AD TMP12

FEATURES
Temperature Sensor Includes 100 Ω Heater
Heater Provides Power IC Emulation
Accuracy 63°C typ. from 240°C to 1100°C
Operation to 1150°C
5 mV/°C Internal Scale-Factor
Resistor Programmable Temperature Setpoints
20 mA Open-Collector Setpoint Outputs
Programmable Thermal Hysteresis
Internal 2.5 V Reference
Single 5 V Operation
400 µA Quiescent Current (Heater OFF)
Minimal External Components
Airflow and Temperature Sensor
TMP12*
FUNCTIONAL BLOCK DIAGRAM
VREF
HYSTERESIS
CURRENT
The TMP12 is a silicon-based airflow and temperature sensor
designed to be placed in the same airstream as heat generating
components that require cooling. Fan cooling may be required
continuously, or during peak power demands, e.g. for a power
supply, and if the cooling systems fails, system reliability and/or
safety may be impaired. By monitoring temperature while emulating a power IC, the TMP12 can provide a warning of cooling
system failure.
The TMP12 generates an internal voltage that is linearly proportional to Celsius (Centigrade) temperature, nominally
15 mV/°C. The linearized output is compared with voltages
from an external resistive divider connected to the TMP12’s
2.5 V precision reference. The divider sets up one or two reference voltages, as required by the user, providing one or two
temperature setpoints. Comparator outputs are open-collector
transistors able to sink over 20 mA. There is an on-board hysteresis generator provided to speed up the temperature-setpoint
output transitions, this also reduces erratic output transitions in
noisy environments. Hysteresis is programmed by the external
resistor chain and is determined by the total current drawn from
the 2.5 V reference. The TMP12 airflow sensor also incorporates a precision, low temperature coefficient 100 Ω heater
resistor that may be connected directly to an external 5 V supply. When the heater is activated it raises the die temperature in
IHYS
SET
HIGH
-
OVER
+
WINDOW
COMPARATOR
SET
LOW
GND
+
VOLTAGE
REFERENCE
AND
SENSOR
1kΩ
UNDER
HYSTERESIS
VOLTAGE
HEATER
100Ω
APPLICATIONS
System Airflow Sensor
Equipment Over-Temperature Sensor
Over-Temperature Protection
Power Supply Thermal Sensor
Low-Cost Fan Controller
GENERAL DESCRIPTION
V+
CURRENT
MIRROR
+
a
PINOUTS
DIP And SO
VREF
1
8
V+
SET HIGH
2
7
OVER
6
UNDER
5
HEATER
SET LOW
3
GND
4
TOP VIEW
(Not to Scale)
the DIP package approximately 20°C above ambient (in still
air). The purpose of the heater in the TMP12 is to emulate a
power IC, such as a regulator or Pentium CPU which has a high
internal dissipation.
When subjected to a fast airflow, the package and die temperatures of the power device and the TMP12 (if located in the
same airstream) will be reduced by an amount proportional to
the rate of airflow. The internal temperature rise of the TMP12
may be reduced by placing a resistor in series with the heater, or
reducing the heater voltage.
The TMP12 is intended for single 5 V supply operation, but will
operate on a 12 V supply. The heater is designed to operate from
5 V only. Specified temperature range is from 240°C to 1125°C,
operation extends to 1150°C at 5 V with reduced accuracy.
The TMP12 is available in 8-pin plastic DIP and SO packages.
*Protected by U.S. Patent No. 5,195,827.
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
© Analog Devices, Inc., 1995
One Technology Way, P.O. Box 9106, Norwood. MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
TMP12–SPECIFICATIONS (V = 15 V, 240°C ≤ T ≤ 1125°C unless otherwise noted.)
S
Parameter
ACCURACY
Accuracy (High, Low Setpoints)
Accuracy (High, Low Setpoints)
Internal Scale Factor
Power Supply Rejection Ratio
Linearity
Repeatability
Long Term Stability
Symbol
Conditions
PSRR
TA = 125°C
TA = 240°C to 1100°C
TA = 240°C to 1100°C
4.5 V ≤ 1VS ≤ 5.5 V
TA = 240°C to 1125°C
TA = 240°C to 1125°C
TA = 1125°C for 1 k Hrs
SETPOINT INPUTS
Offset Voltage
Output Voltage Drift
Input Bias Current
VOS
TCVOS
IB
VREF OUTPUT
Output Voltage
Output Voltage
VREF
VREF
Output Drift
Output Current, Zero Hysteresis
Hysteresis Current Scale Factor
OPEN-COLLECTOR OUTPUTS
Output Low Voltage
Output Low Voltage
Output Leakage Current
Fall Time
HEATER
Resistance
Temperature Coefficient
Maximum Continuous Current
POWER SUPPLY
Supply Range
Supply Current
A
Min
14.9
Typ
Max
Units
62
63
15
0.1
0.5
0.3
0.3
63
65
15.1
0.5
°C
°C
mV/°C
°C/V
°C
°C
°C
0.25
3
25
TA = 125°C, No Load
TA = 240°C to 1100°C,
No Load
2.49
TCVREF
IVREF
SFHYS
VOL
VOL
IOH
tHL
I SINK = 1.6 mA
I SINK = 20 mA
VS = 12 V
See Test Load
RH
IH
TA = 125°C
TA = 240°C to 1125°C
See Note 1
1VS
ISY
ISY
Unloaded at 15 V
Unloaded at 112 V2
97
100
mV
µV/°C
nA
2.50
2.51
2.5 60.015
V
V
210
7
5
ppm/°C
µA
µA/°C
0.25
0.6
1
40
0.4
100
100
103
100
60
4.5
400
450
5.5
600
V
V
µA
ns
Ω
ppm/°C
mA
V
µA
µA
NOTES
1Guaranteed but not tested.
