REV. B

a

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Airflow and Temperature Sensor

FUNCTIONAL BLOCK DIAGRAM

HYSTERESISCURRENT

CURRENTMIRROR

TMP12

IHYS

WINDOWCOMPARATOR

HYSTERESISVOLTAGE

1k

100

VOLTAGEREFERENCE

ANDSENSOR

VREF

SETHIGH

SETLOW

GND

V+

OVER

UNDER

HEATER

PIN CONNECTION

8-Lead SOIC

TOP VIEW(Not to Scale)

8

7

6

5

1

2

3

4

VREF V+

TMP12SET HIGH OVER

SET LOW UNDER

GND HEATER

FEATURES

Temperature Sensor Includes 100 Heater

Heater Provides Power IC Emulation

Accuracy 3 Typ from –40C to +100COperation to 150C5 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 Overtemperature Sensor

Overtemperature Protection

Power Supply Thermal Sensor

Low Cost Fan Controller

TMP12

GENERAL DESCRIPTIONThe TMP12 is a silicon-based airflow and temperature sensordesigned to be placed in the same airstream as heat generatingcomponents that require cooling. Fan cooling may be requiredcontinuously or during peak power demands. For example, ifthe cooling systems of a power supply fails, system reliabilityand/or safety may be impaired. By monitoring temperaturewhile emulating a power IC, the TMP12 can provide a warningof cooling system failure.

The TMP12 generates an internal voltage that is linearly propor-tional to Celsius (Centigrade) temperature, nominally 5 mV/°C.The linearized output is compared with voltages from an exter-nal resistive divider connected to the TMP12’s 2.5 V precisionreference. The divider sets up one or two reference voltages, asrequired by the user, providing one or two temperature setpoints.Comparator outputs are open-collector transistors able to sinkover 20 mA. There is an on-board hysteresis generator providedto speed up the temperature-setpoint output transitions; thisalso reduces erratic output transitions in noisy environments.Hysteresis is programmed by the external resistor chain andis determined by the total current drawn from the 2.5 V reference.The TMP12 airflow sensor also incorporates a precision, lowtemperature coefficient 100 Ω heater resistor that may be con-nected directly to an external 5 V supply. When the heater isactivated, it raises the die temperature approximately 20°C

above ambient (in still air). The purpose of the heater in theTMP12 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 tempera-tures of the power device and the TMP12 (if located in thesame airstream) will be reduced by an amount proportional tothe rate of airflow. The internal temperature rise of the TMP12may be reduced by placing a resistor in series with the heater, orby reducing the heater voltage.

The TMP12 is intended for single 5 V supply operation, but willoperate on a 12 V supply. The heater is designed to operate from5 V only. Specified temperature range is from –40°C to +125°C,and operation extends to 150°C at 5 V with reduced accuracy.

The TMP12 is available in 8-lead SOIC packages.

OBSOLETE

REV. B–2–

TMP12–SPECIFICATIONSParameter Symbol Conditions Min Typ Max Unit

ACCURACYAccuracy (High, Low Setpoints) TA = 25°C ±2 ±3 °C

TA = –40°C to +100°C ±3 ±5 °CInternal Scale Factor TA = –40°C to +100°C 4.9 5 5.1 mV/°CPower Supply Rejection Ratio PSRR 4.5 V ≤ VS ≤ 5.5 V 0.1 0.5 °C/VLinearity TA = –40°C to +125°C 0.5 °CRepeatability TA = –40°C to +125°C 0.3 °CLong Term Stability TA = 125°C for 1 k Hrs 0.3 °C

SETPOINT INPUTSOffset Voltage VOS 0.25 mVOutput Voltage Drift TCVOS 3 µV/°CInput Bias Current IB 25 100 nA

VREF OUTPUTOutput Voltage VREF TA = 25°C, No Load 2.49 2.50 2.51 V

VREF TA = –40°C to +100°C, No Load 2.5 ± 0.015 VOutput Drift TCVREF –10 ppm/°COutput Current, Zero Hysteresis IVREF 7 µAHysteresis Current Scale Factor SFHYS 5 µA/°C

OPEN-COLLECTOR OUTPUTSOutput Low Voltage VOL ISINK = 1.6 mA 0.25 0.4 V

VOL ISINK = 20 mA 0.6 VOutput Leakage Current IOH VS = 12 V 1 100 µAFall Time tHL See Test Load 40 ns

HEATERResistance RH TA = 125°C 97 100 103 ΩTemperature Coefficient TA = –40°C to +125°C 100 ppm/°CMaximum Continuous Current1 IH 60 mA

POWER SUPPLYSupply Range VS 4.5 5.5 VSupply Current ISY Unloaded at 5 V 400 600 µA

ISY Unloaded at 12 V2 450 µA

NOTES1Guaranteed 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.

