Texas Instruments | Precise Temperature Measurements With the TMP116 and TMP117 (Rev. A) | Application notes | Texas Instruments Precise Temperature Measurements With the TMP116 and TMP117 (Rev. A) Application notes

Texas Instruments Precise Temperature Measurements With the TMP116 and TMP117 (Rev. A) Application notes
Application Report
SNOA986A – April 2018 – Revised August 2019
Precise Temperature Measurements With the TMP116 and
TMP117
Mihail Gurevitch
ABSTRACT
Engineers must carefully consider the overall system design when designing high-precision temperature
measurement applications. This application note provides recommendations on how to design a precise
temperature measuring system based on the TMP116 and TMP117 temperature sensors. By following this
application note, the user should be able to design a precise measuring system which adheres to the
performance specifications of the TMP116/117.
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Contents
Introduction ................................................................................................................... 2
TMP116 and TMP117 Device Differences ............................................................................... 2
PCB Considerations ......................................................................................................... 3
Measuring Solid Surface Temperature ................................................................................... 3
Measuring Human Body Temperature ................................................................................... 5
Measuring Still Air Temperature .......................................................................................... 6
Measuring Moving Air Temperature ...................................................................................... 6
Measuring Thermal Resistance in Different Environments ............................................................ 8
Soldering to PCB ............................................................................................................ 9
Self-Heating ................................................................................................................. 11
Self-Heating Estimation Example ........................................................................................ 12
Supply Voltage Change .................................................................................................. 14
Data Averaging ............................................................................................................. 14
Summary ................................................................................................................... 15
List of Figures
1
Simplified Schematic of Temperature Flow During Solid Surface Measurement ................................... 3
2
PCB Layout Example for Rigid Surface Temperature Measuring ..................................................... 5
3
Moving Air Temperature Measurements Noise. Air Speed 0.5, 1 and 2 Meter/Sec. Averaging 8 Samples
Per Reading. 5 Consecutive Measurements at Room Temperature. ................................................. 7
4
PCB Layout Example for Air Temperature Measuring .................................................................. 8
5
Printed-Circuit Boards Used
6
Soldering Shift at +25ºC and Supply 3.3 V With Thermal Pad Soldered on a Rigid PCB.
7
............................................................................................... 9
...................... 10
Soldering Shift for TMP116/117 Without the Thermal Pad Soldered to the PCB. +25ºC, V = +3.3 V .......... 10
8
Device Consumption Power vs Temperature and Part Supply Voltage in Continuous Conversion Mode.
No Pauses Between Conversions, No I2C Bus Activity. ............................................................. 11
9
Supply Current vs. Pin Input Voltage and Device Supply Voltage for Any Digital Pin Input Cell. ............... 12
10
Device Supply Current vs. I2C Bus Clocking Frequency and Supply Voltage. Part is in Shutdown Mode,
but SCL, SDA, and ADD0 Pins are Under Constant I2C Data Flow. ............................................... 12
11
TMP116/117 Coupon Board Self-Heating Effect vs. Time and Supply Voltage in Still +25ºC Air.
12
The TMP116/117 Sampling Distribution for 3 Different Oil Bath Temperatures and 3.3-V Supply Voltage.
No Data Averaging. ........................................................................................................ 14
13
The TMP116/117 Sampling Distribution for 3 Different Supply Voltages at +25ºC. No Data Averaging. ...... 15
14
Temperature Sampling Noise With 8, 32, and 64 Internal Averages. Temperature +25ºC and V = +3.3 V.
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1
Introduction
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All trademarks are the property of their respective owners.
1
Introduction
There are many system factors which can negatively affect the precision of temperature measurements,
and these must be addressed to achieve a high accuracy. The main parameters that affect measurement
precision with the corresponding source of their control are:
• The accuracy of the temperature sensor itself as its accuracy, stability, and repeatability, are set by the
manufacturer and out of the designer’s control.
• The system engineer controls the supply voltage range and noise, the sensor conversion mode, the
system power consumption, the data sampling rate, the communication bus voltage, the I2C bus
frequency, and data flow over it.
• The PCB designer controls the mounting and position of the sensor on the PCB, the temperature
resistance between the sensor and the measured object, and the temperature “leakage” from the
sensor to surrounding air.
These parameters are important for precise temperature measurements and must be analyzed during the
system design. The purpose of this article is to provide recommendations to the system designer, based
on experience obtained in part characterization and device use in real applications.
When using the TMP116/117 for precise temperature measurements, there are a few critical
considerations that must be accounted for by the system designer:
• Proper PCB sensor location and orientation in the system. The proper location must provide the
precise temperature measurement with minimal offset and minimal time delay.