2TMP12 is specified for operation from a 5 V supply. However, operation is allowed up to a 12 V supply, but not tested at 12 V. Maximum heater supply is 6 V.
Specifications subject to change without notice.
TEST LOAD
1kΩ
20pF
–2–
REV. 0
TMP12
WAFER TEST LIMITS (V
S
= 15 V, GND = O V, TA = 125°C, unless otherwise noted.)
Parameter
Symbol
ACCURACY
Accuracy (High, Low Setpoints)
Internal Scale Factor
Conditions
Min
Typ
Max
Units
TA = 125°C
TA = 125°C
14.9
15
63
15.1
°C
mV/°C
100
nA
2.51
V
0.4
100
V
µA
103
Ω
5.5
600
V
µA
SETPOINT INPUTS
Input Bias Current
IB
VREF OUTPUT
Output Voltage
VREF
T A = 125°C, No Load
OPEN-COLLECTOR OUTPUTS
Output Low Voltage
Output Leakage Current
VOL
IOH
ISINK = 1.6 mA
VS = 12 V
HEATER
Resistance
RH
TA = 125°C
POWER SUPPLY
Supply Range
Supply Current
1VS
ISY
Unloaded at 15 V
2.49
97
4.5
100
NOTE
Electrical tests are performed at wafer probe to the limits shown. Due to variations in assembly methods and normal yield loss, yield after packaging is not guaranteed
for standard product dice. Consult factory to negotiate specifications based on dice lot qualification through sample lot assembly and testing.
DICE CHARACTERISTICS
Die Size 0.078 3 0.071 inch, 5,538 sq. mils
(1.98 3 1.80 mm, 3.57 sq. mm)
Transistor Count: 105
8
7
6
5
1. VREF
2. SET HIGH INPUT
3. SET LOW INPUT
4. GND
5. HEATER
6. UNDER OUTPUT
7. OVER OUTPUT
8. V1
1
2
3
4
For additional DICE ordering information, refer to databook.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the TMP12 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
REV. 0
–3–
WARNING!
ESD SENSITIVE DEVICE
TMP12
ABSOLUTE MAXIMUM RATINGS*
FUNCTIONAL DESCRIPTION
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . 20.3 V to 115 V
Heater Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 V
Setpoint Input Voltage . . . . . . . . . . . 20.3 V to [(V1) 10.3 V]
Reference Output Current . . . . . . . . . . . . . . . . . . . . . . . . 2 mA
Open-Collector Output Current . . . . . . . . . . . . . . . . . . 50 mA
Open-Collector Output Voltage . . . . . . . . . . . . . . . . . . . 115 V
Operating Temperature Range . . . . . . . . . . 255°C to 1150°C
Dice Junction Temperature . . . . . . . . . . . . . . . . . . . . . 1175°C
Storage Temperature Range . . . . . . . . . . . . 265°C to 1160°C
Lead Temperature(Soldering, 60 sec) . . . . . . . . . . . . . 1300°C
The TMP12 incorporates a heating element, temperature sensor, and two user-selectable setpoint comparators on a single
substrate. By generating a known amount of heat, and using the
setpoint comparators to monitor the resulting temperature rise,
the TMP12 can indirectly monitor the performance of a
system’s cooling fan.
Package Type
ΘJA
ΘJC
Units
8-Pin Plastic DIP (P)
8-Lead SOIC (S)
1031
1582
43
43
°C/W
°C/W
NOTES
1
ΘJA is specified for device in socket (worst case conditions).
2
ΘJA is specified for device mounted on PCB.
The TMP12 temperature sensor section consists of a bandgap
voltage reference which provides both a constant 2.5 V output
and a voltage which is proportional to absolute temperature
(VPTAT). The VPTAT has a precise temperature coefficient of
5 mV/K and is 1.49 V (nominal) at 125°C. The comparators
compare VPTAT with the externally set temperature trip points
and generate an open-collector output signal when one of their
respective thresholds has been exceeded.
The heat source for the TMP12 is an on-chip 100 Ω low
tempco thin-film resistor. When connected to a 5 V source, this
resistor dissipates:
CAUTION
1. Stresses above those listed under “Absolute Maximum
Ratings” may cause permanent damage to the device. This is a
stress rating only and functional operation at or above this
specification is not implied. Exposure to the above maximum
rating conditions for extended periods may affect device reliability.
2. Digital inputs and outputs are protected, however, permanent
damage may occur on unprotected units from high-energy
electrostatic fields. Keep units in conductive foam or packaging
at all times until ready to use. Use proper antistatic handling
procedures.
3. Remove power before inserting or removing units from their
sockets.
PD =
52 V
V2
=
= 0.25 W ,
100 Ω
R
which generates a temperature rise of about 32°C in still air for
the SO packaged device. With an airflow of 450 feet per minute
(FPM), the temperature rise is about 22°C. By selecting a temperature setpoint between these two values, the TMP12 can provide
a logic-level indication of problems in the cooling system.
A proprietary, low tempco thin-film resistor process, in conjunction with production laser trimming, enables the TMP12 to
provide a temperature accuracy of 63°C (typ) over the rated
temperature range. The open-collector outputs are capable of
sinking 20 mA, allowing the TMP12 to drive small control relays directly. Operating from a single 15 V supply, the quiescent
current is only 600 µA (max), without the heater resistor current.
