28.Bru¨ ck K. Heat production and temperature regulation. In: Stave U, editor. Pernatal Physiology. New York: Plenum Press; 1978. p 474.

29.Day RL. Respiratory metabolism in infancy and childhood. Am J Child 1943;65:376.

30.Hey EN, Katz G. Evaporative water loss in the newborn baby. J Physiol (London) 1969;200:605.

31.Sulyok E, Je´quier E, Ryser G. Effect of relative humidity on thermal balance of the newborn infant. Biol Neonate 1972; 21:210.

32.Belgaumkar TR, Scott KE. Effects of low humidity on small premature infants in servo control incubators. Biol Neonate 1975;26:337.

¨

33. Hammarlund K, Nilsson GE, Oberg PA, Sedin G. Transepidermal water loss in newborn infants. Acta Paediatr Scand 1977;66:553.

¨

34. Hammarlund K, Nilsson GE, Oberg PA, Sedin G. Transepidermal water loss in newborn infants: Evaporation from the skin and heat exchange during the first hours of life. Acta Paediatr Scan 1980;69:385.

35. Hey EN, Mount LE. Heat losses from babies in incubators. Arch Dis Child 1967;42:75.

36. Bru¨ ck K. Temperature regulation in the newborn infant. Biol Neonate 1961;3:65.

37. Grausz JP. The effects of environmental temperature changes on the metabolic rate of newborn babies. Acta Paediatr. Scand 1968;57:98.

38. Mesta´n J, Jrai I, Bata G, Fekete M. The significance of facial skin temperature in the chemical heat regulation of premature infants. Biol Neonate 1964;7:243.

39. Agate FJ, Silverman WA. The control of body temperature in the small newborn infant by low-energy infra-red radiation. Pediatrics 1963;37:725.

40. Perlstein PH, Edwards NK, Sutherland JM. Apnea in premature infants and incubator air temperature changes. N Engl J Med 1970;282:461.

41. Silverman WA. The lesson of retrolental fibroplasias. Sci Am 1977;236:100.

42. Schaal B, Hummel T, Soussignan R. Olfaction in the fetal and premature infant: Functional status and clinical implications. Clin Perinatol. 2004;31:261.

43. Morris BH, Philbin MK, Bose C. Physiologic effects of sound on the newborn. J Perinatol 2000;20:S55.

44. Robertson A, Stuart A, Walker L. Transmission loss of sound into incubators: implications for voice perception by infants. J Perinatol 2001;21:236.

45. Johnson AN. Neonatal response to control of noise inside the incubator. Pediatr Nurse 2001;27:600.

46. Bellini CV, et al., Use of sound-absorbing panel to reduce noisy incubator reverberating effects. Biol Neonate 2003;84:293.

47. Hellstrom-Westas L, et al., Short-term effects of incubator covers on quiet sleep in stable premature infants. Acta Paediatr 2001;90:1004.

48. Rivkees SA. Emergence and influences of circadian rhythmicity in infants. Clin Perinatol 2004;31:217.

49. Bellieni CV, et al. Reduction of exposure of newborns and caregivers to very high electromagnetic fields produced by incubators. Med Phys 2005;32:149.

50. Bellieni CV, et al. Increasing the engine-mattress distance in neonatal incubators: A way to decrease exposure of infants to electromagnetic fields. Ital J Pediatr 2005;29:74.

51. Blumi S, et al. MR imaging of newborns by using an MRcompatible incubator with integrated radiofrequency coils: Initial experience. Radiology, 2004;231:594.

52. Whitby EH, et al. Ultrafast magnetic resonance imaging of the neonate in a magnetic resonance-compatible incubator with a built-in coil. Pediatrics 2004;113:e150.

INTEGRATED CIRCUIT TEMPERATURE SENSOR

157

Further Reading

Adamsons K. The role of thermal factors in fetal and neonatal life. Pediatr Clin North Am 1966;13:599.

Ahlgren EW. Environmental control of the neonate receiving intensive care. Int Anesthesiol Clin 1974;12:173.

