Datasheet

MCP1801
DS22051D-page 18  2010 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, as a result of 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.
Package
Package Type: SOT-23-5
Input Voltage
V
IN
= 2.4V to 5.0V
LDO Output Voltages and Currents
V
OUT
= 1.8V
I
OUT
=50mA
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)
) x I
OUT(MAX)
P
LDO
= (5.0V - (0.98 x 1.8V)) x 50 mA
P
LDO
= 161.8 milli-Watts
T
J
T
JRISE
T
A
+=
Where:
T
J
= Junction Temperature
T
J(RISE)
= Rise in device junction
temperature over the ambient
temperature
T
A
= Ambient temperature
T
J(RISE)
=P
TOTAL
x Rq
JA
T
JRISE
= 161.8 milli-Watts x 256.0°C/Watt
T
JRISE
=41.42°C