ORDERING GUIDE
Model/Grade
Temperature
Range1
Package
Description
Package
Option
TMP12FP
TMP12FS
TMP12GBC
XIND
XIND
125°C
Plastic DIP
SOIC
Die
N-8
SO-8
NOTE
1XIND = 240°C to 1125°C
–4–
REV. 0
140
35
V = 5V
SO–8 SOLDERED TO .5 "
TRANSITION FROM 100°C STIRRED
BATH TO FORCED 25°C AIR
.3" Cu PCB
120
30
a
25
a. 0 FPM
20
b. 250 FPM
c. 450 FPM
15
c
d. 600 FPM
d
10
5V, NO LOAD, HEATER OFF
.3" Cu PCB
PDIP SOLDERED TO 2"
1.31" Cu PCB
80
60
a. PDIP
PCB
b. SOIC
PCB
a
40
b
20
5
0
50
100
150
200
HEATER RESISTOR POWER DISSIPATION – mW
0
0
250
200
100
300
400
500
AIR VELOCITY – FPM
600
700
Figure 4. Package Thermal Time Constant in Forced Air
Figure 1. SOIC Junction Temperature Rise vs. Heater
Dissipation
120
25
V = 5V
PDIP SOLDERED TO 2"
c
b
20
100
a
a. 0 FPM
b. 250 FPM
15
c. 450 FPM
d. 600 FPM
d
10
5
TRANSITION FROM STILL 25°C
AIR TO STIRRED 100°C BATH
110
1.31" Cu PCB
JUNCTION TEMPERATURE – °C
JUNCTION TEMPERATURE RISE ABOVE AMBIENT – °C
100
=
SO–8 SOLDERED TO .5"
AIR FLOW RATES
0
AIR FLOW RATES
a
90
80
V
70
b
60
=
5V, NO LOAD, HEATER OFF
SO–8 SOLDERED TO .5"
.3" Cu PCB
PDIP SOLDERED TO 2"
1.31" Cu PCB
50
40
30
a. SO–8
PCB
20
b. PDIP
PCB
10
0
0
0
50
100
150
200
HEATER RESISTOR POWER DISSIPATION – mW
0
250
Figure 2. PDIP Junction Temperature Rise vs. Heater
Dissipation
2
4
6
8
10
12
TIME – sec
14
16
18
20
Figure 5. Thermal Response Time in Stirred Oil Bath
102
70
a. SO–8, HTR @ 5V
b. PDIP, HTR @ 5V
c. SO–8, HTR @ 3V
d. PDIP, HTR @ 3V
65
60
55
a
101.5
V+ = +5V
b
HEATER RESISTANCE – Ω
JUNCTION TEMPERATURE – °C
V
b
TIME CONSTANT – sec
JUNCTION TEMPERATURE RISE ABOVE AMBIENT – °C
TMP12
50
45
40
35
30
25
c
20
V = 5V RHEATER TO EXTERNAL
SUPPLY TURNED ON @ t = 5 sec
SO–8 SOLDERED TO .5" .3" COPPER PCB
PDIP SOLDERED TO 2" 1.31 COPPER PCB
15
10
d
101
100.5
100
99.5
99
98.5
5
0
0
10
20
30
40
50
60 70 80
TIME – sec
98
-75
90 100 110 120 130
Figure 3. Junction Temperature Rise in Still Air
REV. 0
-25
25
75
TEMPERATURE – °C
125
Figure 6. Heater Resistance vs. Temperature
–5–
175
TMP12
2.52
6
V
=
5V, NO LOAD, HEATER OFF
5
4
3
ACCURACY ERROR – °C
REFERENCE VOLTAGE – V
2.515
2.51
2.505
2.5
a. MAXIMUM LIMIT
2
b. ACCURACY ERROR
b
a
1
c. MINIMUM LIMIT
0
-1
-2
c
-3
-4
2.495
-5
2.49
-75
-25
25
75
TEMPERATURE – °C
125
-6
-50
175
Figure 7. Reference Voltage vs. Temperature
0
25
50
75
TEMPERATURE – °C
100
125
Figure 10. Accuracy Error vs. Temperature
5
500
START-UP VOLTAGE DEFINED AS OUTPUT
READING BEING WITHIN 5 °C OF OUTPUT AT
450
5V
400
4.5
SUPPLY CURRENT – µA
START-UP SUPPLY VOLTAGE – V
-25
NO LOAD, HEATER OFF
4
3.5
350
300
250
200
150
Ta =
25°C, NO LOAD, HEATER OFF
100
50
3
-75
-25
25
75
TEMPERATURE – °C
125
0
175
Figure 8. Start-up Voltage vs. Temperature
0
1
2
3
4
5
SUPPLY VOLTAGE – V
6
7
8
Figure 11. Supply Current vs. Supply Voltage
500
0.5
V
=
5V, NO LOAD, HEATER OFF
POWER SUPPLY REJECTION – °C/V
475
SUPPLY CURRENT – µA
450
425
400
375
350
325
300
-75
-25
25
75
TEMPERATURE – °C
Figure 9. Supply Current vs. Temperature
125
V
0.4
4.5 TO
5.5V
0.3
0.2
0.1
0
-75
175
=
NO LOAD, HEATER OFF
-25
25
75
TEMPERATURE – °C
125
175
Figure 12. VPTAT Power Supply Rejection vs.
Temperature
–6–
REV. 0
TMP12
40
OPEN–COLLECTOR OUTPUT VOLTAGE – mV
700
OPEN COLLECTOR SINK CURRENT – mA
38
36
34
32
30
28
VOL =
26
1V, V
=
5V
24
22
20
-75
-25
25
75
TEMPERATURE – °C
125
600
Figure 13. Open-Collector Output Sink Current vs.