Bru¨ ck K. Heat production and temperature regulation. In: Stave U, editor. Perinatal Physiology New York: Plenum Press; 1978 p 455.

Dawes GS, Oxygen consumption and temperature regulation in the newborn. I Foetal and Neonatal Physiology. Chicago: Year Book Medical Publisher; 1968. p 191.

Delue NA. Climate and environment concepts. Clin Perinatal 1976;3:425.

Hey EN, Katz G. The optimum thermal environment for naked babies. Arch Dis Child 1970;45:328.

Holman JP. Heat Transfer, New York: McGraw-Hill; 1981. Klaus M, Fanaroff A, Martin RJ. The physical environment. In:

Klaus MH, Fanaroff AA, editors. Care of the High Risk Neonate. Philadelphia: Saunders; 1979. p 94.

Lutz L, Perlstein PH. Temperature control in newborn babies. Nurs Clin North Am 1971;6:15.

Mayr O. The Origins of Feedback Control. Cambridge, (MA): MIT Press; 1970.

Ogata K. Modern Control Engineering, Englewood Cliffs (NJ): Prentice-Hall; 1970.

Oliver TK. Temperature regulation and heat production in the newborn. Pediatr Clin North Am 1965;12:765.

Oppenheim AV, Willsky A, Young IT. Signals and Systems, Englewood Cliffs (NJ): Prentice-Hall; 1983.

Perstein PH. Thermal regulation. In: Fanaroff AA, Martin RJ, editors. Behrman’s Neonatal-Perinatal Medicine, 3rd ed. St. Louis (MO): Mosby; 1983. p 259–277.

Scopes JW. Thermoregulation in the newborn. In: Avery GB, editors. Neonatology, Philadelphia: Lippincott; 1975. p 99.

Sinclair JC. The effect of the thermal environment on neonatal mortality and morbidity. In: Adamson K, Fox HA, editors. Preventability of Perinatal Injury. New York: Alan R. Liss; 1975. p 147.

Sinclair JC. Metabolic rate and temperature control. In: Smith CA, Nelson NM, editors. The Physiology of the Newborn Infant. Springfield (IL) : Thomas; 1976. p 354.

Todd JP, Ellis HB. Applied Heat Transfer. New York: Harper & Row; 1982.

See also BIOHEAT TRANSFER; NEONATAL MONITORING; TEMPERATURE MONITORING.

INFANT INCUBATORS. See INCUBATORS, INFANT.

INFORMATION SYSTEMS FOR

RADIOLOGY. See RADIOLOGY INFORMATION SYSTEMS.

INFUSION PUMPS. See DRUG INFUSION SYSTEMS.

INTEGRATED CIRCUIT TEMPERATURE SENSOR

TATSUO TOGAWA

Waseda University

Saitama, Japan

INTRODUCTION

Temperature can affect the electronic characteristics of semiconductor devices. Although this is a disadvantage

158 INTEGRATED CIRCUIT TEMPERATURE SENSOR

in many applications, especially for analogue devices, it may be turned into an advantage if such a device is used as a temperature sensor. In principle, any parameter in such a device having a temperature coefficient can be used for temperature measurement. For example, a temperature telemetry capsule, in which a blocking oscillator frequency varies with temperature, has been developed for measuring gastrointestinal temperature (1). In this system, the temperature affects the reverse-bias base-collector current, which determines the period of relaxation oscillation. However, it has been shown that the voltage across a p–n junction of a diode or transistor under a constant forwardbias current shows excellent linear temperature dependency over a wide temperature range. Many conventional or specially designed diodes or transistors composed of Ge, Si, or GaAs have been studied for use as thermometers (2–4).

The advantages of diodes and transistors as temperature sensors are their high sensitivity and low nonlinearity. The temperature sensitivity under normal operation is ca 2 mV/K, which is 50 times higher than that of a copper-constantan thermocouple. The nonlinearity is low enough for many applications, although its value depends on the structure and material of the device. It is known that a Schottky diode, which has a structure composed of a rectifying metal-semiconductor contact, possesses good voltage–temperature linearity (5). Some transistors used as two-terminal devices by connecting the base to the collector also possess good linearity (6,7), and a transistor that has been especially developed for temperature sensing is commercially available (8). This has a linearity that is comparable to that of a platinum-resistance temperature sensor.