1k

20pF

Figure 1. Test Load

(VS = 5 V, –40C ≤ TA s ≤ 125C, unless otherwise noted.)

OBSOLETE

REV. B

TMP12

–3–

CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readilyaccumulate on the human body and test equipment and can discharge without detection. Althoughthe TMP12 features proprietary ESD protection circuitry, permanent damage may occur ondevices subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions arerecommended to avoid performance degradation or loss of functionality.

WARNING!

ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS*

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +11 VHeater Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 VSetpoint Input Voltage . . . . . . . . . . . –0.3 V to [(V+) + 0.3 V]Reference Output Current . . . . . . . . . . . . . . . . . . . . . . . 2 mAOpen-Collector Output Current . . . . . . . . . . . . . . . . . 50 mAOpen-Collector Output Voltage . . . . . . . . . . . . . . . . . . . . 15 VOperating Temperature Range . . . . . . . . . . –55°C to +150°CStorage Temperature Range . . . . . . . . . . . . –65°C to +160°CLead Temperature (Soldering, 60 sec) . . . . . . . . . . . . 300°C

Package Type JA JC Unit

8-Lead SOIC (S) 1581 432 °C/W

NOTES1JA is specified for device in socket (worst-case conditions).2JC is specified for device mounted on PCB.

*CAUTION1. Stresses above those listed under Absolute Maximum Ratings

may cause permanent damage to the device. This is a stressrating only and functional operation at or above this specifi-cation is not implied. Exposure to the above maximum ratingconditions for extended periods may affect device reliability.

2. Digital inputs and outputs are protected; however, permanentdamage may occur on unprotected units from high-energyelectrostatic fields. Keep units in conductive foam or packag-ing at all times until ready to use. Use proper antistatic handlingprocedures.

3. Remove power before inserting or removing units from theirsockets.

ORDERING GUIDE

Temperature Package PackageModel/Grade Range Description Option

TMP12FS –40°C to +85°C SOIC R-8TMP12FS-REEL –40°C to +125°C SOIC R-8

FUNCTIONAL DESCRIPTIONThe 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 setpointcomparators to monitor the resulting temperature rise, the TMP12can indirectly monitor the performance of a system’s cooling fan.

The TMP12 temperature sensor section consists of a band gapvoltage reference that provides both a constant 2.5 V output anda voltage that is proportional to absolute temperature (VPTAT).The VPTAT has a precise temperature coefficient of 5 mV/Kand is 1.49 V (nominal) at 25°C. The comparators compareVPTAT with the externally set temperature trip points andgenerate an open-collector output signal when one of theirrespective thresholds has been exceeded.

The heat source for the TMP12 is an on-chip 100 Ω low tempcothin-film resistor. When connected to a 5 V source, this resistordissipates

P V

RV

WD = = =2 25

1000 25

Ω.

which generates a temperature rise of about 32°C in still airfor the SOIC packaged device. With an airflow of 450 feetper minute (FPM), the temperature rise is about 22°C. Byselecting a temperature setpoint between these two values,the TMP12 can provide a logic-level indication of problemsin the cooling system.

A proprietary, low tempco thin-film resistor process, in conjunctionwith production laser trimming, enables the TMP12 to provide atemperature accuracy of ±3°C (typ) over the rated temperaturerange. The open-collector outputs are capable of sinking 20 mA,allowing the TMP12 to drive small control relays directly. Oper-ating from a single 5 V supply, the quiescent current is only600 µA (max), without the heater resistor current.

OBSOLETE

REV. B

TMP12

–4–

–Typical Performance Characteristics

HEATER RESISTOR POWERDISSIPATION (mW)

JUN

CT

ION

TE

MP

ER

ATU

RE

RIS

EA

BO

VE

AM

BIE

NT

(C

)

35

0

30

25

20

15

10

5

050 100 150 200 250

V+ = 5VSOIC-8 SOLDERED TO0.5 0.3 CU PCB

0 FPM

250 FPM

600 FPM

AIR FLOW RATES

450 FPM

TPC 1. SOIC Junction TemperatureRise vs. Heater Dissipation

TIME (sec)

JUN

CT

ION

TE

MP

ER

ATU

RE

(C

)

120

0

110

100

90

80

70

60

50

40

30

20

10

02 4 6 8 10 12 14 16 18 20

TRANSITION FROM STILL 25CAIR TO STIRRED 100C BATH

V+ = 5V, NO LEAD, HEATER OFFSOIC-8 SOLDERED TO 0.5 0.3 CU PCB

SOIC AND PCB

TPC 4. Thermal Response Time inStirred Oil Bath

TEMPERATURE (C)

STA

RT-

UP

SU

PP

LY V

OLT

AG

E (

V)