Temperature
a
b. LOAD = 5mA
500
c. LOAD = 1mA
400
b
300
200
V
=
c
5V
100
0
-75
175
a. LOAD = 10mA
-25
25
75
TEMPERATURE – °C
125
175
Figure 14. Open-Collector Voltage vs. Temperature
APPLICATIONS INFORMATION
Temperature Hysteresis
A typical application for the TMP12 is shown in Figure 15. The
TMP12 package is placed in the same cooling airflow as a
high-power dissipation IC. The TMP12’s internal resistor produces a temperature rise which is proportional to air flow, as
shown in Figure 16. Any interruption in the airflow will produce
an additional temperature rise. When the TMP12 chip temperature exceeds a user-defined setpoint limit, the system controller
can take corrective action, such as: reducing clock frequency,
shutting down unneeded peripherals, turning on additional fan
cooling, or shutting down the system.
The temperature hysteresis at each setpoint is the number
of degrees beyond the original setpoint temperature that
must be sensed by the TMP12 before the setpoint comparator will be reset and the output disabled. Hysteresis
prevents “chatter” and “motorboating” in feedback control
systems. For monitoring temperature in computer systems,
hysteresis prevents multiple interrupts to the CPU which
can reduce system performance.
PGA
PACKAGE
PGA
SOCKET
POWER I.C.
Figure 17 shows the TMP12’s hysteresis profile. The hysteresis is programmed, by the user, by setting a specific load
current on the reference voltage output VREF. This output
current, IREF, is also called the hysteresis current. IREF is mirrored internally by the TMP12, as shown in the functional
block diagram, and fed to a buffer with an analog switch.
AIR FLOW
PC BOARD
TMP12
HYSTERESIS
LOW
HYSTERESIS
HIGH
HI
Figure 15. Typical Application
HYSTERESIS HIGH =
HYSTERESIS LOW
OUTPUT
VOLTAGE
OVER, UNDER
65
a
DIE TEMPERATURE (°C)
60
b
LO
55
TEMPERATURE
c
50
TSETLOW
d
Figure 17. TMP12 Hysteresis Profile
45
a. TMP12 DIE TEMP NO AIR FLOW
b. HIGH SET POINT
c. LOW SET POINT
d. TMP12 DIE TEMP MAX AIR FLOW
e. SYSTEM AMBIENT TEMPERATURE
40
After a temperature setpoint has been exceeded and a comparator tripped, the hysteresis buffer output is enabled. The
result is a current of the appropriate polarity which generates a hysteresis offset voltage across an internal 1 kΩ
resistor at the comparator input. The comparator output
remains “on” until the voltage at the comparator input,
now equal to the temperature sensor voltage VPTAT
summed with the hysteresis effect, has returned to the programmed setpoint voltage. The comparator then returns
e
35
0
50
100
150
200
250
TMP12 PD (mW)
Figure 16. Choosing Temperature Setpoints
REV. 0
TSETHIGH
–7–
TMP12
LOW, deactivating the open-collector output and disabling the
hysteresis current buffer output. The scale factor for the programmed hysteresis current is:
I = IVREF = 5 µA/°C 1 7 µA
Thus, since VREF = 2.5 V, a reference load resistance of 357 kΩ
or greater (output current of 7 µA or less) will produce a temperature setpoint hysteresis of zero degrees. For more details, see
the temperature programming discussion below. Larger values of
load resistance will only decrease the output current below 7 µA,
but will have no effect on the operation of the device. The
amount of hysteresis is determined by selecting an appropriate
value of load resistance for VREF, as shown below.
Programming the TMP12
The basic thermal monitoring application only requires a simple
three-resistor ladder voltage divider to set the high and low
setpoints and the hysteresis. These resistors are programmed in
the following sequence:
1. Select the desired hysteresis temperature.
2. Calculate the hysteresis current, IVREF
3. Select the desired setpoint temperatures.
4. Calculate the individual resistor divider ladder values needed
to develop the desired comparator setpoint voltages at the
Set High and Set Low inputs.
For example, setting the high setpoint for 180°C, the low
setpoint for 155°C, and hysteresis for 3°C produces the
following values:
IHYS = IVREF = (3°C 3 5 µA/°C) 1 7 µA = 15 µA 1 7 µA =
22 µA
VSETHIGH = (TSETHIGH 1 273.15)(5 mV/°C) = (80°C 1
273.15)(5 mV/°C) = 1.766 V
VSETLOW = (TSETLOW 1 273.15)(5 mV/°C) = (55°C 1 273.15)
(5 mV/°C) = 1.641 V
R1 (kΩ) = (VREF 2 VSETHIGH)/IVREF = (2.5 V 2 1.766 V)/
22 µA = 33.36 kΩ
R2 (kΩ) = (VSETHIGH 2 VSETLOW)/IVREF = (1.766 V 2 1.641 V)/
22 µA = 5.682 kΩ
R3 (kΩ) = VSETLOW / IVREF = (1.641 V)/22 µA = 74.59 kΩ
The total of R1 1 R2 1 R3 is equal to the load resistance
needed to draw the desired hysteresis current from the
reference, or IVREF .
The nomograph of Figure 19 provides an easy method of
determining the correct VPTAT voltage for any temperature.
Simply locate the desired temperature on the appropriate scale
(K, °C or °F) and read the corresponding VPTAT value from
the bottom scale.
