Analog Devices TMP12FS Datasheet

a
TOP VIEW
(Not to Scale)
8
7
6
5
1
2
3
4
V
REF
V+
TMP12
SET HIGH
OVER
SET LOW
UNDER
GND
HEATER
Airflow and Temperature Sensor
FEATURES Temperature Sensor Includes 100 Heater Heater Provides Power IC Emulation Accuracy 3 Typ from –40C to +100C Operation to 150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 A Quiescent Current (Heater Off) Minimal External Components
APPLICATIONS System Airflow Sensor Equipment Over-Temperature Sensor Over-Temperature Protection Power Supply Thermal Sensor Low-Cost Fan Controller
GENERAL DESCRIPTION
The TMP 12 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 5 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 heater resistor that may be con­nected directly to an external 5 V supply. When the heater is activated, it raises the die temperature in the DIP package
TMP12
*

FUNCTIONAL BLOCK DIAGRAM

V
REF
SET
HIGH
SET
LOW
GND
HYSTERESIS
CURRENT
CURRENT
MIRROR
VO LTAG E
REFERENCE
AND
SENSOR
WINDOW
COMPARATOR
1k
TMP12
I
HYS
HYSTERESIS VO LTAG E
100
V+
OVER
UNDER
HEATER
PIN CONNECTIONS
8-Lead SOIC
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 –40°C to +125°C, operation extends to 150°C at 5 V with reduced accuracy.
The TMP12 is available in 8-pin SO packages.
*Protected by U.S. Patent No. 5,195,827.
Pentium is a registered trademark of Intel Corporation.
REV. A
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 that may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 2002
TMP12–SPECIFICATIONS
(VS = 5 V, –40C TA s 125C unless otherwise noted.)
Parameter Symbol Conditions Min Typ Max Unit
ACCURACY
Accuracy (High, Low Setpoints) TA = 25°C ± 2 ± 3 °C
T
= –40°C to +100°C ±3 ± 5 °C
A
Internal Scale Factor T Power Supply Rejection Ratio PSRR 4.5 V ≤ V Linearity T Repeatability T
= –40°C to +100°C 4.9 5 5.1 mV/°C
A
= –40°C to +125°C 0.5 °C
A
= –40°C to +125°C 0.3 °C
A
5.5 V 0.1 0.5 °C/V
S
Long Term Stability TA = 125°C for 1 k Hrs 0.3 °C
SETPOINT INPUTS
Offset Voltage V
OS
Output Voltage Drift TCV Input Bias Current I
B
OS
0.25 mV 3 µV/°C 25 100 nA
VREF OUTPUT
Output Voltage VREF T
VREF T Output Drift TC Output Current, Zero Hysteresis I Hysteresis Current Scale Factor SF
VREF
VREF
HYS
= 25°C, No Load 2.49 2.50 2.51 V
A
= –40°C to +100°C, No Load 2.5 ± 0.015 V
A
–10 ppm/°C 7 µA 5 µA/°C
OPEN-COLLECTOR OUTPUTS
Output Low Voltage V
Output Leakage Current I Fall Time t
OL
V
OL
OH
HL
I
= 1.6 mA 0.25 0.4 V
SINK
I
= 20 mA 0.6 V
SINK
VS = 12 V 1 100 µA See Test Load 40 ns
HEATER
Resistance R
H
Temperature Coefficient T Maximum Continuous Current1I
H
TA = 125°C 97 100 103
= –40°C to +125°C 100 ppm/°C
A
60 mA
POWER SUPPLY
Supply Range V Supply Current I
NOTES
1
Guaranteed but not tested.
2
TMP12 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.
S
SY
I
SY
Unloaded at 5 V 400 600 µA Unloaded at 12 V
2
4.5 5.5 V
450 µA
1k
20pF
Figure 1. Test Load
–2–
REV. A
TMP12
WARNING!
ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS*

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +11 V
Heater Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 V
Setpoint Input Voltage . . . . . . . . . . . –0.3 V to [(V+) + 0.3 V]
Reference Output Current . . . . . . . . . . . . . . . . . . . . . . . 2 mA
Open-Collector Output Current . . . . . . . . . . . . . . . . . 50 mA
Open-Collector Output Voltage . . . . . . . . . . . . . . . . . . . . 15 V
Operating Temperature Range . . . . . . . . . . –55°C to +150°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +160°C
Lead Temperature (Soldering, 60 sec) . . . . . . . . . . . . 300°C
Package Type
8-Lead SOIC (S) 158
NOTES
1
JA is specified for device in socket (worst case conditions).
2
JC is specified for device mounted on PCB.
JA
2
JC
Unit
43 °C/W
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.