• Proper device electrical and communication interface mode, which can minimize measurement noise,
minimize part self-heating and ensure measurements stability.
• Proper PCB material and thickness, PCB mounting, and PCB layout. All these should provide a
minimal temperature difference between the sensor and the measured object, and should minimize
sensor response time when an object temperature is changing.
2
TMP116 and TMP117 Device Differences
The TMP116 and TMP117 have a similar internal schematic, register map, and electrical characteristics.
The main differences between two devices are shown in Table 1.
Table 1. Parameter Differences Between the TMP116 and TMP117
PARAMETER
TMP116
TMP117
Ensured precision at room temperature
(°C)
±0.2
±0.1
Temperature range (°C)
–40 to +125
–55 to +150
Supply voltage range (V)
1.9 to 5.5
1.8 to 5.5
Shut down current at +25ºC (+125ºC) (µA) 0.25 (3)
0.15 (0.8)
Typical PSRR (m°C/V)
10
6
Package
DRV-6
DRV-6 and WCSP
Thermal mass (mJ/°C)
5.1
5.1 and 0.8 (WCSP)
Price on Jun 2019 (1 Ku) ($)
0.99
1.6
Additionally, the TMP117 has a register to compensate the temperature offset and a reset bit in the
configuration register. Both parts are in the same 6-pin DRV package, but the TMP117 also has a smaller
WCSP-6 package version with a 1.5-mm × 1.0-mm × 0.5-mm die size. All conclusions found for either
device listed in this application note will apply to both the TMP116 and TMP117.
2
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PCB Considerations
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3
PCB Considerations
There are two main tasks in temperature measurements: measuring air (gas) temperature and measuring
temperature of a solid surface. A liquid temperature measurement usually falls in one of the above,
because the sensor is often placed inside a metallic probe for liquid measurements. These two different
tasks dictate two different approaches to device mounting. However, in all cases, these common rules
must be applied:
• To get the manufacturer ensured measurement precision, the 0.1-µF bypass capacitor must be placed
no more than 5 mm (200 mils) away from the device
• To avoid possible heat influence coming from the pullup resistor on the SDA pin and the pullup resistor
on the SCL and ALERT pins (if present), the pins must be placed at least 10 mm (400 mils) away from
the device.
• If there is a risk that the board may bend during PCB mounting, all efforts to prevent the mechanical
tension on the device package must be taken. Guard holes in the PCB around the part can help in this
case.
4
Measuring Solid Surface Temperature
Measuring the temperature of a solid surface is the most common type of temperature measurement, and
the standard approach is to make a tiny rigid PCB, solder the device on one side of the PCB, and attach
the opposite side of the PCB to object surface. Figure 1 shows a cross-sectional view on how to mount
the TMP116/117 sensor to a PCB, along with a simplified schematic of the thermal processes required for
surface temperature measurement.
Figure 1. Simplified Schematic of Temperature Flow During Solid Surface Measurement
On this schematic:
• Tobj is the measured object temperature.
• Tair is the environment temperature (typically air).
• Ts is the sensor temperature.
• Rso is the thermal resistance between the sensor and the object.
• Rsa is the thermal resistance between the sensor and the air (environment).
• Ps is the averaged power dissipated by the sensor during the measurement.
• Mt is the combined thermal mass of device, plus the surrounding PCB area.
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Measuring Solid Surface Temperature
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The most important formula for the precise temperature measurement is:
Tofs
Ts
§ Tobj Tair ·
¨
¸ u R so
© R so + R sa ¹
Tobj Tofs
(1)
where
•
Tofs is a temperature offset between the measured object and sensor.
(2)
Equation 1 shows that the sensor temperature offset is zero only in two cases: if Rso is zero or Rsa is
infinite. If there is a difference between Tobj and Tair (and despite all efforts to make Rsa >> Rso), however,
there will always be some offset between sensor and object temperature. This shift will increase when the
difference between Tobj and Tair is larger, or when Rsa becomes smaller and approaches to Rso value.
Let's calculate temperature offsets for two metallic object temperatures (+50°C and +100°C) where still air
temperature stays the same +25°C and the temperature resistance from sensor to object surface
assumed from line 6 of Table 2 (140m°C/mWt). Let's also assume that the temperature resistance
between sensor and air is equal to line 1 of Table 2 (300m°C/Wt). For object temperatures +50°C and
+100°C, the measurement offset, according to Equation 1, will be 7.9°C and 23.8°C accordingly, which is
not acceptable for precise measurements. TI recommends to use a thinner PCB with a better layout, and
cover the top surface of PCB with thermal isolating foam. The best solution to avoid temperature leakage
to the surrounding air may be to make a cave-kind cavern in the object body and put the PCB of the
sensor inside it, but this kind solution is not always available.