The hysteresis current is readily calculated, as shown above. For
example, to produce 2 degrees of hysteresis IVREF should be set
to 17 µA. Next, the setpoint voltages VSETHIGH and VSETLOW are
determined using the VPTAT scale factor of 5 mV/K = 5 mV/
(°C 1 273.15), which is 1.49 V for 125°C. Finally, the divider
resistors are calculated, based on the setpoint voltages.
218
248
–55
–25 –18
273
298
323
348
373
398
0
25
50
75
100
125
K
°C
–67
–25
0
32 50
77
100
150
200 212
257
1.865
1.99
°F
1.09
1.24
1.365
1.49
1.615
1.74
VPTAT
The setpoint voltages are calculated from the equation:
VSET = (TSET 1 273.15)(5 mV/°C)
Figure 19. Temperature 2 VPTAT Scale
This equation is used to calculate both the VSETHIGH and the
VSETLOW values. A simple 3-resistor network, as shown in Figure
18, determines the setpoints and hysteresis value. The equations
used to calculate the resistors are:
The formulas shown above are also helpful in understanding the
calculations of temperature setpoint voltages in circuits other
than the standard two-temperature thermal/airflow monitor. If a
setpoint function is not needed, the appropriate comparator input should be disabled. SETHIGH can be disabled by tying it
to V1 or VREF, SETLOW by tying it to GND. Either output
can be left disconnected.
R1 (kΩ) = (VREF 2 VSETHIGH )/IVREF = (2.5 V 2 VSETHIGH )/IVREF
R2 (kΩ) = (VSETHIGH 2 VSETLOW)/IVREF
R3 (kΩ) = VSETLOW/IVREF
VREF
= 2.5 V
Selecting Setpoints
8
V+
7
OVER
3
6
UNDER
4
5
HEATER
1
(VREF – VSETHIGH) / IVREF = R1
IVREF
VSETHIGH
2
TMP12
(VSETHIGH – VSETLOW) / IVREF = R2
VSETLOW
VSETLOW / IVREF = R3
GND
Figure 18. TMP12 Setpoint Programming
Choosing the temperature setpoints for a given system is an empirical process, because of the wide variety of thermal issues in
any practical design. The specific setpoints are dependent on
such factors as airflow velocity in the system, adjacent component location and size, PCB thickness, location of copper
ground planes, and thermal limits of the system.
The TMP12’s temperature rise above ambient is proportional to
airflow (Figures 1, 2 and 16). As a starting point, the low
setpoint temperature could be set at the system ambient temperature (inside the enclosure) plus one half of the temperature
rise above ambient (at the actual airflow in the system). With
this setting, the low limit will provide a warning either if the fan
output is reduced or if the ambient temperature rises (for example, if the fan’s cool air intake is blocked). The high setpoint
could then be set for the maximum system temperature to provide a final system shutdown control.
–8–
REV. 0
TMP12
eters which are adjusted to 8-bit resolution via a 3-wire serial interface. The controller simply sweeps the wiper of
potentiometer 1 from the A1 terminal to the B1 terminal
(digital value = 0), while monitoring the comparator output
at Pin 7 of the TMP12. When Pin 7 goes low, the voltage
at Pin 2 equals the VPTAT voltage. This Circuit sweeps
Pin 2's voltage from maximum to minimum, so that the
TMP12's setpoint hystersis will not affect the reading.
Measuring the TMP12 Internal Temperature
As previously mentioned, the TMP12’s VPTAT generator represents the chip temperature with a slope of 5 mV/K. In some cases,
selecting the setpoints is made easier if the TMP12’s internal
VPTAT voltage (and therefore the chip temperature) is known.
For example, the case temperature of a high power microprocessor
can be monitored with a thermistor, thermocouple, or other measurement method. The case temperature can then be correlated
with the TMP12’s temperature to select the setpoints.
The circuit of Figure 21 provides approximately 1°C of
resolution. The two potentiometers divide VREF by two,
and the 8-bit potentiometer further divides VREF by 256,
so the resolution is:
The TMP12’s VPTAT voltage is not available externally, so indirect methods must be used. Since the VPTAT voltage is applied to
the internal comparators, measuring the voltage at which the digital
output changes state will reflect the VPTAT voltage.
TMP12
1
VPTAT
2
R1
Resolution =
V+
SET
HIGH
OVER
330
+5V
LED
200K
3
R1
SET
LOW
UNDER
GND
HEATER
6
2
= 4.9 mV
28
NC
VPTAT = 1.25 V + (Digital Count
200K
4
=
where VREF is the voltage reference output (Pin 1 of the
TMP12) and N is the resolution of the AD8402. Since the
VPTAT has a slope of 5 mV/K, the AD8402 provides 1°C
of resolution. The adjustment range of this circuit extends
from VREF/2 (i.e. 1.25 V, or 223°C) to VREF 2 1 LSB
(i.e. 2.5 V 2 4.9 mV, or 226°C). The VPTAT is therefore:
8
7
2
2N
+5V
VREF
2.5 V
VREF
A simple method of measuring the TMP12 VPTAT is shown in
Figure 20. To measure VPTAT, adjust potentiometer R1 until
the LED turns ON. The voltage at Pin 2 of the TMP12 will
then match the TMP12’s internal VPTAT.
5
4.9 mV)
+5V
where Digital Count is the value sent to the AD8402 which
caused the setpoint 1 output to go LOW.
A third way to measure the VPTAT voltage is to close a
feedback loop around one of the TMP12’s comparators.
This causes the comparator to oscillate, and in turn forces
the voltage at the comparator input to equal the VPTAT
voltage. Figure 22 is a typical circuit for this measurement.
An OP193 operational amplifier, operating as an integrator,
provides additional loop-gain to ensure that the TMP12
comparator will oscillate.
