal Power dissipation is the product of the LDO
output current times the voltage across the LDO
(V
IN
to V
OUT
).
P
LDO(MAX)
=(V
IN(MAX)
-V
OUT(MIN)
)xI
OUT(MAX)
P
LDO
= (3.2V - (0.97 x 1.8V)) x 150 mA
P
LDO
= 218.1 milli-Watts
T
J
T
JRISE
T
A
+=
T
J
= Junction Temperature
T
J(RISE)
= Rise in the device’s junction
temperature over the ambient
temperature
T
A
= Ambient temperature
T
J(RISE)
=P
TOTAL
xR
JA
T
J(RISE)
= 218.1 milli-Watts x 230.0°C/Watt
T
J(RISE)
=50.2°C
T
J
=T
J(RISE)
+T
A(MAX)
T
J
=90.2°C
2x2 DFN-6 (91°C/Watt = R
JA
)
P
D(MAX)
= (125°C - 40°C) / 91°C/W
P
D(MAX)
= 934 milli-Watts
SOT-23 (230.0°C/Watt = R
JA
)
P
D(MAX)
= (125°C - 40°C) / 230°C/W
P
D(MAX)
= 369.6 milli-Watts
SOT-89 (52°C/Watt = R
JA
)
P
D(MAX)
= (125°C - 40°C) / 52°C/W
P
D(MAX)
= 1.635 Watts
TO-92 (131.9°C/Watt = R
JA
)
P
D(MAX)
= (125°C - 40°C) / 131.9°C/W
P
D(MAX)
= 644 milli-Watts