If the sensor temperature shift from the object temperature is too big and cannot be ignored, a system
calibration is needed. In some cases, it should be done for different combinations of Tobj and Tair. This
happens because Rsa is not a linear parameter, and instead depends on the air speed, air moisture, air
temperature, PCB orientation, and so on. All this makes the Rsa value estimation very difficult to find.
However, by making Rso as small as possible and Rsa as big as possible, it would be much easier to
minimize the temperature shift.
Another important aspect is when designers can trust the sensor readings, like when the object
temperature changes from Tobj1 to Tobj2. To estimate or understand the process of this temperature change,
we can use a Gaussian formula for an ideal case. In reality, the object temperature rarely changes
instantly, and therefore the sensor follows the object temperature slower than Equation 3 shows.
Ts
Tobj1
Tobj2
Tobj1 u e
t / tr
where
•
•
tr
t is a time passing from beginning object temperature change.
tr is a response time.
R so u M t
(3)
(4)
Here we can assume that Rsa >> Rso and ignore the temperature leakage to environment. According to
formula, to have minimal measurement delay, it is important to have a small response time (tr), which
means the Rso and Mt should be kept at minimal value, especially if the object temperature changes fast.
Because the device is dissipating some power during the measurements, the sensor is heating itself. The
self-heating temperature shift Tsh is calculate by Equation 5.
Tsh
Ps u R so
(5)
The influence of self-heating on measurement precision is discussed in Section 10.
The following are the recommendations for systems measuring rigid surface temperature:
• Use a PCB with minimal thickness.
• The side of the PCB that makes contact with the surface to be measured should be covered with an
exposed copper layer (and not covered with a solder mask). To prevent copper oxidation, a gold or
melted solder paste cover should be used.
• To improve thermal contact to the surface, consider adding a thermal conducting paste or sticky
thermal film between the surface and the PCB.
4
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Measuring Human Body Temperature
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•
•
•
•
•
•
Place additional vias to connect copper layers on both sides of the PCB. Generally, a via has 400
times less thermal-resistance than the same area of regular PCB material. Using a filled via further
decreases the thermal resistance.
If the PCB internal layers are not used under the device, it is recommended to create internal copper
polygons under the sensors to reduce the PCB side-to-side thermal resistance.
To increase the temperature resistance to surrounding air, minimize the amount of copper wires on top
of the board.
To increase thermal resistance to surrounding air, the sensor and the PCB surface exposed to the air
must be covered with thermal-isolating foam, film, or at least with some stain. This protection is
especially important for precise measurements when air around the sensor is moving.
To minimize the convection air influence, the PCB should be located horizontally and out of any air
flow.
Soldering the device's thermal pad (TP) to the PCB may be a good choice only for systems which
undergo calibration. The negative aspects of TP soldering are described in Section 9. If the TP is
soldered, it should be connected to ground or left floating. Connecting the package TP to a voltage
other than system ground can lead to permanent device damage.
Figure 2 shows an example of a PCB layout for surface temperature measuring.
(1)
Alert pin is not used and grounded. I2C bus pullup resistors are located on master board.
Figure 2. PCB Layout Example for Rigid Surface Temperature Measuring
5
Measuring Human Body Temperature
When making human body temperature measurements, it is important to understand the two cases that
may affect the performance of the system.
The first case is when the thermometer is exposed to the surrounding air temperature before it is pressed
to the body. The goal is to make precise body temperature measurements in the shortest amount of time
when the sensor temperature is changing rapidly at the beginning of measurement. In this case, the
minimal combined thermal mass will allow the sensor to reach a body temperature in the shortest amount
of time. Take care to avoid temperature “leakage” from the sensor to surrounding air. TI recommends to
have a temperature stabilization check before a measurement report is done. As an example, the
stabilization check can be to verify that the temperature didn’t change more than 0.2°C during the last 5
seconds. It is easy to achieve a good thermal contact to the object in this case, and therefore there is less
need to worry about the sensor self-heating. The conversion mode with a small standby time is
recommended.
• Use rigid PCB with minimal thickness to minimize the sensor-to-body thermal resistance.
• Cover the PCB side that makes direct contact with the body with a copper plane. Remove the solder
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Measuring Still Air Temperature
•
•
•
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masks above the planes. To avoid oxidation, cover the exposed copper plane with gold or a melted
soldering material.
Use a bypass capacitor with minimal dimensions to reduce thermal mass.
Place pullup resistors away from the sensor.
Depending on the design, cover the sensor and top side of PCB with a thermal-isolating compound.