5.0

–75

4.5

4.0

3.5

–25 25 75 125 1753.0

START-UP VOLTAGE DEFINED ASOUTPUT READING BEING WITHINC OF OUTPUT AT 5V

NO LEAD, HEATER OFF

TPC 7. Start-Up Voltage vs.Temperature

TIME (sec)JU

NC

TIO

N T

EM

PE

RAT

UR

E (

C)

70

0

65

60

5550

45

40

35

30

25

20

15

10

5

010 20 30 40 50 60 70 80 90 100 110120130

SOIC-8, HTR @ 5V

SOIC-8, HTR @ 3V

V+ = 5V RHEATER TO EXTERNALSUPPLY TURNED ON @ t = 5 secSOIC-8 SOLDERED TO 0.5 0.3COPPER PCB

TPC 2. Junction Temperature Rise inStill Air

TEMPERATURE (C)

HE

ATE

R R

ES

ISTA

NC

E (

)

102.0

–75

101.5

101.0

100.5

100.0

99.5

99.0

98.5

98.0–25 25 75 125 175

V+ = 5V

TPC 5. Heater Resistance vs.Temperature

TEMPERATURE (C)

SU

PP

LY C

UR

RE

NT

(

A)

500

–75

475

450

425

400

375

350

325

300–25 25 75 125 175

V+ = 5VNO LEADHEATER OFF

TPC 8. Supply Current vs.Temperature

AIR VELOCITY (FPM)

TIM

E C

ON

STA

NT

(se

c)

140

0

130

120110

100

90

807060

50

40

302010

0100 200 300 400 500 600 700

TRANSITION FROM 100C STIRREDBATH TO FORCED 25C AIR

V+ = 5V, NO LEAD, HEATER OFFSOIC-8 SOLDERED TO 0.5 0.3 CU PCB

SOIC AND PCB

TPC 3. Package Thermal TimeConstant in Forced Air

TEMPERATURE (C)

RE

FE

RE

NC

E V

OLT

AG

E (

V)

2.520

–75

2.515

2.510

2.505

2.500

2.495

2.490–25 25 75 125 175

V+ = 5VNO LOADHEATER OFF

TPC 6. Reference Voltage vs.Temperature

TEMPERATURE (C)

AC

CU

RA

CY

ER

RO

R (

C)

6

–50

5

4

3

2

1

0

–1

–2

–3

–4

–5

–6–25 0 25 50 75 100 125

MAXIMUM LIMIT

ACCURACY ERROR

MINIMUM LIMIT

TPC 9. Accuracy Error vs.Temperature

OBSOLETE

REV. B –5–

TMP12

SUPPLY VOLTAGE (V)

SU

PP

LY C

UR

RE

NT

(

A)

500

0

450

400

350

300

250

200

150

100

50

01 2 3 4 5 6 7 8

TA = 25CNO LOADHEATER OFF

TPC 10. Supply Current vs. SupplyVoltage

TEMPERATURE (C)

PO

WE

R S

UP

PLY

RE

JEC

TIO

N (

C/V

)

0.5

–75

0.4

0.3

0.2

0.1

0–25 25 75 125 175

V+ = 4.5V TO 5.5VNO LOADHEATER OFF

TPC 11. VPTAT Power SupplyRejection vs. Temperature

TEMPERATURE (C)

OP

EN

CO

LL

EC

TIO

N S

INK

CU

RR

EN

T (

mA

) 40

–75

36

32

28

24

20–25 25 75 125 175

38

34

30

26

22

VOL = 1VV+ = 5V

TPC 12. Open-Collector Output SinkCurrent vs. Temperature

APPLICATIONS INFORMATIONA typical application for the TMP12 is shown in Figure 2. TheTMP12 package is placed in the same cooling airflow as a highpower dissipation IC. The TMP12’s internal resistor produces atemperature rise that is proportional to air flow, as shown inFigure 3. Any interruption in the air flow will produce an addi-tional temperature rise. When the TMP12 chip temperatureexceeds a user-defined setpoint limit, the system controller cantake corrective action, such as reducing clock frequency, shuttingdown unneeded peripherals, turning on additional fan cooling,or shutting down the system.

PGAPACKAGE

PGASOCKET

POWER IC

AIR FLOW

PC BOARD TMP12

Figure 2. Typical Application

TEMPERATURE (C)

OP

EN

CO

LL

EC

TIO

N S

INK

CU

RR

EN

T (

mA

) 700

–75

500

300

100

–25 25 75 125 175

600

400

200

0

LOAD = 10mA

LOAD = 5mA

LOAD = 1mA

TPC 13. Open-Collector Voltage vs.Temperature

TMP12 PD (mW)

65

60

350 25050

DIE

TE

MP

ER

ATU

RE

(C

)

100 150 200

55

50

45

40

A

B

C

D

E

A. TMP12 DIE TEMP NO AIR FLOWB. HIGH SETPOINTC. LOW SETPOINTD. TMP12 DIE TEMP MAX AIR FLOWE. SYSTEM AMBIENT TEMPERATURE

Figure 3. Choosing Temperature Setpoints

Temperature HysteresisThe temperature hysteresis at each setpoint is the number ofdegrees beyond the original setpoint temperature that must besensed by the TMP12 before the setpoint comparator will bereset and the output disabled. Hysteresis prevents chatter andmotorboating in feedback control systems. For monitoring tem-perature in computer systems, hysteresis prevents multipleinterrupts to the CPU which can reduce system performance.