It is advantageous to have a diode and a transistor temperature sensor fabricated on a chip with associated interfacing electronics using integrated circuit (IC) technology. Several integrated temperature sensors that provide either analogue or digital outputs have been developed and are commercially available. A diode or transistor temperature sensor fabricated on a central processing unit (CPU) chip is especially useful when used to monitor the temperature of the chip. Such a sensor has been used to detect overheating, and to protect the CPU system by controlling a fan used to cool the chip or to slow down the clock frequency.

THEORY

The characteristics of p–n junctions are well known (9,10). In p–n junction diodes, the current flowing through the forward-biased junction is given by

I ¼ IsðeqV=mkT 1Þ

ð1Þ

where Is is the saturation current, q is the electron charge, V is the voltage across the junction, k is the Boltzmann constant, m is the ideality factor having a value between 1 and 2, which is related to the dominant current component under the operating conditions used, and T is the absolute temperature. At a temperature close to room temperature, and when the current is relatively high, so that the current

due to the diffusion of the carrier dominates, m ¼ 1, and so the second term in Eq. 1 given in parentheses can be neglected. Equation 1 can then be simplified to

I ¼ IseqV=kT

ð2Þ

The temperature dependence of the saturation current, Is, is given by

Is ¼ Ae Eg =kT

ð3Þ

where Eg is the bandgap energy at T ¼ 0 K, and A is a constant dependent on the geometry and material of the device. Strictly speaking, A also depends on the temperature. However, the temperature dependency is very weak compared to the exponential term in Eq. 3. Thus,

I ¼ AeðqV Eg Þ=kT

ð4Þ

For a constant current, I, (qV–Eg)/kT is constant. Thus, the voltage across a p–n junction, V, is a linear function of the absolute temperature, T. On extrapolating to T ¼ 0, then

qV ¼ Eg.

The temperature coefficient of V can be derived from Eq. 4 as

dV		¼	V		E =q
dT	I		const ¼		T g		ð5Þ

Since the value of qV–Eg is always negative, V decreases with increasing T. For silicon, Eg 1.17 eV, and for T 300 K, V 600 mV, and dV/dT 1.9 mV/K. In actual diodes, the current–voltage characteristics have been studied in detail over a wide temperature range. The forward voltage exhibits a linear dependence for T > 40 K for a constant current (11). The observed value of dV/dT in a typical small signal silicon p–n junction diode ranges between 1.3 and2.4 mV/K for I ¼ 100 mA (11). In germanium and gallium arsenide p–n junction diodes, and for silicon Schottky diodes, the forward voltage exhibits a similar sensitivity (3–5).

In most p–n junctions, the current through the junction contains components other than those due to carrier diffusion, and therefore, Eq. 4 does not hold. The base-emitter p–n junction in transistors is advantageous in this respect. Here, the diffusion component forms a larger fraction of the total current than that in diodes, even for a diode connection in which the base is connected to the collector. The nonlinear temperature dependence in the forward voltage in diode-connected transistors is lower than that of most diodes (7). Further improvement in linearity is attained under constant collector current operation, since only the diffusion component flows to the collector, while other components flow to the base (12).

From Eq. 2, one can obtain the following expression

ln I ¼ ln Is þ qV=kT

ð6Þ

The value of T can be obtained from the gradient of a plot of ln I versus V, as q and k are known universal constants. This implies that the current–voltage characteristics can be used as an absolute thermometer (6). If ln I is a linear function of V, only two measurements are required to determine the gradient. If V1 and V2 are voltages corre-

sponding to different current levels, I1 and I2, the difference between these two voltages is calculated using

V1 V2 ¼ ðkT=qÞlnðI1=I2Þ

ð7Þ

Thus, the difference in voltage corresponding to the different current levels for a constant ratio is proportional to the absolute temperature, without any offset. Using this relationship, a thermometer providing an output proportional to the absolute temperature can be realized, either by applying a square wave current to a p–n junction (12), or by using two matched devices operating at different current levels (13).

