AN871

M
AN871
Solving Thermal Measurement Problems Using The TC72
And TC77 Digital Silicon Temperature Sensors
Author:
Jim Lepkowski,
Microchip Technology Inc.
INTRODUCTION
The TC72 and TC77 are CMOS silicon temperature
sensors that provide an accurate digital temperature
measurement to solve thermal measurement problems.
Data is converted from an internal diode temperaturesensing element to a digital format that can be directly
interfaced to a microcontroller, as shown in Figure 1.
The TC72 and TC77 sensors offer many system-level
advantages, including the integration of the sensor and
signal conditioning circuitry in a small Integrated Circuit
(IC) package.
The main distinguishing feature of the TC72 is its Oneshot Operating mode, which performs a single temperature measurement and then goes into power-saving
Shutdown mode. The One-shot mode makes the TC72
sensor a good choice for power-critical, portable applications. The main feature of the TC77 sensor is its
excellent temperature accuracy specification of 1°C
from +25°C to +65°C (max.), making this device an
excellent choice for precision temperature-sensing
applications.
VDD
The circuitry inside the TC72 and TC77 will be analyzed to illustrate the principles that these sensors use
to accurately measure temperature. In addition, application guidelines will be provided that can maximize
the accuracy of the temperature measurement.
SILICON IC SENSOR
FUNDAMENTALS
Temperature Measurement Diode
IC sensors measure temperature by monitoring the
voltage across a diode. The TC72 and TC77 use a
bipolar temperature-sensing diode that is built from the
substrate of a CMOS IC process. The bipolar diode is
created from a PNP transistor that is formed by
combining the appropriate P and N junctions, as shown
in Figure 2. This method of creating the bipolar
substrate diode is also used by the band gap voltage
reference circuit that is used in almost every analog
and digital IC.
VDD
0.1 µF
0.1 µF
VDD
VDD
PICmicro®
MCU
TC72
CE
AN0
SCK
SCK
SDO
SDI
SDI
SDO
GND
FIGURE 1:
TC77
PICmicro®
MCU
CS
AN0
SCK
SCK
SI/O
SDI
VSS
SDO
Typical Applications of the TC72 and TC77 Temperature Sensors.
 2003 Microchip Technology Inc.
DS00871A-page 1
AN871
threshold voltage, which is process-dependent. Since it
is difficult to obtain an accurate sensor with a MOSFET
diode, most silicon sensors use the substrate bipolar
diode as the temperature-sensing element.
A bipolar diode is used for the temperature measurement because its electrical characteristics are better
than a MOSFET diode. The current and voltage
relationship of a MOSFET diode is dependent on the
Collector
Base
p+
n+
Emitter
p+
n+
p+
⇔
n Well
p Substrate
PNP Transistor
Equivalent Diode
PNP Transistor in N-Well CMOS Technology
FIGURE 2:
Temperature-Sensing Substrate Diode.
Fundamental Diode Equations
The voltage and current equations for a diode are listed
in Figure 3. These equations show that a diode has a
voltage that is proportional to temperature and the
constants k and q. However, the process-dependant
constants of η and IS are also in the equation. IC
temperature sensors solve the process-dependent
issue with a voltage proportional to temperature
(VPTAT) voltage generator circuit, which is similar to a
band gap voltage reference.
If
+
Vf
-
 Vf 
  ---------

V
----------
  ηkT

  ------f  
q




VT 

If = Is e
– 1 ≅ I s e










I
kT
V f = ------ In  ----f 
q I 
s
I
= V T In  ----f 
I 
s
FIGURE 3:
DS00871A-page 2
The non-ideality constant (η) for a silicon diode varies
from 0.95 to 1.05. However, η will be assumed to be
equal to one. The assumption of η not being equal to
one produces a temperature gain and offset error. This
error is minimized in the sensor’s calibration
procedure.
The IS variable must be eliminated because IS varies
with temperature and also from wafer to wafer. The IS
variable in the diode’s voltage equation can be
eliminated by two different methods. The first method
eliminates IS using two different current sources and a
single diode, while the second method uses a single
current source and two different diodes.
where:
If
IS
k
= Forward Current
= Saturation Current
= Boltzmann's Constant
= 1.38 x 10-23 joules/°K
η
= Diode Non-Ideality Constant
= Emission Coefficient in SPICE
q
= Electron Charge
= 1.6 x 10-19 Coulombs
T
= Absolute Temperature (Kelvin)
= Forward Voltage
Vf
= Thermal Voltage
VT
= kT/q
≅ 26 mV @ 25°C
Assumption:
η
= 1
Fundamental Diode Equations.
 2003 Microchip Technology Inc.
AN871
Creating A Voltage Proportional To
Temperature
The TC72 and TC77 use the two current sources with
a single diode method to eliminate IS. Figure 4 provides
a simplified schematic of the circuit that measures the
voltage resulting from multiplexing two current sources
across a diode. The equations illustrate that the IS
variable is cancelled by either subtracting the voltages
or equivalently by calculating the ratio of the logarithmic
equations.