Figure 20. Measuring VPTAT with a Potentiometer
The method described in Figure 20 can be automated by replacing the discrete resistors with a digital potentiometer. The
improved circuit, shown in Figure 21, permits the VPTAT voltage to be monitored with a microprocessor or other digital
controller. The AD8402-100 provides two 100 kΩ potentiomµC
INTERFACE
OVER
+5V
6
10
SHDN
RS
11
V
AD8402–100
SDI
CLK
9
8
A1
13
W1
12
B1
14
1
DD
SERIAL
DATA
INTERFACE
7
A2
3
W2
4
VPTAT
8
7
2
3
NC
CS
B2
VREF TEMPERATURE
SENSOR &
VOLTAGE
REFERENCE
4
WINDOW
COMPARATOR
HYSTERESIS
GENERATOR
2
100
TMP12
AGND
1
DGND
5
Figure 21. Measuring VPTAT with a Digital Potentiometer
REV. 0
–9–
6
NC
5
+5V
TMP12
due to IR voltage drops and coupling of external noise sources.
In any case, a 0.1 µF capacitor for power supply bypassing is
always recommended at the chip.
Understanding Error Sources
The accuracy of the VPTAT sensor output is well characterized
and specified, however preserving this accuracy in a thermal
monitoring control system requires some attention to minimizing the various potential error sources. The internal sources of
setpoint programming error include the initial tolerances and
temperature drifts of the reference voltage VREF, the setpoint
comparator input offset voltage and bias current, and the hysteresis current scale factor. When evaluating setpoint programming
errors, remember that any VREF error contribution at the comparator inputs is reduced by the resistor divider ratios. Each
comparator’s input bias current drops to less than 1 nA (typ)
when the comparator is tripped. This change accounts for some
setpoint voltage error, equal to the change in bias current multiplied by the effective setpoint divider ladder resistance to ground.
Safety Considerations in Heating and Cooling System Design
Designers should anticipate potential system fault conditions
that may result in significant safety hazards which are outside
the control of and cannot be corrected by the TMP12-based circuit. Governmental and Industrial regulations regarding safety
requirements and standards for such designs should be observed
where applicable.
Self-Heating Effects
In some applications the user should consider the effects of selfheating due to the power dissipated by the open-collector outputs,
which are capable of sinking 20 mA continuously. Under full load,
the TMP12 open-collector output device is dissipating:
PDISS = 0.6 V
The thermal mass of the TMP12 package and the degree of
thermal coupling to the surrounding circuitry are the largest factors in determining the rate of thermal settling, which ultimately
determines the rate at which the desired temperature measurement accuracy may be reached (see graph in Figure 3). Thus,
one must allow sufficient time for the device to reach the final
temperature. The typical thermal time constant for the SOIC
plastic package is approximately 70 seconds in still air. Therefore, to reach the final temperature accuracy within 1%, for a
temperature change of 60 degrees, a settling time of 5 time constants, or 6 minutes, is necessary. Refer to Figure 4.
0.020 A = 12 mW
which in a surface-mount SO package accounts for a temperature increase due to self-heating of:
∆T = PDISS
JA
= 0.012 W
158°C/W = 1.9°C
This increase is for still air, of course, and will be reduced at
high airflow levels. However, the user should still be aware that
self-heating effects can directly affect the accuracy of the
TMP12. For setpoint 2, self-heating will add to the setpoint
temperature (that is, in the above example the TMP12 will
switch the setpoint 2 output off 1.9 degrees early). Self-heating
will not affect the temperature at which setpoint 1 turns on, but
will add to the hysteresis. Several circuits for adding external
driver transistors and other buffers are presented in following
sections of this data sheet. These buffers will reduce self-heating
and improve accuracy.
External error sources to consider are the accuracy of the external
programming resistors, grounding error voltages, and thermal gradients. The accuracy of the external programming resistors directly
impacts the resulting setpoint accuracy. Thus, in fixed-temperature
applications the user should select resistor tolerances appropriate
to the desired programming accuracy. Since setpoint resistors are
typically located in the same air flow as the TMP12, resistor temperature drift must be taken into account also. This effect can be
minimized by selecting good quality components, and by keeping all components in close thermal proximity. Careful circuit
board layout and component placement are necessary to minimize common thermal error sources. Also, the user should take
care to keep the bottom of the setpoint programming divider
ladder as close to GND (Pin 4) as possible to minimize errors
Buffering the Voltage Reference
The reference output VREF is used to generate the temperature
setpoint programming voltages for the TMP12. Since the hysteresis is set by the reference current, external circuits which draw
current from the reference will increase the hysteresis value.
+5V
1uF
NC
1
V+
VREF
2 SET
HIGH
3 SET
LOW
4
GND
OVER
UNDER
HEATER
5k
8
200k
7
6
5
300k
NC
+5V
+5V
OP193
10k
~1.5V
TMP12
VPTAT
130k
0.1UF
Figure 22. An Analog Measurement Circuit for VPTAT
–10–
REV. 0
TMP12
The on-board VREF output buffer is typically capable of 500 µA
output drive into as much as 50 pF load (max). Exceeding this
load will affect the accuracy of the reference voltage, could cause
thermal sensing errors due to excess heat build-up, and may induce
oscillations. External buffering of VREF with a low-drift voltage
follower will ensure optimal reference accuracy. Amplifiers which
offer low drift, low power consumption, and low cost appropriate
to this application include the OP284, and members of the OP113
and OP193 families.
