2010 Microchip Technology Inc. DS22008E-page 15
MCP1702
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.
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 +40°C
Ambient Temperature Assuming Minimal Copper
Usage.
6.4 Voltage Reference
The MCP1702 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 MCP1702 LDO. The low-cost, low quiescent
current and small ceramic output capacitor are all
advantages when using the MCP1702 as a voltage
reference.
FIGURE 6-2: Using the MCP1702 as a
Voltage Reference.
Package
Package Type = SOT-23A
Input Voltage
V
IN
= 2.8V to 3.2V
LDO Output Voltages and Currents
V
OUT
=1.8V
I
OUT
=150mA
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
= (3.2V - (0.97 x 1.8V)) x 150 mA
P
LDO
= 218.1 milli-Watts
T
J(RISE)
=P
TOTAL
x Rq
JA
T
JRISE
= 218.1 milli-Watts x 336.0°C/Watt
T
JRISE
= 73.3°C
T
J
=T
JRISE
+ T
A(MAX)
T
J
=113.3°C
SOT-23 (336.0°C/Watt = R
JA
)
P
D(MAX)
= (+125°C - 40°C) / 336°C/W
P
D(MAX)
= 253 milli-Watts
SOT-89 (153.3°C/Watt = R
JA
)
P
D(MAX)
= (+125°C - 40°C) / 153.3°C/W
P
D(MAX)
= 0.554 Watts
TO92 (131.9°C/Watt = R
JA
)
P
D(MAX)
= (+125°C - 40°C) / 131.9°C/W
P
D(MAX)
= 644 milli-Watts
PIC
®
MCP1702
GND
V
IN
C
IN
1µF
C
OUT
1µF
Bridge Sensor
V
OUT
V
REF
ADO
AD1
Ratio Metric Reference
2 µA Bias
Microcontroller
