Texas Instruments | Temperature Sensing With PTC Circuit (Rev. A) | Application notes | Texas Instruments Temperature Sensing With PTC Circuit (Rev. A) Application notes

Texas Instruments Temperature Sensing With PTC Circuit (Rev. A) Application notes
Analog Engineer's Circuit: Amplifiers
SBOA322A – December 2018 – Revised May 2019
Temperature Sensing With PTC Circuit
Design Goals
Temperature
Output voltage
Supply
TMin
TMax
VoutMin
VoutMax
Vdd
Vee
Vref
0℃
50 ℃
0.05V
3.25V
3.3V
0V
1.646V
Design Description
This temperature sensing circuit uses a resistor in series with a positive–temperature–coefficient (PTC)
thermistor to form a voltage–divider, which has the effect of producing an output voltage that is linear over
temperature. The circuit uses an op amp in a non–inverting configuration with inverting reference to offset
and amplify the signal, which helps to utilize the full ADC resolution and increase measurement accuracy.
VDD
VDD
R2 1k
Vdd
3.3 R1
10k
R3 11.5k
Vref
1.646
+
Vin
PTC
Vout
+
U1 TLV9002
VDD
Design Notes
1. Use the op amp in a linear operating region. Linear output swing is usually specified under the AOL test
conditions.
2. The connection, Vin, is a positive temperature coefficient output voltage. To correct a
negative–temperature–coefficient (NTC) output voltage, switch the position of R1 and PTC resistor.
3. Choose R1 based on the temperature range and the PTC’s value.
4. Vref can be created using a DAC or voltage divider. If a voltage divider is used the equivalent resistance
of the voltage divider will alter the gain of the circuit and should be accounted for.
5. Using high–value resistors can degrade the phase margin of the amplifier and introduce additional
noise in the circuit. It is recommended to use resistor values around 10kΩ or less.
6. A capacitor placed in parallel with the feedback resistor will limit bandwidth, improve stability and help
reduce noise.
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Design Steps
1. Calculate the value of R1 to produce a linear output voltage. Use the minimum and maximum values of
the PTC to obtain a range of values for R1.
2. Calculate the input voltage range.
3. Calculate the gain required to produce the maximum output swing.
4. Select R2 and calculate R3 to set the gain calculated in Step 3.
5. Calculate the actual gain based on standard values of R2 and R3.
6. Calculate the output voltage swing based on the actual gain.
7. Calculate the maximum output voltage when the output voltage is symmetrical around mid–supply.
8. Calculate the reference voltage.
2
Temperature Sensing With PTC Circuit
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Design Simulations
DC Transfer Results
T
3.30
Output (V)
Temp=50°C,Vout=3.239V
Temp=0°C,Vout=0.059V
1.65
Temp=25°C,Vout=1.674V
0.00
0.00
25.00
Temperature (°C)
50.00
AC Simulation Results
Gain (dB)
24.00
BW=84.04kHz
12.00
Phase [deg]
0.00
0.00
-45.00
-90.00
1.00
10.00
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100.00
1.00k
Frequency (Hz)
10.00k
100.00k
1.00M
Temperature Sensing With PTC Circuit
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References:
1. Analog Engineer's Circuit Cookbooks
2. SPICE Simulation File SBOMAV5
3. TI Precision Labs
Design Featured Op Amp
TLV9002
Vcc
1.8 V to 5.5 V
VinCM
Rail–to–rail
Vout
Rail–to–rail
Vos
1.5mV
Iq
0.06mA
Ib
5pA
UGBW
1MHz
SR
2V/µs
#Channels
1, 2, 4
http://www.ti.com/product/TLV9002
Design Alternate Op Amp
OPA333
Vcc
1.8 V to 5.5 V
VinCM
Rail–to–rail
Vout
Rail–to–rail
Vos
2µV
Iq
17µA
Ib
70pA
UGBW
350kHz
SR
0.16V/µs
#Channels
1, 2, 4
http://www.ti.com/product/OPA333
Design Featured Thermistor
TMP61
VCC
Up to 5.5 V
R25
10 kΩ
RTOL
1%
ISNS
400 µA
Operating Temperature Range
–40°C to 125°C
http://www.ti.com/product/TMP61
4
Temperature Sensing With PTC Circuit
SBOA322A – December 2018 – Revised May 2019
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (December 2018) to A Revision ................................................................................................ Page
•
Added Design Featured Thermistor table .............................................................................................. 4
SBOA322A – December 2018 – Revised May 2019
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Revision History
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