ORDERING GUIDE

Temperature Package Package
Model/Grade Range
TMP12FS
NOTES
1
XIND = –40°C to +125°C
2
Not for new design, obsolete April 2002.
2
1
Description Option
XIND SOIC SO-8

FUNCTIONAL DESCRIPTION

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.
The TMP12 temperature sensor section consists of a band gap 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 25°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 low tempco thin-film resistor. When connected to a 5 V source, this resistor dissipates:
V
P
== =
D
R
100
W
025Ω.
2
2
V
5
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 ±3°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 5 V supply, the quiescent current is only 600 µA (max), without the heater resistor current.
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. A
–3–
TMP12
TEMPERATURE – C
REFERENCE VOLTAGE – V
2.520
–75
2.515
2.510
2.505
2.500
2.495
2.490 –25 25 75 125 175
V+ = 5V NO LOAD HEATER OFF
Typical Performance Characteristics
35
V+ = 5V SO-8 SOLDERED TO
30
0.5 0.3 CU PCB
25
20
15
10
ABOVE AMBIENT – 8C
JUNCTION TEMPERATURE RISE
5
0
0
50 100 150 200 250
HEATER RESISTOR POWER
250 FPM
0 FPM
450 FPM
600 FPM
AIR FLOW RATES
DISSIPATION – mW
TPC 1. SOIC Junction Temperature Rise vs. Heater Dissipation
120
TRANSITION FROM STILL 25C
110
AIR TO STIRRED 100C BATH
100
90
80
70
60
50
40
30
20
JUNCTION TEMPERATURE – C
V+ = 5V, NO LEAD, HEATER OFF
10
SO-8 SOLDERED TO 0.5  0.3 CU PCB
0
2 4 6 8 10 12 14 16 18 20
0
SOIC AND PCB
TIME – sec
70
65
60
55 50
45
40
35
30
25
20
15
JUNCTION TEMPERATURE – C
10
5
0
10 20 30 40 50 60 70 80 90 100 110120130
0
SO-8, HTR @ 5V
SO-8, HTR @ 3V
V+ = 5V RHEATER TO EXTERNAL SUPPLY TURNED ON @ t = 5 sec SO-8 SOLDERED TO 0.5 8 0.3 COPPER PCB
TIME – sec
TPC 2. Junction Temperature Rise in Still Air
102.0
101.5
101.0
100.5
100.0
HEATER RESISTANCE –
99.5
99.0
98.5
98.0
25 25 75 125 175
75
TEMPERATURE – C
V+ = 5V
140
TRANSITION FROM 100C STIRRED
130
BATH TO FORCED 25C AIR
120
V+ = 5V, NO LEAD, HEATER OFF
110
SO-8 SOLDERED TO 0.5  0.3 CU PCB
100
90
80 70 60
50
40
TIME CONSTANT – sec
30 20 10
0
0
SOIC AND PCB
100 200 300 400 500 600 700
AIR VELOCITY – FPM
TPC 3. Package Thermal Time Constant in Forced Air
TPC 4. Thermal Response Time in Stirred Oil Bath
5.0 START-UP VOLTAGE DEFINED AS
OUTPUT READING BEING WITHIN C OF OUTPUT AT 5V
NO LEAD, HEATER OFF
4.5
4.0
3.5
START-UP SUPPLY VOLTAGE – V
3.0
25 25 75 125 175
75
TEMPERATURE C
TPC 7. Start-Up Voltage vs. Temperature
TPC 5. Heater Resistance vs. Temperature
500
V+ = 5V NO LEAD
475
HEATER OFF
450
425
400
375
350
SUPPLY CURRENT – A
325
300
25 25 75 125 175
75
TEMPERATURE C
TPC 8. Supply Current vs. Temperature
TPC 6. Reference Voltage vs. Temperature
6
5
4
3
2
1
0
1
2
3
ACCURACY ERROR C
4
5
6
50
MAXIMUM LIMIT
ACCURACY ERROR
MINIMUM LIMIT
–25 0 25 50 75 100 125
TEMPERATURE – C
TPC 9. Accuracy Error vs. Temperature
–4–
REV. A
TMP12
500
TA = 25C
450
NO LOAD HEATER OFF
400
350
300
250
200
150
SUPPLY CURRENT – A
100
50
0
12345678
0
SUPPLY VOLTAGE – V
TPC 10. Supply Current vs. Supply Voltage
0.5 V+ = 4.5V TO 5.5V NO LOAD HEATER OFF
0.4
0.3
0.2
0.1
POWER SUPPLY REJECTION – C/V
0
25 25 75 125 175
75
TEMPERATURE C
TPC 11. VPTAT Power Supply Rejection vs. Temperature
700
600
500
400
300
200
100
OPEN COLLECTION SINK CURRENT – mA
0
75
25 25 75 125 175
LOAD = 10mA
LOAD = 5mA
LOAD = 1mA
TEMPERATURE – C
TPC 13. Open-Collector Voltage vs. Temperature
40
VOL = 1V
38
V+ = 5V
36
34
32
30
28
26
24
22
OPEN COLLECTION SINK CURRENT – mA
20
25 25 75 125 175
75
TEMPERATURE C
TPC 12. Open-Collector Output Sink Current vs. Temperature