OBSOLETE

REV. B

TMP12

–6–

Figure 4 shows the TMP12’s hysteresis profile. The hysteresis isprogrammed by the user by setting a specific load current on thereference voltage output, VREF. This output current, IREF, is alsocalled the hysteresis current. IREF is mirrored internally by theTMP12, as shown in the Functional Block Diagram, and isfed to a buffer with an analog switch.

HYSTERESISLOW

HYSTERESISHIGH

HYSTERESIS HIGH =HYSTERESIS LOW

TEMPERATURETSETLOW TSETHIGH

HI

LO

OUTPUTVOLTAGE

OVER, UNDER

Figure 4. Hysteresis Profile

After a temperature setpoint has been exceeded and a compara-tor tripped, the hysteresis buffer output is enabled. The result isa current of the appropriate polarity, which generates a hyster-esis offset voltage across an internal 1 kΩ resistor at thecomparator input. The comparator output remains on until thevoltage at the comparator input, now equal to the temperaturesensor voltage VPTAT summed with the hysteresis effect,returns to the programmed setpoint voltage. The comparatorthen returns LOW, deactivating the open-collector output anddisabling the hysteresis current buffer output. The scale factorfor the programmed hysteresis current is

I I A C AVREF= = ° +5 7µ µ/

Thus, since VREF = 2.5 V, a reference load resistance of 357 kΩor greater (output current of 7 µA or less) will produce a tempera-ture setpoint hysteresis of zero degrees. For more details, see thetemperature programming discussion that follows. Largervalues of load resistance will only decrease the output currentbelow but will have no effect on the operation of the device. Theamount of hysteresis is determined by selecting an appropriatevalue of load resistance for VREF, as shown below.

Programming the TMP12The basic thermal monitoring application only requires a simplethree-resistor ladder voltage divider to set the high and lowsetpoints and the hysteresis. These resistors are programmed inthe 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 neededto develop the desired comparator setpoint voltages at theSet High and Set Low inputs.

The hysteresis current is readily calculated, as shown above. Forexample, to produce two degrees of hysteresis, IVREF should beset to 17 µA. Next, the setpoint voltages VSETHIGH and VSETLOW

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 calcu-lated, based on the setpoint voltages.

The setpoint voltages are calculated from the equation

V T mV CSET SET= +( ) °( )273 15 5. /

This equation is used to calculate both the VSETHIGH and theVSETLOW values. A simple three-resistor network, as shown inFigure 5, determines the setpoints and the hysteresis value.The equations used to calculate the resistors are

R1 k V –V / I

V –V / I

REF SETHIGH VREF

SETHIGH VREF

Ω( ) = ( ) =

( )2.5

R2 k V –V /ISETHIGH SETLOW VREFΩ( ) = ( )

R3 k V /ISETLOW VREFΩ( ) =

V+

TMP12VSETHIGH OVER

VSETLOW UNDER

GND HEATER

1 8

2 7

3 6

4 5

|VREF(VREF – VSETHIGH)/IVREF = R1

(VSETLOW – VREF)/IVREF = R3

VREF = 2.5V

(VSETHIGH – VSETLOW)/IVREF = R2

Figure 5. Setpoint Programming

For example, setting the high setpoint for 80°C, the low setpointfor 55°C, and hysteresis for 3°C produces the following values:

I I C A C A

A A A

HYS VREF= = ° × °( ) + =

+ =

3 5 7

15 7 22

µ µ

µ µ µ

/

V T mV C

C mV C V

SETHIGH SETHIGH= +( ) °( ) =

° +( ) °( ) =

273 15 5

80 273 15 5 1 766

. /

. / .

V T mV C

C mV C V

SETLOW SETLOW= +( ) °( ) =

° +( ) °( ) =

273 15 5

55 273 15 5 1 641

. /

. / .

R VREF V I

V V A

SETHIGH VREF1 k

k

Ω

Ω

( ) = ( ) =

( ) =

– /

. – . / .2 5 1 766 22 33 36µ

R V V I

V V A

SETHIGH SETLOW VREF2 k

k

Ω

Ω

( ) = ( ) =

( ) =

– /

. – . / .1 766 1 641 22 5 682µ

R V I V ASETLOW VREF3 k kΩ Ω( ) = = ( ) =/ . / .1 641 22 74 59µ

The total of R1 + R2 + R3 is equal to the load resistance needed todraw the desired hysteresis current from the reference, or IVREF.