The second case is a monitoring case where sensor attached to the body for a long period of time. In this
scenario, the temperature is changing very slowly and samples are taken less frequently (like once every
16 sec). It is easy to make a good thermal contact to a body and minimize temperature leakage. Bigger
sensor thermal mass may be useful as a low-frequency filter working to reduce temperature fluctuation
(noise). This can reduce the averaging number down to 1 during sampling, which lowers the power
consumption and extends the battery life. Bigger sensor thermal mass also reduces device self-heating
during conversions.
• Use a flexible PCB to make better temperature contact to the body.
• Cover the PCB side that makes direct contact with the human body with a copper plane. Remove the
solder masks above the planes. To avoid oxidation, cover the exposed copper plane with gold or a
melted soldering material.
• To make PCB maximal flexible and to increase PCB reliability, use the smallest size capacitor and
place the pullup resistors away from the sensor.
• To prevent temperature leakage and protect device contacts from oxidation, cover the top side of the
board with a thermal-isolating protection compound.
6
Measuring Still Air Temperature
The main feature of still air measurement is that the temperature changes slowly (usually less than a
degree per minute), and this is primarily due to air convection. When temperature change is slow, it is not
critical to have minimal thermal mass for sensor and surrounding PCB. Even with increased thermal mass,
the sensor will be able to follow the air temperature with minimal lag. The thermal resistance between slow
moving or still air and the PCB (including mounted sensor) is very high. Therefore, the designer should try
to improve the thermal contact with the air while simultaneously excluding any heat transfer from other
heat sources located on the same PCB. Due to the slow temperature change, there is no need to keep the
device running continuously. The update rate of one sample per second or less may be sufficient for most
use cases. When the device spends most of the time in standby or in shutdown mode, the power
consumption is minimal and self-heating is negligible.
• Place the PCB vertically. This will improve convection air flow and reduce dust collection over time.
The layer of accumulated dust works as a thermal-isolating barrier between the air and the PCB.
• To make a better PCB thermal contact with the air, place copper planes on both sides of PCB.
• Remove the solder masks above the planes. To prevent oxidation, cover the exposed copper plane
with melted soldering material or gold.
• Thermal isolation is required to avoid thermal coupling from the heat sources through the PCB. Use air
gaps between the sensor and PCB heat sources, if needed.
7
Measuring Moving Air Temperature
The main feature of moving air temperature measurement is high thermal noise, which is coming from the
temperature fluctuation inside air stream. Figure 3 shows the measurement noise of the room air flow,
which is moving along the rigid coupon board with a mounted TMP117 sensor at different speeds. As
seen in the graph, the measurements are still noisy even with an internal averaging of 8 temperature
samples.
6
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Figure 3. Moving Air Temperature Measurements Noise. Air Speed 0.5, 1 and 2 Meter/Sec. Averaging 8
Samples Per Reading. 5 Consecutive Measurements at Room Temperature.
The standard approach to reduce the noise is to increase the sample average number, but an alternate
method is to increase the sensor thermal response time in Equation 4.
Increased response time works as a low-pass filter, and it reduces the measurements noise. Knowing
response time tr, the designer can calculate the filter 3db cut-off frequency, Fc=1/tr. However, it is difficult
to estimate the effective combined thermal mass and effective thermal resistance between the sensor and
moving air, due to its dependents of many non-linear factors.
Moving air provides a good thermal contact to the sensor, and there can be a rare case where the sensor
can have the same temperature as a measured object. Low thermal resistance to moving air also
minimizes the device self-heating effect.
• Because moving air temperature usually has a lot of fluctuations, the PCB increased thermal mass can
reduce measurement noise. Therefore, it is acceptable in these cases to use a PCB with increased
thickness.
• Place the PCB vertically along air flow. This makes air flow smooth and prevent air “shades”.
• Design PCB soldering pads bigger than usual, especially the package corner pads. This will improve
the thermal contact from package to air.
• Cover both side of unused board space with a copper layer,
• Use a PCB with thicker copper layers, if possible. This improves thermal conductivity along the PCB,
and it allows better “average” temperature fluctuations from different parts of the board.
• If air (or gas) is expected to contain moisture or includes some corrosive components, the device pins
must be protected by a stain to avoid corrosion or moisture accumulation on the pins.
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Measuring Thermal Resistance in Different Environments
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Figure 4 is an example of a PCB layout for air temperature measuring.
(1)
Alert pin is not used and grounded. I2C pullup resistors are located on master board.
Figure 4. PCB Layout Example for Air Temperature Measuring
8
Measuring Thermal Resistance in Different Environments
As mentioned earlier, the thermal resistance between the sensor and measured object is a parameter
sensitive to PCB layout, board mounting. and environment condition. This parameter is not easily
calculable upfront. A more practical way is to measure the thermal resistance between the sensor and
object, and the sensor and environment, in already designed system. Knowing Ps, Rso, Rsa and using
Equations 1-5, it is possible to estimate the measurement error and sensor response time for different
temperatures and apply a necessary system correction. To measure Rso or Rsa, the system designer may
do the following:
• The environment or object temperature is fixed and well controlled.