APPLICATIONS INFORMATION

A typical application for the TMP12 is shown in Figure 2. The TMP12 package is placed in the same cooling airflow as a high­power dissipation IC. The TMP12s internal resistor produces a temperature rise that is proportional to air flow, as shown in Figure 3. Any interruption in the air flow 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.
PGA
PGA
SOCKET
PACKAGE
POWER I.C.
AIR FLOW
PC BOARD
TMP12
Figure 2. Typical Application
65
60
55
50
45
DIE TEMPERATURE – C
A. TMP12 DIE TEMP NO AIR FLOW B. HIGH SET POINT
40
C. LOW SET POINT D. TMP12 DIE TEMP MAX AIR FLOW E. SYSTEM AMBIENT TEMPERATURE
35
0 25050
100 150 200
TMP12 PD – mW
A
D
E
B
C
Figure 3. Choosing Temperature Setpoints
Temperature Hysteresis
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.
REV. A
–5–
TMP12
Figure 4 shows the TMP12s hysteresis profile. The hysteresis is programmed, by the user, by setting a specific load current on the reference voltage output V also called the hysteresis current. I
. This output current, I
REF
is mirrored internally by
REF
REF
, is
the TMP12, as shown in the Functional Block Diagram, and fed to a buffer with an analog switch.
OUTPUT
VO LTAG E
OVER, UNDER
HYSTERESIS
HI
LO
LOW
HYSTERESIS HIGH =
HYSTERESIS LOW
T
SETLOW
Figure 4. Hysteresis Profile
TEMPERATURE
HYSTERESIS
HIGH
T
SETHIGH
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 k 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 LOW, deactivating the open-collector output and disabling the hysteresis current buffer output. The scale factor for the programmed hysteresis current is:
==°+57µµ/
II AC A
VREF
Thus, since VREF = 2.5 V, a reference load resistance of 357 kQ or greater (output current of 7 µA or less) will produce a tempera- ture 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 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, I
VREF
.
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.
The hysteresis current is readily calculated, as shown above. For example, to produce two degrees of hysteresis, I set to 17 µA. Next, the setpoint voltages V
SETHIGH
should be
VREF
and V
SETLOW
are determined using the VPTAT scale factor of
5 5 273 15mV K mV C//.+
(
)
which is 1.49 V for 25°C. Finally, the divider resistors are calcu­lated, based on the setpoint voltages.
The setpoint voltages are calculated from the equation:
V T mV C
=+
(
SET SET
273 15 5./
This equation is used to calculate both the V V
values. A simple 3-resistor network, as shown in Figure 5,
SETLOW
°
(
)
)
SETHIGH
and the
determines the setpoints and hysteresis value. The equations used to calculate the resistors are:
Rk V V I
125Ω
=
(
)
VV I
. /
(
Rk V V I
2
(
)
Rk V I
3
(
)
(V
– V
REF
(V
SETHIGH
(V
SETLOW
/
(
REF SETHIGH VREF
)
SETHIGH VREF
=
(
SETHIGH SETLOW VREF
= /
SETLOW VREF
V
REF
)/|V
SETHIGH
– V
SETLOW
– V
Figure 5. Setpoint Programming
REF
)/|V
)/|V
V
REF
REF
REF
V
)
/
= 2.5V
= R1
SETHIGH
= R2
SETLOW
= R3
GND
=
)
18
|V
REF
TMP12
27
36
45
V+
OVER
UNDER
HEATER
For example, setting the high setpoint for 80°C, the low setpoint for 55°C, and hysteresis for 3°C produces the following values:
==°×°
II CACA
HYS VREF
+=
15 7 22
µµ µ
AA A
V T mV C
SETHIGH SETHIGH
80 273 15 5 1 766
CmVCV
°+
(
V T mV C
SETLOW SETLOW
55 273 15 5 1 641
CmVCV
°+
(
R k VREF V I
1
=
(
)
. – ./ .µ
VVA k
2 5 1 766 22 33 36ΩΩ
(
Rk V V I
2
(
)
. – ./ .µ
VVA k
1 766 1 641 22 5 682ΩΩ