The nomograph of Figure 6 provides an easy method of determin-ing the correct VPTAT voltage for any temperature. Simply locatethe desired temperature on the appropriate scale (K, °C, or °F)and read the corresponding VPTAT value from the bottom scale.

OBSOLETE

REV. B

TMP12

–7–

K

C

F

VPTAT

218 248 273 298 323 348 373 398

–55 –25–18 0 25 50 75 100 125

–67

1.09

–25 0 32 50 77 100 150 200 212 257

1.24 1.365 1.49 1.615 1.74 1.865 1.99

Figure 6. Temperature, VPTAT Scale

The preceding formulas are also helpful in understanding thecalculations of temperature setpoint voltages in circuits otherthan the standard two-temperature thermal/airflow monitor. If asetpoint function is not needed, the appropriate comparatorinput should be disabled. SETHIGH can be disabled by tying itto V+ or to VREF; SETLOW by tying it to GND. Eitheroutput can be left disconnected.

Selecting SetpointsChoosing the temperature setpoints for a given system is anempirical process because of the wide variety of thermal issuesin any practical design. The specific setpoints are dependent onfactors such as airflow velocity in the system, adjacent componentlocation and size, PCB thickness, location of copper groundplanes, and thermal limits of the system.

The TMP12’s temperature rise above ambient is proportionalto airflow (TPC 1 and Figure 2). As a starting point, the lowsetpoint temperature could be set at the system ambient tem-perature (inside the enclosure) plus one-half of the temperaturerise above ambient (at the actual airflow in the system). Withthis setting, the low limit will provide a warming either if the fanoutput is reduced or if the ambient temperature rises (for example,if the fan’s cool air intake is blocked). The high setpoint couldthen be set for the maximum system temperature to provide afinal system shutdown control.

Measuring the TMP12 Internal TemperatureAs previously mentioned, the TMP12’s VPTAT generator rep-resents the chip temperature with a slope of 5 mV/K. In somecases, selecting the setpoints is made easier if the TMP12’s internalVPTAT voltage (and therefore, the chip temperature) is known.

For example, the case temperature of a high power micropro-cessor can be monitored with a thermistor, thermocouple, orother measurement method. The case temperature can then becorrelated with the TMP12’s temperature to select thesetpoints.

The TMP12’s VPTAT voltage is not available externally, so indi-rect methods must be used. Since the VPTAT voltage is appliedto the internal comparators, measuring the voltage at which thedigital output changes state will reflect the VPTAT voltage.

A simple method of measuring the TMP12 VPTAT is shown inFigure 7. To measure VPTAT, adjust potentiometer R1 untilthe LED turns on. The voltage at Pin 2 of the TMP12 willthen match the TMP12’s internal VPTAT.

TMP12

VREF

SETHIGH

SETLOW

GND

V+

OVER

UNDER

HEATER

1 8

2 7

3 6

4 5

VPTAT

R1200k

R1200k

3305V

5V

LED

NC

5V

NC = NO CONNECT

Figure 7. Measuring VPTAT with a Potentiometer

The method described in Figure 7 can be automated by replacingthe discrete resistors with a digital potentiometer. The improvedcircuit, shown in Figure 8, permits the VPTAT voltage to bemonitored with a microprocessor or other digital controller. TheAD8402-100 provides two 100 kΩ potentiometers, which areadjusted to 8-bit resolution via a 3-wire serial interface. Thecontroller simply sweeps the wiper of potentiometer l from theAl terminal to the Bl terminal (digital value = 0), while monitor-ing the comparator output at Pin 7 of the TMP12. When Pin 7goes low, the voltage at Pin 2 equals the VPTAT voltage. Thiscircuit sweeps Pin 2’s voltage from maximum to minimum, sothat the TMP12’s setpoint hysteresis will not affect the reading.

NC

5V

NC = NO CONNECT

NC

13

12

14

3

4

2

A1

W1

B1

A2

W2

B2

1 5

AD8402–100

DGNDAGND

SHDN RS VDD

5V

9

8

7

OVER

CS

CLK

SDI

CINTERFACE

8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

Figure 8. Measuring VPTAT with a Digital Potentiometer

OBSOLETE

REV. B

TMP12

–8–

The circuit of Figure 8 provides approximately 1°C of resolution.The two potentiometers divide VREF by 2, and the 8-bitpotentiometer further divides VREF by 256, so the resolution is

Resolution VREF

NV

mV= ÷ = ÷ =22

2 5 228

4 9.

