Datasheet

© 2010 Microchip Technology Inc. DS22075B-page 19
MCP1790/MCP1791
5.3 Power Dissipation Example
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.
5.3.1 POWER DISSIPATION EXAMPLE
5.3.1.1 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
Application”, (DS00792), for more information
regarding this subject.
5.3.1.2 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.
5.3.1.3 Maximum Package Power
Dissipation at +40°C Ambient
Temperature
5.4 Pulsed Load Applications
For some applications, there are pulsed load current
events that may exceed the specified 70 mA maximum
specification of the MCP1790/MCP1791. The internal
current foldback feature of the MCP1790/MCP1791 will
prevent high peak load demands from causing
non-recoverable damage. The Current Foldback
feature of the device will limit the output voltage and
output current during pulsed applications. As the
current rises above the foldback current threshold, the
output voltage will decrease.
Package:
Package Type = SOT-223-5
Input Voltage:
V
IN
= 8V to 24V
LDO Output Voltages and Currents:
V
OUT
=5.0V
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
= (24V - (0.98 x 5.0V)) x 50 mA
P
LDO
= 955 milli-Watts
T
J(RISE)
=P
TOTAL
x Rq
JA
T
JRISE
= 955 milli-Watts x 62°C/Watt
T
JRISE
= 59.2°C
T
J
=T
JRISE
+ T
A(MAX)
T
J
= 99.2°C
SOT-223-5 (62°C/Watt = Rθ
JA
)
P
D(MAX)
= (125°C - 40°C) / 62°C/W
P
D(MAX)
= 1.371 Watts
DDPAK-5 (32°C/Watt = Rθ
JA
)
P
D(MAX)
= (125°C - 40°C) / 32°C/W
P
D(MAX)
= 2.656 Watts