(
Rk V I V A k
3 1 641 22 74 59ΩΩ
()
35 7
(
=+
(
./.
)
=+
(
./.
)
(
273 15 5
(
273 15 5
(
°
/
SETHIGH VREF
)
=
(
SETHIGH SETLOW VREF
/
)
==
SETLOW VREF
+=
µµ
/
)
./
(
)
°
=
)
./
(
)
=
)
)
=
)
=
°
=
)
°
=
)
=
=
()
=/./ .µ
The total of R1 + R2 + R3 is equal to the load resistance needed to draw the desired hysteresis current from the reference, or I
VREF
.
The nomograph of Figure 6 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.
–6–
REV. A
TMP12
218 248 273 298 323 348 373 398
K
–55 –25–18 0 25 50 75 100 125
C
67
25 0 32 50 77 100 150 200 212 257
F
VPTAT
1.09
1.24 1.365 1.49 1.615 1.74 1.865 1.99
Figure 6. Temperature, VPTAT Scale
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 V+ or VREF, SETLOW by tying it to GND. Either output can be left disconnected.
Selecting Setpoints
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 TMP12s temperature rise above ambient is proportional to airflow (TPC 1 and Figure 2). 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 warming either if the fan output is reduced or if the ambient temperature rises (for example, if the fans cool air intake is blocked). The high setpoint could then be set for the maximum system temperature to provide a final system shutdown control.
Measuring the TMP12 Internal Temperature
As previously mentioned, the TMPl2s VPTAT generator repre­sents the chip temperature with a slope of 5 mV/K. In some cases, selecting the setpoints is made easier if the TMPl2s inter­nal VPTAT voltage (and therefore, the chip temperature) is
C
INTERFACE
OVER
5V
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 TMP12s temperature to select the setpoints.
The TMPl2s 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.
A simple method of measuring the TMP12 VPTAT is shown in Figure 7. To measure VPTAT, adjust potentiometer R1 until the LED turns ON. The voltage at Pin 2 of the TMP12 will then match the TMP12s internal VPTAT.
5V
V
VPTAT
200k
200k
1
REF
R1
R1
SET
2
HIGH
SET
3
LOW
GND
4
NC = NO CONNECT
TMP12
UNDER
HEATER
V+
OVER
8
330
7
LED
6
NC
5
5V
5V
Figure 7. Measuring VPTAT with a Potentiometer
The method described in Figure 7 can be automated by replacing the discrete resistors with a digital potentiometer. The improved circuit, shown in Figure 8, permits the VPTAT voltage to be monitored with a microprocessor or other digital controller. The AD8402-100 provides two 100 k potentiometers which are adjusted to 8-bit resolution via a 3-wire serial interface. The controller simply sweeps the wiper of potentiometer l from the Al terminal to the Bl terminal (digital value = 0), while monitor­ing the comparator output at Pin 7 of the TMP 12. When Pin 7 goes low, the voltage at Pin 2 equals the VPTAT voltage. This circuit sweeps Pin 2s voltage from maximum to minimum, so that the TMP12s setpoint hysteresis will not affect the reading.
REV. A
SDI
CLK
CS
SHDN RS
AD8402–100
9
8
7
15
V
DD
13
A1
12
W1
14
B1
3
A2
W2
4
NC
2
B2
DGNDAGND
NC = NO CONNECT
V
REF
1
2
3
4
TEMPERATURE
SENSOR AND
VO LTAG E
REFERENCE
WINDOW
COMPARATOR
HYSTERESIS
GENERATOR
TMP12
Figure 8. Measuring VPTAT with a Digital Potentiometer
–7–
100
VPTAT
8
7
6
NC
5
5V
TMP12
The circuit of Figure 8 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
25 2
V
28
÷
mV=
=
49..
÷
Resolution
where VREF is the voltage reference output (Pin l 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 –23°C) to VREF – 1 LSB (i.e., 2.5 V – 4.9 mV, or