The two current, one diode method is used to eliminate
IS because it is relatively easy to build current sources
that are a ratio of each other. In practice, the two
currents are chosen to have a ratio of ten, which
produces a voltage with a temperature coefficient of
approximately 200 µV/°C. The ∆VEB equation is
important because it contains three constants (k, q and
N) and the temperature variable T. This equation
establishes a voltage that is proportional to a constant
multiplied by temperature, while eliminating the
process-dependent variable, IS.
Voltage ∆VEB is also referred to as VPTAT, or the voltage
which is proportional to absolute temperature. Figure 5
shows a graphical representation of the VPTAT voltage,
which is linear with a slope, or temperature coefficient,
equal to approximately 200 µV/°C with N = 10. The
absolute value of the current source is not in the
temperature equation. It is only important that the ratio
(N) of the two current sources track each other over
temperature. Note that it has been assumed that ∆VEB
is only a function of the current and thermal voltage VT
(VT = kT/q). While the complete equation for ∆VEB is
more complex, this complication can be neglected as a
second order effect.
An alternative method to eliminate the IS term in the
diode’s voltage equation is accomplished by measuring
the voltage of two different diodes created from a single
current source, as shown in Figure 6. This method to
eliminate the process variable IS is used because the
magnitude of the currents can be controlled by the
dimensions of a transistor. The current ratio circuit can
be created by using a parallel circuit of N transistors
identical to the first. Reference [4] provides further
details on the current ratio circuit shown in Figure 6.
The total current is shared equally between the
transistors and the voltage VEB(N) is established. A
second method to implement this circuit is to scale the
emitter area of the transistors.
∆V EB = V EB ( I ) – V EB ( I
2
1)
N×I
I
------ In  --------------1  – kT
= kT
------ In  ---1- 



q
q
IS
IS 
I1
I2 = N x I1
  N × I 1 
  -------------I S  
= kT
------ In  ------------------q   I1  
 ---- 
  IS  
+
k
= --- In ( N ) × T
q
VEB
= CONSTANT × T
-
where:
N = Integer number,
VEB = emitter-to-base junction voltage
FIGURE 4:
Creating a Voltage Proportional to Temperature with Two Current Sources and One
Diode.
 2003 Microchip Technology Inc.
DS00871A-page 3
AN871
VEB
VEB(I2)
∆VEB = VPTAT
VEB(I1)
I2
I1
FIGURE 5:
IC
Graphical Representation of the VPTAT Voltage Created with Two Current Sources and
One Diode.
I
V EB = kT
------ In  ---1- 
q  IS 
I1 
kT
V EB ( N ) = ------ In  -------------q  N × IS 
I1
∆V EB = V EB – V EB ( N )
I
I1 
------ In  ---1-  – kT
------ In  -------------= kT



q
q
IS
N × IS 
N Transistors
+
VEB
-
+
VEB(N)
-
  I 1 
  --I S 
kT
= ------ In  ---------------------q   I1  
 -------------- 
 N × I 
S
k
= --- In ( N ) × T
q
= CONSTANT × T
FIGURE 6:
DS00871A-page 4
Creating a Voltage Proportional to Temperature with One Current Source and Two
Diodes.
 2003 Microchip Technology Inc.
AN871
TC72 AND TC77 BUILDING BLOCKS
Figure 7 provides simplified block diagrams of the
TC72 and TC77. Details of the temperature building
blocks will be analyzed to demonstrate how a silicon
sensor accurately measures temperature. In addition,
the review of the circuitry inside the temperature sensor
will provide an understanding of the advantages and
disadvantages of silicon sensors as compared to other
temperature sensor technologies.
VDD
Internal
Diode
Temperature
Sensor
10-Bit
Delta-Sigma
A/D Converter
Temperature
Register
GND
FIGURE 7:
Calibration
Registers
The TC72 and TC77 sensors offer many system-level
advantages, including the integration of the sensor and
the signal conditioning circuitry. Advancements in
CMOS IC fabrication processes has enabled the
integration of the temperature sensor, ADC and digital
registers on a single chip that is connected to the
processor through a serial data bus. The serial I/O
communication interface to a microcontroller allows the
user the ability to select either the Continuous
Temperature Conversion, One-shot or the powersaving Shutdown operating mode, in addition to
reading the temperature and manufacturer ID
registers.
VDD
TC72
Manufacturer
ID Register
Serial
Port
Interface
Control
Register
TC77
Internal
Diode
Temperature
Sensor
13-Bit
Delta-Sigma
A/D Converter
CE
SCK
SDO
SDI
Temperature
Register
VSS
Calibration
Registers
Manufacturer
ID Register
CS
Serial
Port
Interface
SI/O
SCK
Configuration
Register
TC72 and TC77 Simplified Block Diagrams.
 2003 Microchip Technology Inc.
DS00871A-page 5
AN871
Internal Diode Temperature Sensor
coefficient of the voltage created from a VPTAT circuit,
as shown in Figure 8. The voltage VEB has a
temperature coefficient of -2.2 mV/°C, while the VPTAT
voltage has a temperature coefficient of +0.085 mV/°C.
Next, VPTAT is amplified by K so that the temperature
coefficient is scaled to +2.2 mV/°C. When V EB is added
to the scaled VPTAT signal, the two temperature
coefficients cancel and an output voltage results that is
independent of temperature.
BAND GAP VOLTAGE REFERENCE
A band gap voltage reference circuit is used to create
a reference voltage that is stable over temperature.