With excellent drift and noise characteristics, VREF offers a good
voltage reference for data acquisition and transducer excitation applications as well. Output drift is typically better than 210 ppm/°C,
with 315 nV/Hz (typ) noise spectral density at 1 kHz.
Preserving Accuracy Over Wide Temperature Range Operation
The TMP12 is unique in offering both a wide-range temperature
sensor and the associated detection circuitry needed to implement
a complete thermostatic control function in one monolithic device.
The voltage reference, setpoint comparators, and output buffer
amplifiers have been carefully compensated to maintain accuracy
over the specified temperature ranges in this application. Since the
TMP12 is both sensor and control circuit, in many applications the
external components used to program and interface the device are
subjected to the same temperature extremes. Thus, it is necessary
to place components in close thermal proximity minimizing large
temperate differentials, and to account for thermal drift errors
where appropriate, such as resistor matching temperature coefficients, amplifier error drift, and the like. Circuit design with the
TMP12 requires a slightly different perspective regarding the thermal behavior of electronic components.
Switching Loads with the Open-Collector Outputs
In many temperature sensing and control applications some
type of switching is required. Whether it be to turn on a
heater when the temperature goes below a minimum value
or to turn off a motor that is overheating, the open-collector
outputs can be used. For the majority of applications, the
switches used need to handle large currents on the order of
1 Amp and above. Because the TMP12 is accurately measuring temperature, the open-collector outputs should
handle less than 20 mA of current to minimize self-heating.
Clearly, the trip point outputs should not drive the equipment directly. Instead, an external switching device is
required to handle the large currents. Some examples of
these are relays, power MOSFETs, thyristors, IGBTs, and
Darlington transistors.
This section shows a variety of circuits where the TMP12
controls a switch. The main consideration in these circuits,
such as the relay in Figure 23, is the current required to activate the switch.
+12V
R1
3
Thermal Response Time
The time required for a temperature sensor to settle to a
specified accuracy is a function of the thermal mass of the
sensor, and the thermal conductivity between the sensor and
the object being sensed. Thermal mass is often considered
equivalent to capacitance. Thermal conductivity is commonly
specified using the symbol Q, and is the inverse of thermal
resistance. It is commonly specified in units of degrees per
watt of power transferred across the thermal joint. Figures 3
and 5 illustrate the typical RC time constant response to a
step change in ambient temperature. Thus, the time required
for the TMP12 to settle to the desired accuracy is dependent
on the package selected, the thermal contact established in
the particular application, and the equivalent thermal conductivity of the heat source. For most applications, the
settling-time is probably best determined empirically.
REV. 0
MOTOR
SHUTDOWN
7
2
R2
WINDOW
COMPARATOR
2604-12-311
COTO
NC
6
R3
140Ω
4
PC Board Layout Considerations
The TMP12 also requires a different perspective on PC board layout. In many applications, wide traces and generous ground planes
are used to extract heat from components. The TMP12 is slightly
different, in that ideal path for heat is via the cooling system air
flow. Thus, heat paths through the PC traces should be minimized.
This constraint implies that minimum pad sizes and trace widths
should be specified in order to reduce heat conduction. At the
same time, analog performance should not be compromised. In
particular, the bottom of the setpoint resistor ladder should be
located as close to GND as possible, as discussed in the Understanding Error Sources section of this data sheet.
VREF TEMPERATURE VPTAT
SENSOR &
1
8
VOLTAGE
IN4001
REFERENCE
OR EQUIV
5
HYSTERESIS
GENERATOR
+12 V
100
TMP12
Figure 23. Reed Relay Drive
It is important to check the particular relay you choose to
ensure that the current needed to activate the coil does not
exceed the TMP12’s recommended output current of
20 mA. This is easily determined by dividing the relay coil
voltage by the specified coil resistance. Keep in mind that
the inductance of the relay will create large voltage spikes
that can damage the TMP12 output unless protected by a
commutation diode across the coil, as shown. The relay
shown has contact rating of 10 Watts maximum. If a relay
capable of handling more power is desired, the larger contacts will probably require a commensurably larger coil,
with lower coil resistance and thus higher trigger current.
As the contact power handling capability increases, so does
the current needed for the coil, In some cases an external
driving transistor should be used to remove the current load
on the TMP12 as explained in the next section.
–11–
TMP12
Power FETs are popular for handling a variety of high current
dc loads. Figure 24 shows the TMP12 driving a P-channel
MOSFET transistor for a simple heater circuit. When the output transistor turns on, the gate of the MOSFET is pulled down
to approximately 0.6 V, turning it on. For most MOSFETs a
gate-to-source voltage or Vgs on the order of -2 V to -5 V is sufficient to turn the device on. Figure 25 shows a similar circuit
for turning on an N-channel MOSFET, except that now the
gate to source voltage is positive. For this reason an external
transistor must be used as an inverter so that the MOSFET will
turn on when the trip point pulls down.
1
VREF TEMPERATURE VPTAT
V+
SENSOR &
8
VOLTAGE
REFERENCE
4.7kΩ
4.7kΩ
MOTOR
CONTROL
7 NC
2
IRGBC40S
3
4
WINDOW
COMPARATOR
2N1711
6
5 +5V
HYSTERESIS
GENERATOR
100
TMP12
1
VREF TEMPERATURE VPTAT
SENSOR &
8
VOLTAGE
REFERENCE
2
3
4
7
WINDOW
COMPARATOR
V+
NC
NC = NO CONNECT
2.4kΩ (12V)
1.2kΩ (6V)
5%
6
5
HYSTERESIS
GENERATOR
Figure 26. Driving an IGBT
IRFR9024
OR EQUIV
The last class of high power devices discussed here are Thyristors,
which include SCRs and Triacs. Triacs are a useful alternative to
relays for switching ac line voltages. The 2N6073A shown in Figure 27 is rated to handle 4 A (rms). The opto-isolated MOC3021
Triac shown features excellent electrical isolation from the noisy ac
line and complete control over the high power Triac with only a
few additional components.