FUNDAMENTAL CIRCUITS AND DEVICES

A schematic drawing of the fundamental circuit of the thermometer with a short-circuited transistor or a diode is shown in Fig. 1. A constant current is applied to the transistor or diode in the forward bias direction, and the voltage across the junction is amplified using a differential amplifier. By adjusting the reference voltage applied to another input of the differential amplifier, an output voltage proportional to either the absolute temperature in kelvin or in degrees Celsius or any other desired scale can be obtained. The operating current of small signal diodes and transistors is typically 40–100 A. If the current becomes too high, a self-heating error may be produced due to the power dissipated in the junction. If the current becomes too small, problems due to leakage and the input current of the first stage amplifier may become significant

(7).

The nonlinearity in the temperature dependency of the forward voltage is not a serious problem for most applications, and it can be reduced by appropriate circuit design. In a Schottky diode, this nonlinearity is < 0.1 K over the

Current

source

	–
I	+	Output
Sensor	Reference
transistor
	voltage
or diode
	input

Figure 1. A fundamental interfacing circuit of a thermometer making use of a transistor or a diode as a temperature sensor to provide a voltage output proportional to temperature, with a zero voltage output at a specific temperature dependent on the reference voltage selected.

INTEGRATED CIRCUIT TEMPERATURE SENSOR

159

Figure 2. A circuit for constant collector current operation in a sensor transistor.

temperature range 65 to 50 8C (5), and a comparable performance is expected for diode-connected silicon transistors (7). Further improvement in the linearity can be attained by linearization of the circuit. Linearization using a logarithmic ratio module reduces the error to <0.05 8C in the temperature range 65 to 100 8C (7). Linearity is also improved using a constant collector current, as pointed out previously. An example of an actual circuit is shown in Fig. 2. In this circuit, the operational amplifier drives the base-emitter voltage to maintain a constant collector current. By applying a square-wave current and measuring the amplitude of the resulting square-wave base-emitter voltage, a linear output proportional to the absolute temperature is obtained, as expected from Eq. 7(12). Further improvement in accuracy can be attained by employing a curve fitting with three-point calibration, the error due to the nonlinearity can be reduced to 0.01 8C in the temperature range of 50 to 125 8C (14).

Three-terminal monolithic IC temperature sensors that provide a voltage output proportional to temperature using the Celsius scale are commercially available, examples being LM45 (National Semiconductor) and AD22100/ 22103 (Analog Devices). The LM45 device operates using a single power supply voltage in the range 4–10 V, and provides a voltage output that corresponds to the temperature in degrees Celsius multiplied by a factor of 10 mV, for example, 250 mV ¼ 25 8C. The AD22100 and AD22103 devices provide a ratiometric output, that is, the output voltage is proportional to the temperature multiplied by the power supply voltage. For example, AD22100 has a sensitivity of 22.5 mV/ 8C giving output voltages of 0.25 V at 50 8C and 4.75 V at 150 8C when the power supply voltage is 5.0 V.

Two matched transistors operated using different collector currents can be used to obtain an output proportional to the absolute temperature (15). The difference in the base-emitter voltages of the two transistors is a linear function of temperature, as shown in Eq. 7. Convenient two-terminal current-output devices using this technique are commercially available. Figure 3 shows an idealized scheme representing such devices. If the transistors Q1 and Q2 are assumed to be identical and have a high commonemitter current gain, their collector currents will be equal, and will constrain the collector currents Q3 and Q4. If Q3 has r-fold base-emitter junctions, and each one is identical

160 INTEGRATED CIRCUIT TEMPERATURE SENSOR

Figure 3. An idealized scheme of a two-terminal IC temperature sensor that provides a current output proportional to the absolute temperature.

to that of Q4, the emitter current of a junction in Q3 is 1/r that of Q4. From Eq. 7, the voltage across resistance R is obtained from