.

where VREF is the voltage reference output (Pin l of the TMP12)and N is the resolution of the AD8402. Since the VPTAT has aslope of 5 mV/K, the AD8402 provides 1°C of resolution. Theadjustment 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, or226°C). The VPTAT is therefore

VPTAT V Digital Count mV= + ×( )1 25 4 9. .

where Digital Count is the value sent to the AD8402 which causedthe setpoint 1 output to go low.

A third way to measure the VPTAT voltage is to close a feed-back loop around one of the TMP12’s comparators. This causesthe comparator to oscillate, and in turn forces the voltage at thecomparator input to equal the VPTAT voltage. Figure 9 is atypical circuit for this measurement. An OP193 operationalamplifier, operating as an integrator, provides additional loop-gainto ensure that the TMP12 comparator will oscillate.

Understanding Error SourcesThe accuracy of the VPTAT sensor output is well characterizedand specified; however, preserving this accuracy in a thermalmonitoring control system requires some attention to minimiz-ing the various potential error sources. The internal sourcesof setpoint programming error include the initial tolerances andtemperature drifts of the reference voltage VREF, the setpointcomparator input offset voltage and bias current, and the hysteresiscurrent scale factor. When evaluating setpoint programmingerrors, remember that any VREF error contribution at the com-parator inputs is reduced by the resistor divider ratios. Eachcomparator’s input bias current drops to less than 1 nA (typ) whenthe comparator is tripped. This change accounts for some setpointvoltage error, equal to the change in bias current multiplied bythe effective setpoint divider ladder resistance to ground.

The thermal mass of the TMP12 package and the degree ofthermal coupling to the surrounding circuitry are the largestfactors in determining the rate of thermal settling, which ultimatelydetermines the rate at which the desired temperature measure-ment accuracy may be reached. (See graph in TPC 2.) Thus,one must allow sufficient time for the device to reach the finaltemperature. The typical thermal time constant for the SOICplastic package is approximately 70 seconds in still air. There-fore, to reach the final temperature accuracy within 1%, for atemperature change of 60°C, a settling time of five time con-stants, or six minutes, is necessary. Refer to TPC 3.

External error sources to consider are the accuracy of the externalprogramming resistors, grounding error voltages, and thermalgradients. The accuracy of the external programming resistorsdirectly impacts the resulting setpoint accuracy. Thus, in fixed-temperature applications, the user should select resistor tolerancesappropriate to the desired programming accuracy. Since setpointresistors are typically located in the same air flow as the TMP12,

resistor temperature drift must be taken into account also. Thiseffect can be minimized by selecting good quality components,and by keeping all components in close thermal proximity.Careful circuit board layout is essential to minimize commonthermal error sources. Also, the user should take care to keepthe bottom of the setpoint programming divider ladder as closeto GND (Pin 4) as possible to minimize errors due to IR voltagedrops and coupling of external noise sources. In any case, a 0.1 µFcapacitor for power supply bypassing is always recommended atthe chip.

Safety Considerations in Heating and Cooling System DesignDesigners should anticipate potential system fault conditionsthat may result in significant safety hazards that are outsidethe control of and cannot be corrected by the TMP12-basedcircuit. Governmental and industrial regulations regardingsafety requirements and standards for such designs should beobserved where applicable.

Self-Heating EffectsIn some applications, the user should consider the effects of selfheating due to the power dissipated by the open-collector out-puts, which are capable of sinking 20 mA continuously. Underfull load, the TMP12 open-collector output device is dissipating

P V A mWDISS = × =0 6 0 020 12. .

which in a surface-mount SOIC package accounts for a tempera-ture increase due to self-heating of

∆T P W C W CDISS JA= × = × ° = °θ 0 012 158 1 9. / .

This increase is for still air, of course, and will be reduced athigh airflow levels. However, the user should still be aware thatself-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 Setpoint2 output off 1.9°C early). Self-heating will not affect the tempera-ture at which Setpoint 1 turns on, but will add to the hysteresis.Several circuits for adding external driver transistors and otherbuffers are presented in the following sections of this data sheet.These buffers will reduce self-heating and improve accuracy.

Buffering the Voltage ReferenceThe reference output VREF is used to generate the temperaturesetpoint programming voltages for the TMP12. Since the hyster-esis is set by the reference current, external circuits which drawcurrent from the reference will increase the hysteresis value.

The on-board VREF output buffer is typically capable of 500 µAoutput drive into as much as 50 pF load (max). Exceeding thisload will affect the accuracy of the reference voltage, couldcause thermal sensing error due to excess heat buildup, andmay induce oscillations. External buffering of VREF with alow-drift voltage follower will ensure optimal reference accu-racy. Amplifiers that offer low-drift, low-power consumption,and low cost appropriate to this application include the OP284and members of the OP113 and OP193 families.

With excellent drift and noise characteristics, VREF offers agood voltage reference for data acquisition and transducer exci-tation applications as well. Output drift is typically better than–10 ppm/°C, with 315 nV/Hz (typ) noise spectral density at 1 kHz.