• When TMP116/117 temperature is stabilized the device temperature T1 is read using minimal
conversion power. Single shot mode, which makes P1 power almost zero, is the best choice.
• The TMP116/117 an average consumption power is increased in any possible way. The simplest way
is to increase the supply voltage from min to max and switch to conversion mode without the standby
time. This will be the device power P2.
• When the device internal temperature is stabilized after power increase, the temperature reading T2 is
taken.
Now the designer can calculate the thermal resistance Rsx, which is Rso or Rsa:
R sx
T2
P2
T1
P1
(6)
In this measurement, it is assumed that the object (environment) temperature is stable during the test and
is not changed due to sensor self-heating.
Using this method, data about thermal resistance between part mounted on a coupon board (CB) to a
different kind of environment has been collected. The 2-layer coupon boards used in the experiments
have board size of 21 mm × 11 mm, a board thickness from 6 to 64 mil (0.15 to 1.62 mm), and an
identical layout. See Figure 5. Each CB has surface mounted 0.1-µF bypass capacitor and 6 contact pins.
Table 2 shows the thermal resistance from the TMP116/117 to a different environment.
Table 2. Thermal Resistance Between TMP116/117 to Differentiate Environment. The CB is 21 mm ×
11 mm.
ENVIRONMENT
Still Air.
8
THERMAL
RESISTANCE
(m°C/mWt)
260-320
COMMENTS
For all CB thickness and all CB orientation
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Soldering to PCB
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Table 2. Thermal Resistance Between TMP116/117 to Differentiate Environment. The CB is 21 mm ×
11 mm. (continued)
ENVIRONMENT
COMMENTS
Moving Air along CB. 0.5 M/Sec
236
64 mill (1.62 mm) rigid CB (1)
Moving Air along CB. 0.5 M/Sec
190
6 mill (0.15 mm) flex CB (1)
Moving Air along CB. 2 M/Sec
200
64 mill (1.62 mm) rigid CB (1)
Moving Air along CB. 2 M/Sec
156
6 mill (0.15 mm) flex CB (1)
CB pressed to flat copper surface.
Device thermal pad is not soldered.
140
64 mill (1.62 mm) rigid CB. Thermal conductive paste between PCB
and copper is used.
CB pressed to flat copper surface.
Device thermal pad is soldered.
75
64 mill (1.62 mm) rigid CB. Thermal conductive paste between PCB
and copper is used.
Oil bath.
40
64 mill (1.62 mm) rigid CB. Oil is under intensive circulation (1).
WCSP die. CB pressed to flat
copper surface.
160
32 mill (0.81 mm) rigid CB
WCSP die. CB pressed to flat
copper surface.
160
6 mill (0.18 mm) flexible CB
(1)
9
THERMAL
RESISTANCE
(m°C/mWt)
The decision to solder or not solder the thermal pad does not make a significant difference.
Soldering to PCB
Soldering the TMP116/117 to a PCB can create significant package stress and degrade the absolute
accuracy. The measuring error of a TMP116/117 device in an oil bath before and after soldering often
shows an increase in the error, especially on rigid PCBs with the thermal pad soldered. This soldering shift
can be significant for precise measurements. Figure 5 shows the boards used in soldering shift tests. All
measurements were made in an oil bath.
Figure 5. Printed-Circuit Boards Used
In Figure 5, Board A is the socketed board used for testing loose devices prior to soldering. Board B is a
flexible PCB, and board C is a rigid PCB. Both used for testing devices after soldering.
Figure 7 shows the impact of soldering for 16 devices soldered to a rigid coupon boards. In Figure 7, parts
were measured in an oil bath at +25ºC with a 3.3-V supply before and after soldering. In this case, the
package thermal pad was also soldered to the coupon board. The average soldering shift in the example
is around 20mºC, but for device #4456, it reaches 50mºC. According to our research, the soldering shift is
not predictable, can be positive or negative, and, in the worst case, can reach ±100mºC.
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Soldering to PCB
130
120
110
100
90
80
70
60
50
40
30
20
10
0
-10
-20
-30
4439
4456
3739
3740
4050
4048
3737
3047
3080
3385
3734
3735
3081
3387
3386
Loose Units in sockets
Coupon Boards
3393
Temperature Error (mC)
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D001
Device ID
Figure 6. Soldering Shift at +25ºC and Supply 3.3 V With Thermal Pad Soldered on a Rigid PCB.
Furthermore, the soldering shift can be different for different temperatures, which makes it even less
predictable.
