Datasheet

MCP1755/1755S
DS25160A-page 22  2012 Microchip Technology Inc.
5.3 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.
EXAMPLE 5-1: POWER DISSIPATION
5.3.1 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 SOT-23 Can Dissipate in an
Application” (DS00792), for more information regarding
this subject.
EXAMPLE 5-2:
5.3.2 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.
EXAMPLE 5-3:
Maximum Package Power Dissipation Examples at
+40°C Ambient Temperature
5.4 Voltage Reference
The MCP1755/1755S 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 MCP1755/1755S
LDO. The low-cost, low quiescent current and small
ceramic output capacitor are all advantages when
using the MCP1755/1755S as a voltage reference.
FIGURE 5-2: Using the MCP1755/1755S
as a Voltage Reference.
Package
Package Type = SOT-23
Input Voltage
V
IN
= 3.6V to 4.8V
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
= (4.8V – (0.97 x 1.8V)) x 50 mA
P
LDO
=152.7mW
T
J(RISE)
=P
TOTAL
xR
JA
T
JRISE
= 152.7 mW x 336.0°C/Watt
T
JRISE
=51.3°C
T
J
=T
JRISE
+T
A(MAX)
T
J
=91.3°C
SOT-23 (336.0°C/Watt = R
JA
)
P
D(MAX)
= (125°C – 40°C)/336°C/W
P
D(MAX)
=253mW
SOT-89 (153.3°C/Watt = R
JA
)
P
D(MAX)
= (125°C – 40°C)/153.3°C/W
P
D(MAX)
=554mW
PIC
®
GND
V
IN
C
IN
1µF
C
OUT
1µF
Bridge Sensor
V
OUT
V
REF
ADO
AD1
Ratio Metric Reference
68 µA Bias
Microcontroller
MCP1755S