The term band gap refers to the theoretical voltage of a
silicon junction at 0°K. Band gap circuits achieve
temperature independence by canceling the negative
temperature coefficient of a PNP transistor’s emitter-tobase diode voltage (VEB) with the positive temperature
V
I1
VEB
VPTAT
KVPTAT
VEB
VREF = 1.25V
+
VPTAT
VREF = V EB + KVPTAT
VREF
KVPTAT
K
T (°C)
Temperature Coefficients (@ +25°C)
VEB = -2.2 mV/°C
VPTAT = +0.085 mV/°C
FIGURE 8:
Band Gap Voltage Reference Concept.
A simplified schematic of a band gap circuit is shown in
Figure 9. This circuit is based on the principle that the
magnitude of currents I1 and I2 are proportional to the
size of the emitter area (AE) of the transistors. A
1.250V reference voltage (V REF) will be produced if the
R1= p
R2 = p
+
I1
I2
FIGURE 9:
DS00871A-page 6
VREF
-
R3 = 1
Q1
AE = 1
emitter area ratio is equal to eight (n = 8) and the resistor ratio is set to ten (p = 10). References [1] and [3]
provide further details on the band gap voltage
reference circuit.
VPTAT
V REF = V E B + KV PTAT
kT
V REF = V E B ( Q 1) + p  ------ In ( n )
 q
Q2
AE = n
Band Gap Voltage Reference Building Block.
 2003 Microchip Technology Inc.
AN871
Delta-Sigma Converter
input signal to a high-frequency digital signal by
functioning as a 1-bit ADC where the output is a digital
pulse stream that is representative of the average
value of the input signal. The comparator then drives a
1-bit DAC, which is essentially a switch that provides a
reference signal to the difference amplifier.
FUNDAMENTALS
The TC72 and TC77 use a Delta-Sigma (∆Σ) analog-todigital converter (ADC). ∆Σ ADCs are used in the
majority of digital temperature sensors because they
are easy to integrate, offer a high bit resolution and
have low power consumption. The TC72 has a 10-bit
ADC with a typical conversion time of 150 ms, while the
TC77 has a 13-bit ADC with a typical conversion time
of 300 ms.
The basic principle of the ∆Σ ADC is to digitize an
analog signal with a very low resolution 1-bit ADC at a
very high sampling rate. This over-sampling technique
effectively increases the resolution of the ADC. The
output of the ∆Σ ADC is a 1-bit data stream that is
converted by a counter or accumulator circuit to a
digital count, which is representative of the measured
temperature. The counter circuit provides the digital
filtering function to restore an output stream of either
ones or zeroes which is representative of the input
data. The filtering is accomplished by counting the
number of pulses in a fixed time window.
A block diagram of the architecture of the ∆Σ ADC is
given in Figure 10. The first part of the ADC is a difference amplifier, followed by an integrator amplifier. The
difference amplifier is used to buffer the analog input
signal and to complete the feedback loop from the
DAC. The integrator is used to provide gain and
functions as a high-pass filter that will minimize the
quantization noise. Next, the comparator converts the
Difference
Amplifier
Analog
Input
+
Delta (∆)
Switched Capacitor
Integrator
Comparator /
1-Bit ADC
∫
+
-
Sigma (Σ)
Digital
Output
To Digital
Filter
1-Bit DAC
VREF
FIGURE 10:
Simplified Delta-Sigma ADC Block Diagram.
SWITCHED CAPACITOR AMPLIFIER
The switched capacitor amplifier provides gain in the
∆Σ ADC. The VPTAT signal created from the VPTAT
voltage generator circuit is amplified with the switched
capacitor integrator to increase the magnitude of the
temperature coefficient. Switched capacitor amplifiers
feature low noise and offset voltages that are needed
to accurately amplify the VPTAT voltage of 200 µV/°C to
a voltage of approximately of 2 mV/°C.
A switched capacitor amplifier is based on the principle
that a capacitor can be used to create an equivalent
resistance in a switching circuit, as shown in Figure 11.
Amplifier circuits can be built using capacitors in place
of resistors and have the advantage of an inherent
“auto-zeroing” feature that minimizes the input offset
 2003 Microchip Technology Inc.
voltage error of the amplifier. The analog switches are
built by using both a N-channel and P-channel
MOSFET in parallel.
Switched capacitor amplifiers are also used because it
is relatively easy to build capacitors that are equal to a
ratio of each other in an IC process. Also, the effective
magnitude of the capacitance can be accurately
controlled using a time multiplexed scheme. For
example, a 2 nF capacitor that is switched into the
circuit with a 50% duty cycle is equivalent to a 1 nF
capacitor.
DS00871A-page 7
AN871
Analog Switch
Switched Capacitor
Integrator
Analog Integrator
C2
φ
VIN
VOUT
C2
R
VIN
+
φ
VIN
VOUT
C1
+
VOUT
1
R EFF = ----------fC C1
–1
V OUT = ---------- ∫ V IN dt
RC 2
FIGURE 11:
fc
Switching Capacitor Circuits.
A switched capacitor, VPTAT amplifier is shown in
Figure 12. See reference [3] for additional information.
For simplicity, the circuit shown in Figure 12 is singleended, while the TC72 and TC77 use a differential
topology. A differential integrator increases the noise
immunity of the amplifier by reducing the common
mode noise of the analog ground signal.