The main reason for the soldering shift is mechanical tension coming to the silicon die through the
package from the PCB and the hardened solder. When the temperature drops in the reflow oven, the
solder hardens and fixes the thermal pad and package pin locations. But package material continues to
contract, and because the solder and the rigid PCB have different contraction coefficients than device
package, it creates the mechanical tension which leads to package bending and therefore creates
tensions in the silicon die. However, when the package thermal pad is not soldered, the bending forces
are applied only to the package pins, which have much less mechanical contact to the silicon die.
Figure 7 shows the effects of soldering when the thermal pad is not soldered to the PCB. In this case, the
accuracy shift is much less and the worst-case offset is only 15 mC.
20
Temperature Error (mC)
15
10
5
0
-5
-10
-15
-20
Loose Units
Coupon Boards
-25
2107
2418
1765
1460
2420
2421
1459
2419
2111
1769
2115
2109
1772
1771
1773
1770
-30
D003
Device Number
Figure 7. Soldering Shift for TMP116/117 Without the Thermal Pad Soldered to the PCB. +25ºC, V = +3.3 V
The reasonable question is: when the thermal pad is not soldered, by how much will the thermal
resistance between the sensor and the PCB going to increase? In conducted experiments, the device was
soldered to a rigid coupon board 11-mm × 22-mm × 1.1-mm size with no vias under the part and a copper
radiator was attached to the opposite side of PCB. (The silicon thermo conductive paste between copper
radiator and PCB back side was applied). The measurements showed that not soldering the package
thermo pad increased the thermal resistance from 75 to 140ºC/Wt. By knowing the thermal resistance and
device thermal mass Mt = 5.1 mJ/ºC, it is possible to calculate the sensor thermal response time with
Equation 4.
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Self-Heating
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The calculated response time values are 0.39 and 0.72 seconds and measured response time matched
the calculated values. Because the device package thermal mass is extremely small, the thermal
response time is also very small and even the 0.72 second value, when the thermal pad is not soldered,
satisfies most users applications.
Here are the recommendations on how to minimize the soldering shift in the TMP116/117 parts:
• To maintain device manufacturer precision, in case the system calibration is not planned, TI highly
recommends not to solder the package thermal pad to avoid a soldering shift.
• Use the standard reflow oven soldering process with a maximum temperature to +250ºC for one
minute.
• Manual soldering is not acceptable because it creates additional stress on the device package,
resulting in soldering shift as large as ±150mºC.
• Using a flexible PCB with thickness less than 6 mil (0.15 mm) creates minimal mechanical tensions
and minimal soldering shift even in the case when the thermal pad is soldered.
• When using a flexible PCB with thickness more than 6 mil (0.15 mm), the thermal pad must not be
soldered. The flexible PCB minimizes the thermal mass and thermal resistance, which may improve
measurement precision.
10
Self-Heating
To achieve the best measurement accuracy, the TMP116/117 part is specially designed to dissipate
minimal power and minimize the part temperature change due to self-heating. In typical conditions (supply
voltage is 3.3 V, 8 samples average, one data collection per second), the TMP116/117 dissipates 53 uWt
at +25°C. However, when operating with a higher supply voltage and taking more frequent measurements,
the power dissipation can increase to almost 1 mWt. Figure 8 shows the power dissipation as a function of
the device temperature at different voltage supplies.
1.1
1
0.9
1.8 V
2.6 V
3.3 V
4.1 V
4.8 V
5.5 V
Power (mWt)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-75
-50
-25
0
25
50
75
Temperature (qC)
100
125
150
D011
Figure 8. Device Consumption Power vs Temperature and Part Supply Voltage in Continuous Conversion
Mode. No Pauses Between Conversions, No I2C Bus Activity.
The power consumption in user measurements is usually significantly less than 1 mWt, but to make the
most accurate measurement and reduce any influence of self-heating, all efforts to reduce the dissipation
power must be taken. Here are recommendations on how to reduce the device power consumption:
• Use the minimal supply voltage acceptable for the system. This is especially important when the
device is in continuous conversion mode without the pauses.
• Use one-shot conversion mode or use a conversion cycle mode where the device goes into standby
after a conversion.
• Use pullup resistors larger than 5 kΩ on the SDA, SCL, and ALERT pins. Place resistors at least 10
mm from the TMP116/117 to reduce any influence from the resistor's heat dissipation.
• Ensure that the SCL and SDA signal levels are below 10% and above 90% of the device supply
voltage. If the SCL, SDA, and ADD0 pin input voltages are close to ground or device supply level, the
current going through the digital pin input cell is low, which minimizes the sensor heating (see
Figure 9). Remember that the I2C bus voltage can go up to 6 V and is not limited by the applied supply
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11
Self-Heating Estimation Example
•
voltage.
