MCP9700/9700A and MCP9701/9701A
4.0
APPLICATIONS INFORMATION
The Linear Active Thermistor™ IC uses an internal
diode to measure temperature. The diode electrical
characteristics have a temperature coefficient that
provides a change in voltage based on the relative
ambient temperature from -40°C to 125°C. The change
in voltage is scaled to a temperature coefficient of
10.0 mV/°C (typ.) for the MCP9700/9700A and
19.5 mV/°C (typ.) for the MCP9701/9701A. The output
voltage at 0°C is also scaled to 500 mV (typ.) and
400 mV (typ.) for the MCP9700/9700A and
MCP9701/9701A, respectively. This linear scale is
described in the first-order transfer function shown in
For higher accuracy using a sensor compensation
technique, refer to AN1001
“IC Temperature Sensor
Accuracy
Compensation
with
a
PICmicro
®
Microcontroller”
(DS01001). The application note
shows that if the MCP9700 is compensated in addition
to room temperature calibration, the sensor accuracy
can be improved to ±0.5°C (typ.) accuracy over the
operating temperature (Figure 4-2).
6.0
100 Samples
4.0
Accuracy (°C)
2.0
0.0
-2.0
-4.0
Spec. Limits
EQUATION 4-1:
SENSOR TRANSFER
FUNCTION
+
Average
-
V
OUT
=
T
C
•
T
A
+
V
0°C
Where:
T
A
= Ambient Temperature
V
OUT
= Sensor Output Voltage
V
0°C
= Sensor Output Voltage at 0°C
T
C
= Temperature Coefficient
-50
-25
0
25
50
75
100
125
Temperature (°C)
FIGURE 4-2:
Sensor Accuracy.
MCP9700/9700A Calibrated
The compensation technique provides a linear
temperature reading. A firmware look-up table can be
generated to compensate for the sensor error.
4.1
Improving Accuracy
The MCP9700/9700A and MCP9701/9701A accuracy
can be improved by performing a system calibration at
a specific temperature. For example, calibrating the
system at +25°C ambient improves the measurement
accuracy to a ±0.5°C (typ.) from 0°C to +70°C, as
shown in Figure 4-1. Therefore, when measuring
relative temperature change, this family measures
temperature with higher accuracy.
4.2
Shutdown Using Microcontroller
I/O Pin
3.0
2.0
Accuracy (°C)
1.0
0.0
-1.0
-2.0
-3.0
-50
-25
0
25
50
T
A
(°C)
75
100
125
V
DD
= 3.3V
10 Samples
The MCP9700/9700A and MCP9701/9701A family of
low operating current of 6 µA (typ.) makes it ideal for
battery-powered
applications.
However,
for
applications that require tighter current budget, this
device can be powered using a microcontroller
Input/Output (I/O) pin. The I/O pin can be toggled to
shut down the device. In such applications, the
microcontroller internal digital switching noise is
emitted to the MCP9700/9700A and MCP9701/9701A
as power supply noise. This switching noise compro-
mises measurement accuracy. Therefore, a decoupling
capacitor and series resistor will be necessary to filter
out the system noise.
4.3
Layout Considerations
FIGURE 4-1:
vs. Temperature.
Relative Accuracy to +25°C
The change in accuracy from the calibration
temperature is due to the output non-linearity from the
first-order equation, as specified in Equation 4-2. The
accuracy can be further improved by compensating for
the output non-linearity.
The MCP9700/9700A and MCP9701/9701A family
does not require any additional components to operate.
However, it is recommended that a decoupling
capacitor of 0.1 µF to 1 µF be used between the V
DD
and GND pins. In high-noise applications, connect the
power supply voltage to the V
DD
pin using a 200Ω
resistor with a 1 µF decoupling capacitor. A high
frequency ceramic capacitor is recommended. It is
necessary for the capacitor to be located as close as
possible to the V
DD
and GND pins in order to provide
effective noise protection. In addition, avoid tracing
digital lines in close proximity to the sensor.
DS21942C-page 8
©
2006 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