I2 = (N-1) x I1
I1
φ1
φ1
C2
C1
A1
φ2
VPTAT
A2
C3
Gain = -1
C
I1 + I2 
kT
V PTAT =  -----1-   ------  In  ------------- C  q   I

2
1
FIGURE 12:
DS00871A-page 8
Switched Capacitor VPTAT Amplifier.
 2003 Microchip Technology Inc.
AN871
Digital Registers
OPERATING MODES
The TC72 has four internal 8-bit registers, while the
TC77 has three 16-bit registers that are used by a
microcontroller for communication. The temperature
measurement data is stored in the Temperature
Register, while the TC72 Control Register or TC77
Configuration Register is used to select the operating
mode of the sensor. The Manufacturer’s Identification
(ID) register is used to identify the sensor as a
Microchip component. Tables 1, 2 and 3 provide the bit
definitions of the registers.
The user configured operating modes of the TC72 and
TC77 include a Continuous Temperature and a Shutdown mode that are selected via the Control/
Configuration Register. In the Continuous Temperature mode, an ADC conversion is performed every
150 ms for the TC72 and every 300 ms for the TC77.
If a Temperature Register read operation is requested
while an ADC conversion is in progress, the previous
completed ADC conversion data will be outputted via
the sensor’s serial I/O port.
The Shutdown mode is used to minimize the power
consumption of the TC72 and TC77 sensors when
active temperature monitoring is not required. The
Shutdown mode disables the temperature conversion
circuitry; however, the serial I/O communication port
remains active. The current consumption of the sensor
will be less than 1 µA when the Shutdown mode is
activated.
The TC72 offers a One-shot mode, which is useful
when only a single temperature recording is required.
The One-shot mode performs a single temperature
measurement and returns to the power-saving
Shutdown mode.
The Calibration Register is used to store the
adjustments that are determined during the sensor’s
acceptance test procedure. The Calibration Registers
are not accessible by the external microcontroller. The
contents of the Calibration Registers are nonvolatile.
TABLE 1:
TC72 DIGITAL REGISTERS
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
Bit
0
Value at
Power-up/Reset
Control
0
0
0
One-shot (OS)
0
1
0
Shutdown (SHDN)
Shutdown mode
LSB Temperature
-1
Register
0
0
0
0
0
0
Temp. = 0.00°C
Sign
26
25
24
23
22
21
20
Temp. = 0°C
0
1
0
1
0
1
0
0
54 hex
2
MSB Temperature
Manufacturer ID
TABLE 2:
2-2
TC72 CONTROL REGISTER TEMPERATURE CONVERSION MODE SELECTION
Control Register Bit 4
One-shot (OS)
Control Register Bit 0
Shutdown (SHDN)
Continuous Temperature Conversion
0
0
Shutdown
0
1
Continuous Temperature Conversion
(One-shot Command is ignored if SHDN = ‘0’)
1
0
One-shot
1
1
Operational Mode
TABLE 3:
TC77 DIGITAL REGISTERS
Bit
15
Bit
14
Configuration **
C15
C14 C13 C12
Temperature
Sign
27
26
25
24
0
1
0
1
0
Register
Manufacturer ID
Bit
13
Bit
12
Bit
11
Bit
10
Bit
9
Bit
8
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
Bit
Value at
0 Power-up/Reset
C11 C10
C9
C8
C7
C6
C5
C4
C3
C2
C1
C0 Continuous
Temperature
Conversion
mode
23
22
21
20
2-1
2-2
2-3
2-4
*
x
x
Temp. = -2°C
1
0
0
0
0
0
0
0
0
x
x
Bit 15 to
Bit 8 = 54 hex
* Temperature Bit 2 = 0 during power-up; otherwise, Bit 2 =1
** C15:C0 = XXXX/XXXX 1111/1111 (Shutdown mode)
C15:C0 = XXXX/XXXX 0000/0000 (Continuous Temperature Conversion mode)
 2003 Microchip Technology Inc.
DS00871A-page 9
AN871
Temperature Data Format
The TC72’s temperature data is represented by a
10-bit two’s complement word with a resolution of
0.25°C per bit, as shown in Table 4. The example
below is of the Temperature Data Registers bit
definition for a temperature of 41.5°C.
The TC77’s temperature data is represented by a
13-bit two’s complement digital word, as shown in
Table 5. The Least Significant Bit (LSb) is equal to
0.0625°C. Note that the last three bits (Bit 0, 1 and 2)
are tri-stated and are represented as a logic ‘1’ in the
table. The example below is of the TC77’s Temperature
Register bit definition for a temperature of 85.125°C.