HEATING
ELEMENT
+5V
100
TMP12
NC = NO CONNECT
1
Figure 24. Driving a P-Channel MOSFET
VREF TEMPERATURE VPTAT
SENSOR &
8
VOLTAGE
REFERENCE
2
1
VREF TEMPERATURE VPTAT
SENSOR &
8
VOLTAGE
REFERENCE
2
3
7
WINDOW
COMPARATOR
7
V+ = 5V
300Ω
NC
1
V+
3
4.7kΩ
WINDOW
COMPARATOR
2
3
6 150Ω
MOC3011 5
4
2N6073A
NC
4
IRF130
5 +5V
HYSTERESIS
GENERATOR
2N1711
6
6
HEATING
ELEMENT
4.7kΩ
AC
LOAD
100
TMP12
4
5
HYSTERESIS
GENERATOR
+5V
NC = NO CONNECT
100
Figure 27. Controlling the 2N6073A Triac
TMP12
Figure 25. Driving an N-Channel MOSFET
Isolated Gate Bipolar Transistors (IGBTs) combine many of the
benefits of power MOSFETs with bipolar transistors and are
used for a variety of high power applications. Because IGBTs
have a gate similar to MOSFETs, turning on and off the devices
is relatively simple as shown in Figure 26. The turn on voltage
for the IGBT shown (IRGB40S) is between 3.0 and 5.5 volts.
This part has a continuous collector current rating of 50 A and a
maximum collector to emitter voltage of 600 V, enabling it to
work in very demanding applications.
–12–
REV. 0
TMP12
High Current Switching
As mentioned earlier, internal dissipation due to large loads on
the TMP12 outputs will cause some temperature error due to
self-heating. External transistors buffer the load from the
TMP12 so that virtually no power is dissipated in the internal
transistors and minimal self-heating occurs. This section shows
several examples using external transistors. The simplest case
uses a single transistor on the output to invert the output signal
is shown in Figure 28. When the open-collector of the TMP12
turns “ON” and pulls the output down, the external transistor
Q1’s base will be pulled low, turning off the transistor. Another
transistor can be added to re-invert the signal as shown in Figure
29. Now, when the output of the TMP12 is pulled down, the
first transistor, Q1, turns off and its collector goes high, which
turns Q2 on, pulling its collector low. Thus, the output taken
from the collector of Q2 is identical to the output of the TMP12.
By picking a transistor that can accommodate large amounts of
current, many high power devices can be switched.
1
VREF TEMPERATURE VPTAT
SENSOR &
8
VOLTAGE
REFERENCE
2
3
4
7
WINDOW
COMPARATOR
3
4
6
100
Figure 28. An External Transistor Minimizes Self-Heating
+12V
RELAY
MOTOR
SWITCH
3
4
4.7kΩ
C
4.7kΩ
TIP-110
2N1711
6
5
HYSTERESIS
GENERATOR
I
V+
7
WINDOW
COMPARATOR
+5V
100
TMP12
Figure 30. Darlington Transistor Can Handle Large Currents
REV. 0
Q2
An example of a higher power transistor is a standard
Darlington configuration as shown in Figure 30. The part chosen, TIP-110, can handle 2 A continuous which is more than
enough to control many high power relays. In fact the
Darlington itself can be used as the switch, similar to
MOSFETs and IGBTs.
Q1
2
Q1
Figure 29. Second Transistor Maintains Polarity of TMP12
Output
C
VREF TEMPERATURE VPTAT
SENSOR &
8
VOLTAGE
REFERENCE
2N1711
100
TMP12
1
C
TMP12
5
HYSTERESIS
GENERATOR
I
4.7kΩ
4.7kΩ
5
HYSTERESIS
GENERATOR
2N1711
6
V+
2N1711
I
4.7kΩ
VREF TEMPERATURE VPTAT
SENSOR &
8
VOLTAGE
REFERENCE
2
V+
7
WINDOW
COMPARATOR
1
–13–
TMP12
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
C2074-10-10/95
8-Pin Epoxy DIP
5
8
0.280 (7.11)
0.240 (6.10)
1
4
0.070 (1.77)
0.430 (10.92)
0.348 (8.84)
0.045 (1.15)
0.325 (8.25)
0.300 (7.62)
0.015
(0.381) TYP
0.210
(5.33)
MAX
0.160 (4.06)
0.115 (2.93)
0.100
(2.54)
0.022 (0.558)
0.014 (0.356)
0.195 (4.95)
0.115 (2.93)
0.130
0.015 (0.381)
(3.30)
MIN
0.008 (0.204)
SEATING
PLANE
0°- 15°
BSC
8-Pin SOIC
5
8
0.1574 (4.00)
0.1497 (3.80)
0.2440 (6.20)
0.2284 (5.80)
4
1
0.1968 (5.00)
0.1890 (4.80)
0.102 (2.59)
0.094 (2.39)
0.0196 (0.50) × 45°
0.0099 (0.25)
0.0098 (0.25)
0.0040 (0.10)
0°8°
0.0192 (0.49) SEATING
0.0138 (0.35) PLANE
0.0098 (0.25)
0.0075 (0.19)
0.0500 (1.27)
0.0160 (0.41)
PRINTED IN U.S.A.
0.0500 (1.27) BSC
–14–
REV. 0