Avoid heavy bypass traffic on the I2C bus. Remember that the intensive communication to other
devices on the same bus increases the TMP116/117 supply current, even if the device is in shutdown
mode (see Figure 10).
Use the highest available communication speed. To increase the SCL and SDA rising edge speeds,
use a bus pullup voltage higher than the device supply voltage.
ISUPPLY (uA)
•
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650
600
550
500
450
400
350
300
250
200
150
100
50
0
5.5 V
4.4 V
3.3 V
2V
0
10
20
30
40
50
60
VIN/V+ (%)
70
80
90
100
D005
Figure 9. Supply Current vs. Pin Input Voltage and Device Supply Voltage for Any Digital Pin Input Cell.
100
5.5 V
4.5 V
3.3 V
1.9 V
90
Supply Current (uA)
80
70
60
50
40
30
20
10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
I2C Bus Frequency (MHz)
0.8
0.9
1
D006
Figure 10. Device Supply Current vs. I2C Bus Clocking Frequency and Supply Voltage. Part is in
Shutdown Mode, but SCL, SDA, and ADD0 Pins are Under Constant I2C Data Flow.
11
Self-Heating Estimation Example
The self-heating impact can be calculated by the simple formula below:
Tsh
P u Rt
where
•
•
•
Tsh is a temperature offset due to sensor self-heating.
P is an averaged power dissipated by the sensor.
Rt is a combined temperature resistance to the environment.
(7)
This implies that another way to reduce the self-heating is to reduce the thermal resistance to the
measured object. On the contrary, the larger the thermal resistance between the sensor and measured
object, the larger the self-heating influence on measurement precision. Below are listened cases when the
self-heating effect can be ignored:
• The desired measurement precision is worse than ±0.2ºC.
12
Precise Temperature Measurements With the TMP116 and TMP117
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Self-Heating Estimation Example
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•
•
•
The system calibration takes care of self-heating and all other effects.
The device average consumption power is less than 0.1 mWt.
The thermal resistance between the sensor and measured object is small.
In this list, the most difficult parameter to estimate is the thermal resistance between the sensor and the
environment. The estimation is difficult because it depends on many poorly controlled factors. Here is a
recommendation on how to estimate the device object thermal resistance in a real application environment
and then calculate a possible self-heating temperature rise for a worst case scenario. The idea is to
measure the self-heating for some fixed supply voltage and fixed environment temperature, and then
extrapolate results over an entire voltage and temperature range.
Figure 11 shows an example of the self-heating effect on positioning the coupon boards horizontally in a
"still air box", with a TMP116/117 placed on top of the board. At time zero, the device is switched from
shutdown mode to continuous conversion mode with a 64 sampling averaging and no pauses between
conversions. There is no heating from the I2C bus activity because the data reading happens only once
per second. The temperature change on Figure 11 happens only due to device dissipated power and
following self-heating. Let's calculate the thermal resistance between the part and its environment.
80
5.5 V
5V
4V
3V
1.9 V
Temperature Change (mC)
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Time (seconds)
70
80
90
D007
Figure 11. TMP116/117 Coupon Board Self-Heating Effect vs. Time and Supply Voltage in Still +25ºC Air.
For example, assume the customer test was done with a 3-V supply and air temperature +25ºC. We see
the device temperature stabilized after 90 seconds with 40mºC self-heating value. According to Figure 8,
the consumption power for this mode is 0.36 mWt for a 3-V supply. So, the thermal-resistance between
the device and surrounding air is Rt = 40mºC/ 0.36 mWt = 111C/Wt. Now, knowing the thermal resistance,
it is possible to calculate the self-heating offset for other situations. For example, if the air temperature is
+125ºC and the supply voltage is 4 V according to Figure 8, the dissipated power would be 0.65mWt and
self-heating temperature offset would be Tsh = 111C/Wt x 0.65mWt = 72m°C. The 80-second long settling
time here is associated with stabilization time of air convection process in the “still air box”. if the box size
changes, the self-heating and stabilization time will also change.
As a reminder, this example above is a worst-case scenario where the thermal resistance between the
device and environment is high and device is continuously converting. It does demonstrate, however, that
self-heating can occur and must be considered when trying to achieve the best precision. If the
experiment is repeated with moving air, the self-heating offset will be much smaller and could become
negligible. But in all cases, the recommendation is the same: minimize the device dissipated power.
The easiest way to minimize the dissipated power is to limit the rate at which the temperature is sampled.
If we used device default mode (8 sample averaging with sampling rate 1 Hz) in the example above, the
average supply current would be 16 µA, the dissipated power would be only 48 µWt, and the self-heating
would only be 5.3m°C, which is less than sensor resolution and is negligible.