Example:
Example:
Temperature
=
41.5°C
Temperature
MSB Temperature Register
=
00101001b
25 + 23 + 20
Temperature Register
=
=
32 + 8 + 1 = 41
=
64 + 16 + 4 + 1 + 0.125
=
10000000b
85.125
=
2-1
=
=
0.5
LSB Temperature Register
TABLE 4:
TABLE 5:
TC72 TEMPERATURE OUTPUT
DATA
Temperature
Bit 7
Binary
MSB / LSB
Bit 0 / Bit 7
85.125°C
=
00101010 10010111b
=
26 + 24 + 22 + 20 + 2-3
TC77 TEMPERATURE OUTPUT
DATA
Temperature
Bit 0
=
Binary
Bit 15
Bit 0
+125°C
0011 1110 1000 0111
+25°C
0000 1100 1000 0111
+0.0625°C
0000 0000 0000 1111
+125°C
0111 1101 / 0000 0000
+25°C
0°C
0000 0000 0000 0111
0001 1001 / 0000 0000
+0.25°C
-0.0625°C
1111 1111 1111 1111
0000 0000 / 0100 0000
0°C
-25°C
1111 0011 1000 0111
0000 0000 / 0000 0000
-0.25°C
-55°C
1110 0100 1000 0111
1111 1111 / 1100 0000
-25°C
1110 0111 / 0000 0000
-55°C
1100 1001 / 0000 0000
Serial Port Interface
The TC72 and TC77 are designed to be compatible
with the Serial Peripheral Interface™ (SPI™) Serial I/O
Specification. This provides a simple communication
interface to a variety of microcontrollers.
The TC72’s serial interface consists of:
•
•
•
•
Chip Enable (CE)
Serial Clock (SCK)
Serial Data Input (SDI)
Serial Data Output (SDO)
The TC77’s serial interface consists of:
• Chip Select (CS)
• Serial Clock (SCK)
• Bidirectional Serial Data (SI/O) signals
Details on the sensor’s SPI protocol are given in the
TC72 data sheet (DS21743) and TC77 data sheet
(DS20092). Note that the SPI configuration defines the
voltage level and timing specifications for the I/O
signals. However, the register bit definitions and the
protocol of the read and write operations are unique for
most silicon IC sensors.
DS00871A-page 10
 2003 Microchip Technology Inc.
AN871
Putting the Building Block Circuits
Together to Create the TC72 and TC77
Details on the circuitry inside the TC72 and TC77 has
been greatly simplified to illustrate the features and
application issues of these sensors. The actual circuity
of the sensor is more complex and has been optimized
to produce a small, power-efficient sensor. For
example, the VPTAT circuit is implemented as part of the
∆Σ ADC circuit.
The main building blocks of the TC72 and TC77 are the
VPTAT generator, band gap voltage reference, DeltaSigma ADC, Digital Registers and the SPI serial I/O
port. Figure 13 shows these building blocks, which
along with an oscillator, logic control unit and voltage
detector, produce a sensing system that can accurately
measure temperature.
The TC72 and TC77 sensors use a sophisticated
network of switched current circuits and precisely
matched capacitors. The timing sequence of the
switching networks is controlled by the control unit,
which is, essentially, a digital state machine. The
voltage detector is used to control the power-up and
power-down sequencing of the digital registers.
The basic temperature measurement is implemented
by the VPTAT circuit, which produces the current IPTAT,
which is proportional to temperature. The band gap
voltage reference produces the reference current, IREF
from the reference voltage, which is not sensitive to
temperature. Next, the ∆Σ ADC compares the IPTAT and
IREF currents to produce a digital output that is
representative of the temperature measurement.
VPTAT Generator
I1
ITEMP
I2
Please refer to the TC72 data sheet (DS21743) and
TC77 data sheet (DS20092) for details on the digital
registers and SPI communication protocol.
Delta-Sigma ADC
+
-
∫
+
-
IREF
Counter /
Accumulator
VREF
VPTAT
Digital Registers
Temperature
Band Gap Voltage
Reference
Control
Logic
Control /
Configuration
Manufacture ID
VREF
Calibration
Registers
Voltage Detector
Oscillator
Serial Port Interface
FIGURE 13:
Detailed Block Diagram of the TC72 and TC77.
 2003 Microchip Technology Inc.
DS00871A-page 11
AN871
GUIDELINES TO MAXIMIZE TC72 AND
TC77 PERFORMANCE
Interpreting the Data Sheet Temperature
Accuracy Specification
3.5
3
2.5
2
1.5
1
0.5
0
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
Upper Specification Limit
Mean
Mean + 3V
Mean - 3V
Lower Specification Limit
-55
-35
The accuracy of the TC72 and TC77 is better than the
specification limits listed in the data sheet. However,
Microchip can only ensure that the devices will meet
the minimum and maximum error limits. The error limits
of the TC72 and TC77 data sheets are determined by
the accuracy of the sensor and production test
equipment. The accuracy plots in the data sheets were
determined by qualifying the IC design with laboratorygrade instrumentation. In contrast, the ovens used in
the high volume IC fabrication have a much larger
temperature variance. Therefore, the data sheet limits
have to be set higher to take into account the error of
the test equipment.
-15
5
25
45
65
85
105 125
Reference Temperature (°C)
FIGURE 14:
TC77’s Temperature
Accuracy (TC77-X.XMXX).
Percentage of Occurances (%)
Temperature Error (°C)
The accuracy of the TC72 and TC77 sensors is
measured by comparing the temperature output of the
sensors to the temperature measured by a calibrated
plantium RTD sensor. The sensors are placed in a
temperature chamber that provides a stable ambient
temperature. Figure 14 shows the temperature
accuracy graph of the TC77 that was created by
measuring a number of sensors and averaging the
temperature error.