OBSOLETE

REV. B

TMP12

–9–

Preserving Accuracy Over Wide Temperature RangeOperationThe TMP12 is unique in offering both a wide-range temperaturesensor and the associated detection circuitry needed to imple-ment a complete thermostatic control function in one monolithicdevice. The voltage reference, setpoint comparators, and outputbuffer amplifiers have been carefully compensated to maintainaccuracy over the specified temperature ranges in this applica-tion. Since the TMP12 is both sensor and control circuit, inmany applications, the external components used to programand interface the device are subjected to the same temperatureextremes. Thus, it is necessary to place components in closethermal proximity minimizing large temperate differentials, andto account for thermal drift errors where appropriate, such asresistor matching temperature coefficients, amplifier error drift,and the like. Circuit design with the TMP12 requires a slightlydifferent perspective regarding the thermal behavior of electroniccomponents.

PC Board Layout ConsiderationsThe TMP12 also requires a different perspective on PC boardlayout. In many applications, wide traces and generous groundplanes are used to extract heat from components. The TMP12is slightly different in that ideal path for heat is via the coolingsystem air flow. Thus, heat paths through the PC traces shouldbe minimized. This constraint implies that minimum pad sizesand trace widths should be specified to reduce heat conduc-tion. At the same time, analog performance should not becompromised. In particular, the bottom of the setpoint resistorladder should be located as close to GND as possible, asdiscussed in the Understanding Error Sources section of thisdata sheet.

Thermal Response TimeThe time required for a temperature sensor to settle to a specifiedaccuracy is a function of the thermal mass of the sensor andthe thermal conductivity between the sensor and the object beingsensed. Thermal mass is often considered equivalent to capaci-tance. Thermal conductivity is commonly specified using thesymbol Q, and is the inverse of thermal resistance. It is commonlyspecified in units of degrees per watt of power transferred acrossthe thermal joint. TPCs 2 and 4 illustrate the typical RC timeconstant response to a step change in ambient temperature. Thus,the time required for the TMP12 to settle to the desired accuracy isdependent on the package selected, the thermal contact establishedin the particular application, and the equivalent thermal conductiv-ity of the heat source. For most applications, the settling time isprobably best determined empirically.

Switching Loads with the Open-Collector OutputsIn many temperature sensing and control applications, sometype of switching is required. Whether it be to turn on a heaterwhen the temperature goes below a minimum value or to turnoff a motor that is overheating, the open-collector outputs canbe used. For the majority of applications, the switches used needto handle large currents on the order of 1 A and above. Because theTMP12 is accurately measuring temperature, the open-collectoroutputs should handle less than 20 mA of current to minimizeself-heating. Clearly, the trip point outputs should not drive theequipment directly. Instead, an external switching device isrequired to handle the large currents. Some examples of theseare relays, power MOSFETs, thyristors, IGBTs, and Darlingtontransistors.

This section shows a variety of circuits where the TMP12 con-trols a switch. The main consideration in these circuits, such asthe relay in Figure 10, is the current required to activate the switch.

NC

12V

NC = NO CONNECT

R1

R2

R3

IN4001OR EQUIV

140

MOTORSHUTDOWN

2604-12-311COTO

12V

8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

Figure 10. Reed Relay Drive

It is important to check the particular relay you choose to ensurethat the current needed to activate the coil does not exceed theTMP12’s recommended output current of 20 mA. This is easilydetermined by dividing the relay coil voltage by the specifiedcoil resistance. Keep in mind that the inductance of the relaywill create large voltage spikes that can damage the TMP12output unless protected by a commutation diode across the coil,as shown. The relay shown has a contact rating of 10 W maxi-mum. If a relay capable of handling more power is desired, thelarger contacts will probably require a commensurably largercoil, with lower coil resistance and thus higher trigger current.As the contact power handling capability increases, so does the

OP193

VPTAT

200kTMP12

VREF

SETHIGH

SETLOW

GND

V+

OVER

UNDER

HEATER

1 8

2 7

3 6

4 5

5V

NC

5V

NC = NO CONNECT

NC

5V300k

130k

1F

–1.5V

10k

0.1F

Figure 9. An Analog Measurement Circuit for VPTAT

OBSOLETE

REV. B

TMP12

–10–

current needed for the coil. In some cases, an external drivingtransistor should be used to remove the current load on theTMP12 as explained in the next section.

Power FETs are popular for handling a variety of high currentdc loads. Figure 11 shows the TMP12 driving a P-channelMOSFET transistor for a simple heater circuit. When the outputtransistor turns on, the gate of the MOSFET is pulled down toapproximately 0.6 V, turning it on. For most MOSFETs, agate-to-source voltage or Vgs on the order of –2 V to –5 V issufficient to turn the device on. Figure 12 shows a similar circuitfor turning on an N-channel MOSFET, except that now thegate-to-source voltage is positive. For this reason, an externaltransistor must be used as an inverter so that the MOSFET willturn on when the trip point pulls down.

