Datasheet

MCP1804
DS20002200D-page 22  2009-2013 Microchip Technology Inc.
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
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
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.
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.
Maximum Package Power Dissipation at +25°C
Ambient Temperature (minimum PCB footprint)
6.4 Voltage Reference
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.
FIGURE 6-2: Using the MCP1804 as a
Voltage Reference.
6.5 Pulsed Load Applications
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 or the maximum 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).
Package:
Package Type = SOT-23
Input Voltage:
V
IN
= 3.8V to 4.6V
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)
)xI
OUT(MAX)
P
LDO(MAX)
= (4.6V - (0.98 x 1.8V)) x 50 mA
P
LDO(MAX)
= 141.8 milli-Watts
T
J(RISE)
=P
TOTAL
xR
JA
T
J(RISE)
= 141.8 milli-Watts x 256.0°C/Watt
T
J(RISE)
=36.3°C
T
J
=T
J(RISE)
+T
A(MAX)
T
J
= 76.3°C
SOT-23 (256°C/Watt = R
JA
):
P
D(MAX)
= (125°C - 25°C) / 256°C/W
P
D(MAX)
= 390 milli-Watts
SOT-89 (180°C/Watt = R
JA
):
P
D(MAX)
= (125°C - 25°C) / 180°C/W
P
D(MAX)
= 555 milli-Watts
PICmicro
®
GND
V
IN
C
IN
1µF
C
OUT
1µF
Bridge Sensor
V
OUT
V
REF
AD0
AD1
Ratio Metric Reference
50 µA Bias
Microcontroller
MCP1804