The TC72 data sheet (DS21743) and TC77 data sheet
(DS20092) also provide histograms of the temperature
accuracy of the sensor. The histogram data shows the
variance of the temperature error over a sample of
sensors. Figure 15 shows a histogram of the
temperature error of the TC77 at +25°C.
50
Sample Size = 108
TA = +25°C
45
40
35
30
25
20
15
10
5
0
-1
-0.75
-0.5
-0.25
0
0.25
0.5
0.75
1
Temperature Error (°C)
FIGURE 15:
Histogram of TC77’s
Temperature Accuracy at +25°C
(TC77-X.XMXX).
DS00871A-page 12
 2003 Microchip Technology Inc.
AN871
The TC72 is specified with a VDD range of 2.65V to
5.5V. However, the temperature accuracy is tested and
calibrated at either 2.8V, 3.3V or 5.0V. The TC77 has a
supply voltage specification of 2.7V to 5.5V and the
temperature accuracy is tested and calibrated at either
3.3V or 5.0V. As V DD varies from the the voltage used
during the cailibartion procedure, the accuracy may be
degraded as shown in Figures 16 and 17.
The user should select a TC72 or TC77 sensor that has
a calibration voltage that is as close as possible to the
system voltage on the PCB to maximize the temperature accuracy of the sensor. Note that the temperature
error resulting from using a V DD voltage is different than
what was used during the calibration procedure and
varies as a function of temperature. In addition, the
temperature error versus VDD curve varies slightly from
sensor to sensor. Therefore, it is typically not possible
to compensate for this error at the microcontroller
system level.
Temperature Change (°C)
1
VDD = 2.7V
TC77-3.3MXX
0.5
0
VDD = 3.3V
-0.5
-1
VDD = 5.0V
-1.5
VDD = 5.5V
-2
-55 -35
-15
5
25
45
65
85
Temperature Change (°C)
Temperature Accuracy vs. VDD
0.4
TC77-3.3MXX
0.3
0.2
0.1
TA = +85°C
TA = +25°C
0
-0.1
TA = -25°C
-0.2
-0.3
-0.4
3
3.1
3.2
3.3
3.4
3.5
3.6
Supply Voltage (V)
FIGURE 17:
TC77’s Temperature
Accuracy vs. Supply Voltage Plotted at -25°C,
+25°C and +85°C.
Factory Calibration
The TC72 and TC77 sensors are calibrated during the
manufacturing process to maximize the accuracy of the
temperature measurement. The calibration procedure
can be compared to the y = mx + b representation of a
straight line. An adjustment to the sensor is
accomplished at the factory to calibrate both the slope
(m) or the temperature coefficient and the offset (b) of
the sensor. A simplified representation of the sensor
calibration procedure is shown in Figure 18. Note that
the calibration registers are only accessible during the
testing operation and cannot be reconfigured by the
user.
105 125
Reference Temperature (°C)
FIGURE 16:
TC77’s Temperature
Accuracy vs. Supply Voltage Plotted at
VDD = 2.7V, 3.3V, 5.0V and 5.5V.
2. Temperature Data After Gain Adjustment
3. Temperature Data After Offset Adjustment
Sensor Output
+85°C
1. Temperature Data Before Gain and Offset Adjustment
+25°C
+25°C
+85°C
Oven Temperature
FIGURE 18:
Simplified Factory Calibration Procedure.
 2003 Microchip Technology Inc.
DS00871A-page 13
AN871
Optional User Printed Circuit Board
(PCB) Level Calibration
While the TC72 and TC77’s temperature accuracy is
adequate for most applications, the accuracy of the
sensor can be improved by measuring the output of the
sensor, providing a correction in a microcontoller. Data
is provided in reference [2] that shows that the temperature error curve of a silicon IC is very repeatable. Reference [2] uses a second-order, curve-fitting equation
to compensate for the offset and curvature of the sensor’s error, improving the accuracy of the sensor by a
factor of ten. The sensor correction can also be implemented with a simple microcontroller lookup table routine where the correction temperature offsets are
stored in nonvolatile memory. Note that the temperature error curve will vary from sensor to sensor. Thus,
the optional temperature error adjustment requires a
board-level calibration for each sensor.
ADC Resolution
The resolution of the sensor is defined as the temperature reading per bit of the ADC. Although a higher ADC
resolution typically produces a more accurate sensor,
this may not always be true. The TC72’s 10-bit ADC
has a resolution of 0.25°C/bit, while the TC77’s 13-bit
ADC has a resolution of 0.0625°C/bit. Note that the
TC72’s data is formatted using a range of -128°C to
+127°C, while the TC77’s data is formatted with a
range of -256°C to +255°C. The resolution of the two
sensors is calculated below:
TC72 Resolution
TC77 Resolution
=
(-128°C to +127°C) / 210
=
256 / 210
=
256 / 1024
=
0.25°C/bit
=
(-256°C to +255°C) / 213
=
512 / 213
=
512 / 8192
=
0.0625°C/bit
While the temperature accuracy and resolution of a
silicon sensor are related, they are two different
specifications. Increasing the ADC’s number of bits
decreases the temperature per bit step size, which
reduces the quantization error and effectively averages
out some of the uncertainty error inherent in the
analog-to-digital conversion. In general, higher bit
order ADCs produce a more accurate sensor.
However, the conversion time of a ∆Σ is typically
doubled for each additional ADC bit.
