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MCP1804
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.
T J = T JRISE + T A(MAX)
T J = 76.3°C
Maximum Package Power Dissipation at +25°C
Ambient Temperature (minimum PCB footprint)
6.3.1
POWER DISSIPATION EXAMPLE
SOT-23 (256°C/Watt = R θ JA ):
Package:
Package Type = SOT-23
Input Voltage:
V IN = 3.8V to 4.6V
LDO Output Voltages and Currents:
P D(MAX) = (85°C - 25°C) / 256°C/W
P D(MAX) = 234 milli-Watts
SOT-89 (180°C/Watt = R θ JA ):
P D(MAX) = (85°C - 25°C) / 180°C/W
P D(MAX) = 333 milli-Watts
V OUT = 1.8V
6.4
Voltage Reference
V IN
V OUT
1 μF C OUT
1 μF
I OUT = 50 mA
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 = (4.6V - (0.98 x 1.8V)) x 50 mA
P LDO = 141.8 milli-Watts
6.3.1.1 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 (R θ 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 Ther-
mal Conductivity Test Board for Leaded Surface Mount
Packages”. The standard describes the test method
and board specifications for measuring the thermal
The MCP1804 can be used not only as a regulator, but
also as a low quiescent current voltage reference. In
many microcontroller applications, the initial accuracy
of the reference can be calibrated using production test
equipment or by using a ratio measurement. When the
initial accuracy is calibrated, the thermal stability and
line regulation tolerance are the only errors introduced
by the MCP1804 LDO. The low-cost, low quiescent
current and small ceramic output capacitor are all
advantages when using the MCP1804 as a voltage
reference.
Ratio Metric Reference
MCP1804 PICmicro ?
50 μA Bias Microcontroller
C IN V REF
GND
ADO
AD1
Bridge Sensor
resistance from junction to ambient. The actual thermal
resistance for a particular application can vary depend-
ing on many factors, such as copper area and thick-
ness. Refer to AN792, “A Method to Determine How
FIGURE 6-2:
Voltage Reference.
Using the MCP1804 as a
Much Power a SOT23 Can Dissipate in an Application”
6.5
Pulsed Load Applications
(DS00792), for more information regarding this subject.
T J(RISE) = P TOTAL x Rq JA
T JRISE = 141.8 milli-Watts x 256.0 ° C/Watt
T JRISE = 36.3 ° C
6.3.1.2 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.
? 2011 Microchip Technology Inc.
For some applications, there are pulsed load current
events that may exceed the specified 150 mA
maximum specification of the MCP1804. The internal
current limit of the MCP1804 will prevent high peak
load demands from causing non-recoverable damage.
The 150 mA rating is a maximum average continuous
rating. As long as the average current does not exceed
150 mA nor the max power dissipation of the packaged
device, pulsed higher load currents can be applied to
the MCP1804 . The typical current limit for the
MCP1804 is 200 mA (T A = +25°C).
DS22200C-page 21
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