226°C). The VPTAT is therefore
VPTAT V Digital Count mV=+ ×
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 TMPl2s comparators. This causes the comparator to oscillate, and in turn forces the voltage at the comparator input to equal the VPTAT voltage. Figure 9 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.
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 comparators input bias current drops to less than l 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.
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 TPC 2.) 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 five time constants, or six minutes, is necessary. Refer to TPC 3.
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 fixed­temperature 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,
VREF
125 49..
2
2
=
N
()
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 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 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.
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 self heating 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
PVAmW
=0 6 0 020 12..
DISS
which in a surface-mount SO package accounts for a tempera­ture increase due to self-heating of
TP W CW C
=×= ×°=°θ 0 012 158 1 9./.
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 TMPl2. 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 the following sections of this data sheet. These buffers will reduce self-heating and improve accuracy.
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.
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 error due to excess heat buildup, 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 –10 ppm/°C, with 315 nV/Hz (typ) noise spectral density at 1 kHz.
DISS JA
–8–
REV. A
TMP12
NC
12V
NC = NO CONNECT
R1
R2
R3
IN4001
OR EQUIV
140
MOTOR SHUTDOWN
2604-12-311
COTO
12V
8
7
6
5
1
2
3
4
TEMPERATURE
SENSOR AND
VO LTAG E
REFERENCE
V
REF
WINDOW
COMPARATOR
HYSTERESIS GENERATOR
100
TMP12
VPTAT
NC
5V
V
1
REF
TMP12
SET
2
HIGH
SET
3
LOW
GND
4
NC = NO CONNECT
OVER
UNDER
HEATER
V+
8
7
6
NC
5
5V
200k
5V
–1.5V
300k
OP193
130k
1F
10k
VPTAT
0.1F
Figure 9. An Analog Measurement Circuit for VPTAT
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 A 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 10, is the current required to activate the switch.
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 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.
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. TPC 2 and 4 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. A
It is important to check the particular relay you choose to ensure that the current needed to activate the coil does not exceed the TMP12s 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 a contact rating of 10 W 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
–9–
Figure 10. Reed Relay Drive
TMP12
current needed for the coil. In some cases, an external driving transistor should be used to remove the current load on the TMPl2 as explained in the next section.
Power FETs are popular for handling a variety of high current dc loads. Figure 11 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 12 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.
TEMPERATURE
V
1
2
3
4
REF
HYSTERESIS GENERATOR
SENSOR AND
VO LTAG E
REFERENCE
WINDOW
COMPARATOR
TMP12