NC

NC = NO CONNECT

5V

2.4k (12V)1.2k (6V)5%

HEATINGELEMENT

IRFR9024OR EQUIV

+

V+8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

Figure 11. Driving a P-Channel MOSFET

NC

NC = NO CONNECT

5V

HEATINGELEMENT

4.7k 4.7k

2N1711

IRF130

V+8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

Figure 12. Driving an N-Channel MOSFET

Isolated gate bipolar transistors (IGBTs) combine many of thebenefits of power MOSFETs with bipolar transistors and areused for a variety of high power applications. Because IGBTshave a gate similar to MOSFETs, turning the devices on and offis relatively simple as shown in Figure 13. The turn-on voltagefor the IGBT shown (IRGBC40S) is between 3 V and 5.5 V.This part has a continuous collector current rating of 50 A and amaximum collector to emitter voltage of 600 V, enabling it towork in very demanding applications.

NC

NC = NO CONNECT

5V

4.7k 4.7k

2N1711

IRGBC40S

MOTORCONTROL

V+8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

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 alternativeto relays for switching ac line voltages. The 2N6073A shownin Figure 14 is rated to handle 4 A (rms). The opto-isolatedMOC3021 Triac shown features excellent electrical isolationfrom the noisy ac line and complete control over the high powerTriac with only a few additional components.

NC = NO CONNECT

NC

5V

300

V+ = 5V

MOC3021

150

LOADAC

2N6073A

8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

Figure 14. Controlling the 2N6073A Triac

High Current SwitchingAs mentioned earlier, internal dissipation due to large loads onthe TMP12 outputs will cause some temperature error due toself-heating. External transistors buffer the load from the TMP12so that virtually no power is dissipated in the internal transistorsand minimal self-heating occurs. This section shows severalexamples using external transistors. The simplest case uses asingle transistor on the output to invert the output signal as shownin Figure 15. When the open-collector of the TMP12 turns on andpulls the output down, the external transistor Q1’s base will bepulled low, turning off the transistor. Another transistor can beadded to reinvert the signal as shown in Figure 16. When theoutput of the TMP12 is pulled down, the first transistor, Q1,turns off and its collector goes high, which turns Q2 on, pullingits collector low. Thus, the output taken from the collector of Q2is identical to the output of the TMP12. By picking a transis-tor that can accommodate large amounts of current, manyhigh-power devices can be switched.

OBSOLETE

REV. B

TMP12

–11–

An example of a higher power transistor is a standard Darlingtonconfiguration as shown in Figure 17. The part chosen, TIP-110,can handle 2 A continuous, which is more than enough to con-trol many high-power relays. In fact, the Darlington itself can beused as the switch, similar to MOSFETs and IGBTs.

8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

4.7k

2N1711

V+

IC

Q1

Figure 15. An External Transistor Minimizes Self-Heating

8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

4.7k

2N1711

V+IC

Q1

4.7k

2N1711

Q2

Figure 16. Second Transistor Maintains Polarity of TMP12Output

8

7

6

5

1

2

3

4

TEMPERATURESENSOR AND

VOLTAGEREFERENCE

VREF

WINDOWCOMPARATOR

HYSTERESISGENERATOR

100

TMP12

VPTAT

4.7k

2N1711

V+IC

4.7kTIP-110

5V

MOTORSHUTDOWN

RELAY12V

Figure 17. Darlington Transistor Can Handle Large Currents

OBSOLETE

REV. B–12–

C00

335–

0–12

/03(

B)

TMP12OUTLINE DIMENSIONS

8-Lead Standard Small Outline Package [SOIC]Narrow Body

(R-8)Dimensions shown in millimeters and (inches)

0.25 (0.0098)0.17 (0.0067)

1.27 (0.0500)0.40 (0.0157)

0.50 (0.0196)0.25 (0.0099)

45

80

1.75 (0.0688)1.35 (0.0532)

SEATINGPLANE

0.25 (0.0098)0.10 (0.0040)

8 5

41

5.00 (0.1968)4.80 (0.1890)

4.00 (0.1574)3.80 (0.1497)

1.27 (0.0500)BSC

6.20 (0.2440)5.80 (0.2284)

0.51 (0.0201)0.31 (0.0122)COPLANARITY

0.10

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FORREFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

COMPLIANT TO JEDEC STANDARDS MS-012AA

Revision HistoryLocation Page

12/03—Data Sheet changed from REV. A to REV. B.

Edits to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Edits to TYPICAL PERFORMANCE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Updated OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Edits to ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Edits to TYPICAL PERFORMANCE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Edits to OUTLINE DIMENSIONS Figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12OBSOLETE