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Supply Voltage Change
12
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Supply Voltage Change
Precise measurements usually mean that supply voltage has minimal noise that does not change during
the measurements. In some battery systems, however, the voltage can change significantly with battery
aging. The TMP117 has excellent (almost zero) electrical PSRR, and the supply voltage change has no
effect on the precision of the readings. The only case where a system designer must take precautions is
when the device dissipates some heat in continues conversion mode without the standby. If the thermal
resistance to the object is significant, the supply voltage change from the maximum to minimum can
create sensor self-heating offset change (so named self-heating PSRR). Standard recommendations of
minimizing device average dissipated power and minimizing thermal resistance to the object applies in this
case. For the TMP116, the typical electrical PSRR is around 10mºC/V, and should be considered if the
supply voltage changes. The best recommendation for precise temperature measurements is simple:
stabilize the supply voltage at minimum system acceptable level.
13
Data Averaging
The TMP116/117 can be configured to take multiple measurements and provide the resultant average as
the result. Figure 12 and Figure 13 show the output temperature distribution with no averaging for 3
temperatures, and no averaging for different supply voltages. In all these cases, the standard deviation of
the readings is about 1 LSB, and data distribution covers an area approximately of six neighboring codes,
which match the ±3 sigma rule. This leads to the important conclusion that sensor internal noise is the
same for whole temperature range –55ºC to +150ºC, and the whole supply voltage range 1.9 V (1.8 V) to
5.5 V. Based on this data, the sensor internal noise without averaging in ideal bath condition can be
estimated as ±25m°C.
50
40qC (St. Dev. = 1.12)
25qC (St. Dev. = 1.01)
125qC (St. Dev. = 1.05)
45
40
Appearing in %
35
30
25
20
15
10
5
0
-4
-3
-2
-1
0
Data Distribution (LSB)
1
2
3
4
D008
Figure 12. The TMP116/117 Sampling Distribution for 3 Different Oil Bath Temperatures
and 3.3-V Supply Voltage. No Data Averaging.
14
Precise Temperature Measurements With the TMP116 and TMP117
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Summary
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50
V = 1.9 V (St. Dev. = 0.91)
V = 3.3 V (St. Dev. = 1.01)
V = 5 V (St. Dev. = 0.96)
45
40
Appearing in %
35
30
25
20
15
10
5
0
-4
-3
-2
-1
0
Data Distribution (LSB)
1
2
3
4
D009
Figure 13. The TMP116/117 Sampling Distribution for 3 Different Supply Voltages at +25ºC.
No Data Averaging.
The TMP116/117 provides an internal mechanism for averaging 8, 32, and 64 consequent samples
controlled by the configuration register. As shown in Figure 14, even the 8 samples averaging reduces the
internal noise distribution to a theoretical minimum of 2 LSB. This means that if the measured temperature
changes slowly and has no temperature fluctuations, the supply voltage is stable and has no glitches, and
there is no heavy bypassing traffic on I2C bus, the 8 samples averaging is enough to neutralize the
internal sensor noise and provide stable temperature readings. However, if the measured conditions are
far from ideal, higher averaging numbers are recommended.
80
Average 8 (St. Dev. = 0.51)
Average 32 (St. Dev. = 0.56)
Average 64 (St. Dev. = 0.61)
70
Appearing in %
60
50
40
30
20
10
0
-4
-3
-2
-1
0
Data Distribution (LSB)
1
2
3
4
D010
Figure 14. Temperature Sampling Noise With 8, 32, and 64 Internal Averages.
Temperature +25ºC and V = +3.3 V.
14
Summary
The TMP116/117 provides excellent precision, small power consumption, extremely small thermal mass,
and averaging tools with wide temperature and supply range. To achieve best performance, system
designers must follow the recommendations in this application note and product data sheets.
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15
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 (April 2018) to A Revision .......................................................................................................... Page
•
•
•
•
•
•
•
•
•
•
16
Added references to the TMP117 device .............................................................................................. 1
Added Measuring Solid Surface Temperature section ............................................................................... 3
Added Measuring Human Body Temperature section ............................................................................... 5
Added Measuring Still Air Temperature section ....................................................................................... 6
Added Measuring Human Body Temperature section ............................................................................... 6
Added Measuring Thermal Resistance in Different Environments section ........................................................ 8
Removed Devices Temperature Error Change Due to Device Soldering for 6 Different Temperatures. V = +3.3 V
graph ...................................................................................................................................... 10
Added recommendations on how to minimize the soldering shift in the TMP116/117 parts................................... 10
Changed Device Consumption Power vs Temperature and Part Supply Voltage in Continuous Conversion Mode
graph ...................................................................................................................................... 11
Added Supply Voltage Change section ............................................................................................... 14
Revision History
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