DS00871A-page 14
Noise Immunity
While the TC72 and TC77 sensors do not require any
external components, it is recommended that a 0.1 µF
to 1 µF decoupling capacitor be provided between the
power supply and ground pins. Although the current
consumption of the TC72 and TC77 is modest, the
sensors contain an on-chip data acquisition system
with internal digital switching circuitry. Thus, it is good
design practice to use a decoupling capacitor with the
sensor. A high-frequency ceramic capacitor should be
used and be located as close as possible to the IC
power pins in order to provide effective noise protection
to the sensor.
The TC72 and TC77’s PCB should be designed with
the standard layout guidelines used for a low noise
circuit. The PCB should provide a ground plane or
copper trace as thick as possible at the ground pin. The
temperature sensor’s main thermal path to the PCB is
through the ground connection. Thus, the size of the
ground pad should be as large as possible. An IC
socket or extender board should not be used with the
sensors. The coupling of noise to the serial
communication pins can be minimized by keeping the
digital traces away from any high-frequency clock or
switching power supply signal traces.
Thermal Response Time
Internal diode silicon IC sensors provide an accurate
temperature measurement for a steady state or
relatively constant temperature. However, their
response time to a rapid change in temperature is
relatively poor compared to other temperature sensors,
such as plantium RTDs and thermistors. For example,
IC sensors are an excellent sensor for measuring the
temperature of an electronic enclosure, but they would
not be a good choice for a flow sensor that requires a
thermal response time of a few milliseconds.
Applications requiring a fast thermal response time
should consider using a remote diode sensor. A remote
diode sensor is similar to the internal diode sensor,
except that this sensor measures the temperature of a
remote diode located outside of the silicon IC sensor.
For example, remote diode sensors can be used to
measure the temperature of a remote diode located on
the die of a Pentium ® PC microprocessor.
The thermal response time of a silicon temperature
sensor is specified by mounting the device on a small
PCB, measuring the sensor’s output at a nominal
temperature such as +25°C and quickly exposing the
device to a temperature step such as +125°C. The
thermal time constant, defined as the time required for
the sensor to reach 63.2% of it’s final value, is usually
specified in either still air or a hot oil bath. The thermal
response is also defined by the time required by the
sensor to output a stable measurement equal to the
final temperature of the step response.
 2003 Microchip Technology Inc.
AN871
the surface by using a thermally-conductive adhesive,
such as thermal epoxy. The self-heating error of the
sensor is small because of the TC72 and TC77’s low
power consumption. Therefore, the die temperature
will accurately track the surface temperature.
The time required for a silicon sensor to reach the final
temperature of the step response is typically 1 to 3
minutes in still air and approximately 5 to 20 seconds in
a hot oil bath. Note that the thermal response
measurement of a silicon sensor varies by the size of
the PCB that the sensor is mounted on and also by the
IC’s package.
Silicon sensors provide a “non-contact” temperature
measurement by being in close proximity to a “hot
object”, such as the MOSFETs used in a switching
power supply circuit. The location of the silicon IC
sensor on the PCB can be critical, especially if large
temperature gradients exist. A silicon sensor measures
temperature by monitoring the voltage of a diode
located on the IC die. Thus, the sensor should be
located as close as possible to the external heat
source.
Silicon IC sensors measure temperature by monitoring
the voltage of a diode located on the IC die. Figure 19
provides a cross section of the SOT23 package. The
TC72 and TC77’s die substrate is connected to the
PCB’s ground plane through a bonding wire and the
ground pin of the package. The Ground pin of the IC
provides a low-impedance thermal path between the
die and the PCB, allowing the sensor to effectively
monitor the temperature of the PCB board. The other
non-grounded IC pins also provide a good thermal path
to the die. However, they are not directly connected to
the substrate and have a smaller effect on the die temperature. It is unlikely that a large temperature gradient
exists between the package pins, but if this condition
exists, an additional error will result in the temperature
measurement.
Note that it is difficult to develop a correlation that could
be used to adjust the temperature reading of the
sensor, which is in the vicinity of, but not in direct
contact with, the hot object that requires an accurate
temperature measurement. The accumulation of the
thermal resistances between the sensor and heat
source, in addition to factors such as the air-flow
velocity, will produce a temperature measurement with
too much uncertainty to accurately model. Remote
diode sensors and thermistors offer the advantage that
they are available in a variety of packages, including
parts that can be mounted directly on a hot object, such
as a heatsink.
The thermal path between the top of the package to the
ambient air and between the bottom of the package
and the PCB is not as efficient because the plastic IC
housing package functions as a thermal insulator.
Thus, the ambient air temperature (assuming that a
large temperature gradient exists between the air and
PCB) has only a small effect on the temperature
measured by the silicon sensor.
The TC72 and TC77 can also be located inside a probe
for applications such as a liquid temperature measurement. The sensors can be mounted on a small PCB,
which is then placed inside a sealed tube that is dipped
into the liquid. The temperature sensor and PCB can
be insulated against moisture by using a protective
coating, such as Humiseal.