NC = NO CONNECT
VPTAT
100
V+
8
7
6
5
NC
5V
2.4k (12V)
1.2k (6V) 5%
IRFR9024 OR EQUIV
HEATING ELEMENT
+
Figure 11. Driving a P-Channel MOSFET
TEMPERATURE
V
1
2
3
4
REF
HYSTERESIS GENERATOR
TMP12
SENSOR AND
VO LTAG E
REFERENCE
WINDOW
COMPARATOR
100
NC = NO CONNECT
VPTAT
V+
8
4.7k4.7k
7
NC
6
5
5V
2N1711
HEATING ELEMENT
IRF130
Figure 12. 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 13. The turn on voltage for the IGBT shown (IRGB40S) is between 3.0 V and 5.5 V. 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.
TEMPERATURE
V
1
2
3
4
REF
HYSTERESIS GENERATOR
TMP12
SENSOR AND
VO LTAG E
REFERENCE
WINDOW
COMPARATOR
100
NC = NO CONNECT
VPTAT
8
7
6
5
V+
NC
5V
4.7k4.7k
2N1711
MOTOR CONTROL
IRGBC40S
Figure 13. Driving an IGBT
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 14 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.
TEMPERATURE
V
REF
1
2
3
4
HYSTERESIS GENERATOR
TMP12
NC = NO CONNECT
SENSOR AND
VO LTAG E
REFERENCE
WINDOW
COMPARATOR
VPTAT
100
8
7
6
5
V+ = 5V
NC
5V
300
MOC3011
150
2N6073A
LOAD
AC
Figure 14. Controlling the 2N6073A Triac
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 15. When the open-collector of the TMP12 turns “ON” and pulls the output down, the external transistor Q1s base will be pulled low, turning off the transistor. Another transistor can be added to reinvert the signal as shown in Figure 16. 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.
–10–
REV. A
TMP12
TEMPERATURE
V
1
2
3
4
REF
SENSOR AND
VO LTAG E
REFERENCE
COMPARATOR
HYSTERESIS GENERATOR
VPTAT
WINDOW
100
V+
8
4.7k
7
6
5
2N1711
Q1
I
C
TMP12
Figure 15. An External Transistor Minimizes Self-Heating
TEMPERATURE
V
1
2
3
4
REF
SENSOR AND
VO LTAG E
REFERENCE
COMPARATOR
HYSTERESIS GENERATOR
VPTAT
WINDOW
100
V+
8
7
6
5
4.7k
4.7k
2N1711
Q1
2N1711
Q2
I
C
TMP12
Figure 16. Second Transistor Maintains Polarity of TMP12 Output
An example of a higher power transistor is a standard Darlington configuration as shown in Figure 16. 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.
12V
RELAY
MOTOR SHUTDOWN
I
V
1
2
3
4
TEMPERATURE
REF
SENSOR AND
REFERENCE
COMPARATOR
HYSTERESIS GENERATOR
VO LTAG E
WINDOW
VPTAT
100
V+
8
4.7k
7
6
5
5V
4.7k
2N1711
TIP-110
C
TMP12
Figure 17. Darlington Transistor Can Handle Large Currents
REV. A
–11–
0.1574 (4.00)
0.1497 (3.80)
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Pin SOIC
0.1968 (5.00)
0.1890 (4.80)
85
0.2440 (6.20)
0.2284 (5.80)
41
PIN 1
0.0098 (0.25)
0.0040 (0.10)
SEATING
PLANE
0.0500 (1.27) BSC
0.0192 (0.49)
0.0138 (0.35)
0.0688 (1.75)
0.0532 (1.35)
0.0098 (0.25)
0.0075 (0.19)
0.0196 (0.50)
0.0099 (0.25)
8
0.0500 (1.27)
0
0.0160 (0.41)
45
C00335–0–2/02(A)

Revision History

Location Page
09/01—Data Sheet changed from REV. 0 to REV. A.
Edits to GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Edits to PINOUTS Figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Deleted WAFER TEST LIMITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Deleted DICE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Edits to PACKAGE TYPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Edits to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Edits to TYPICAL PERFORMANCE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Edits to OUTLINE DIMENSIONS Figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
–12–
PRINTED IN U.S.A.
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