Sensor Location
Silicon IC sensors are typically mounted on a PCB and
measure the ambient temperature inside an electronic
enclosure. The TC72 and TC77 can be used to
determine the surface temperature of a hot object, such
as a metal heat sink. The sensors can be connected to
SOT-23-5A
Mold Compound
Die Attach Adhesive
Copper Lead
FIGURE 19:
Die Attach Pad
Die
Gold Wire
Cross section of the TC77’s 5-Pin SOT23 Package.
 2003 Microchip Technology Inc.
DS00871A-page 15
AN871
Power-Up and Power-Down
CONCLUSION
The TC72 and TC77 devices contain a voltage detector
circuit that determines when a power-up or powerdown condition has occurred. A supply voltage lower
than 1.6V (typ.) is considered a power-down state for
the TC72 and TC77. The internal voltage detector
ensures that the sensor is held in a reset condition until
the voltage reaches a high enough level to ensure the
proper operation of the sensor. Also, the voltage
detector resets the digital registers to the power-up
values when a power-down or brown-out condition is
detected.
The TC72 and TC77 are CMOS silicon temperature
sensors that provide an accurate digital temperature
measurement to solve thermal management problems.
The TC72 and TC77 sensors offer many system-level
advantages, including the integration of the sensor and
the signal-conditioning circuitry in a small IC package.
This provides for easy system integration and
minimizes the required PCB space, component count
and design time.
The TC72 and TC77’s minimum supply voltage is specified at 2.65V and 2.7V, respectively. However, the
voltage supply should match the VDD used in the
calibration procedure in order to accurately measure
temperature. The operation of the sensor cannot be
ensured for a steady-state voltage below the minimum
specified V DD voltage.
Self-Heating Errors
The supply current for the TC72 and TC77 is less than
250 µA (typ.); therefore, the self-heating error is less
than 0.2°C. The rise in the die temperature (Tj) due to
the power consumption of the TC77’s SOT-23 5-pin
package operating at 3.3V in the Continuous
Temperature Conversion operating mode is shown
below:
∆Tj
=
PDissapation x θJA
=
(3.3V x 250µA) x 230°C/W
=
0.19°C
Where:
θJA is the package junction-to-air thermal
resistance provided in the data sheet.
The TC72 is a good sensor for power-critical portable
applications. The TC72’s One-shot Operating mode
performs a single temperature measurement and then
goes to the power-saving Shutdown mode. The TC77
offers a 1°C accuracy from +25°C to +65°C, making
this device an excellent choice for precision
temperature sensing applications.
REFERENCES
[1] Allen, P. and Holberg, D., “CMOS Analog Circuit
Design”, Oxford University Press, N.Y., 2002.
[2] Application Note 208 - “Curve Fitting the Error of
a Band Gap Based Temperature Sensor”,
Maxim Semiconductor, 2002.
[3] Bakker, A. and Huijsing, J., “High-Accuracy
CMOS Smart Temperature Sensors”, Kluwer
Academic Publishers, Boston, 2000.
[4] Kester, W., Bryant, W. and Jung, W., “Section 7:
Temperature Sensors”, Practical Design
Techniques for Sensor Signal Conditioning,
Analog Devices, 1999.
[5] Steele, Jerry, “Get Maxim Accuracy From
Temperature Sensors, Electronic Design,
pp. 99-110, August 19, 1996.
A potential for self-heating errors can exist if the SPI
communication lines are heavily loaded. A temperature
accuracy error will result from self-heating if the SPI
communication pins sink/source the maximum current
specified for the sensor. The output loading of the SPI
signals should be minimized to maximize the accuracy
of the temperature measurement.
DS00871A-page 16
 2003 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
use or otherwise. Use of Microchip’s products as critical
components in life support systems is not authorized except
with express written approval by Microchip. No licenses are
conveyed, implicitly or otherwise, under any intellectual
property rights.
Trademarks
The Microchip name and logo, the Microchip logo, dsPIC,
KEELOQ, MPLAB, PIC, PICmicro, PICSTART, PRO MATE and
PowerSmart are registered trademarks of Microchip
Technology Incorporated in the U.S.A. and other countries.
FilterLab, microID, MXDEV, MXLAB, PICMASTER, SEEVAL
and The Embedded Control Solutions Company are
registered trademarks of Microchip Technology Incorporated
in the U.S.A.
Accuron, Application Maestro, dsPICDEM, dsPICDEM.net,
ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, InCircuit Serial Programming, ICSP, ICEPIC, microPort,
Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM,
PICC, PICkit, PICDEM, PICDEM.net, PowerCal, PowerInfo,
PowerMate, PowerTool, rfLAB, rfPIC, Select Mode,
SmartSensor, SmartShunt, SmartTel and Total Endurance are
trademarks of Microchip Technology Incorporated in the
U.S.A. and other countries.
Serialized Quick Turn Programming (SQTP) is a service mark
of Microchip Technology Incorporated in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2003, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received QS-9000 quality system
certification for its worldwide headquarters,
design and wafer fabrication facilities in
Chandler and Tempe, Arizona in July 1999
and Mountain View, California in March 2002.
The Company’s quality system processes and
procedures are QS-9000 compliant for its
PICmicro® 8-bit MCUs, KEELOQ® code hopping
devices, Serial EEPROMs, microperipherals,
non-volatile memory and analog products. In
addition, Microchip’s quality system for the
design and manufacture of development
systems is ISO 9001 certified.
 2003 Microchip Technology Inc.
DS00871A-page 17
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