RI ¼ ðkT=qÞln r

ð8Þ

Thus, the total current, 2I, is proportional to the absolute temperature. Although the actual components are not ideal, practical devices are available as monolithic ICs, such as AD590 and AD592 (Analog Devices) (16). In these devices, r ¼ 8 and R is trimmed to have a sensitivity of about 1 A/K. The output current is unchanged in the sup- ply-voltage range 4.0 to 30 V. A voltage output proportional to the absolute temperature can be obtained by connecting a resistor in series with the ICs. For example, a sensitivity of 1 mV/K is obtained by connecting 1 kV resistor in series. By trimming the series resistor, the error in temperature reading can be adjusted to zero at any desired temperature. After trimming, the maximum error depends on the range in temperature under consideration. For example, a maximum error of <0.1, 0.2, and 0.3 8C is obtained for temperature ranges of 10, 25, and 50 8C, respectively (17).

Monolithic temperature sensors that provide a digital output are also commercially available. For example, TMP06 (Analog Devices) sensors provide a pulse-width modulated output. The output voltage assumes either a high or low level, so that the high period (T1) remains constant at 40 ms for all temperatures, while the low period (T2) varies with temperature. In the normal operation mode, the temperature on the Celsius scale, T, is given by

T ¼ 406 ½731 ðT1=T2Þ&

ð9Þ

According to Analog Devices’ TMP06 data sheet, for an operating supply voltage between 2.7 and 5.5 V, the absolute temperature accuracy is 1 8C in the temperature range 0–70 8C, with a temperature resolution of 0.02 8C.

The National Semiconductor LM75 device is also a monolithic temperature sensor that provides a digital output. It includes a nine-bit analog-to-digital converter, and provides a serial output in binary format so that the least significant bit corresponds to a temperature difference of 0.5 8C.

Newer devices will come along in the future that may be more appropriate than the ones mentioned here. Information about such devices, together with their data sheets, will be available from the internet sites of manufactures.

APPLICATIONS

Although thermistors are still widely used for thermometry in the medical field, IC temperature sensors have potential advantages over thermistors. Integrated circuit sensors can be fabricated using IC technology encompassing interfacing electronics on a single IC chip, and many general purpose IC temperature sensors are now commercially available.

Current-output-type IC temperature sensors, such as AD590, are convenient for use as thermometer probes for body core and skin temperature measurements. Figure 4 shows a scheme for such a simple thermometer. According to the manufacturer’s data sheet, although the sensitivity and zero offset are adjustable independently in this circuit, an accuracy of 0.1 8C is attainable with L- or M-grade AD590 devices using a single-trim calibration if the temperature span is 10 8C or less. If a regulating resistor is included in the probe, interchangeability can be realized. Because of the current output capacity, the resistance of

Figure 4. A simple thermometer that makes use of a twoterminal current output-type IC temperature sensor.

Figure 5. A multiplexing scheme for a current output-type IC temperature sensor.

the cable or connector does not affect the temperature measurement.

This type of device is also convenient for temperature measurements at many other points, especially when the output data are processed using a PC. All the sensors can be connected to a single resistor, as shown in Fig. 5, and by switching the excitation the outputs from each sensor can be multiplexed. To calibrate each sensor individually, all the sensors are maintained at an appropriate temperature, together with a standard thermometer. The outputs from each sensor as well as that from a standard thermometer are input into a PC. Then, the temperature offsets for each sensor can be stored, and all the measurement data can be corrected using these correction factors. Two-point calibration is also realized by using data at two known temperatures. A matrix arrangement of the sensors can be formed using two decoder drivers.

VDD

INTEGRATED CIRCUIT TEMPERATURE SENSOR

161

Temperature measurements at many different points can be performed easier using IC temperature sensors that generate serial digital outputs, such as TMP05/TMP06. Connecting these devices as shown in Fig. 6 allows for the realization of a daisy chain operation. When a start pulse is applied to the input of the first sensor, the temperature data from all the sensors is generated serially, so that the temperatures of each sensor are represented in a ratiometric form, which is the ratio of the duration of the high and low output levels for each period. It is a remarkable advantage of this sensor that a thermometer can be realized without using any analogue parts.

An important application of IC temperature sensors is the monitoring of CPU temperatures to protect a system from overheating. The temperature of a CPU chip can be detected by a p–n junction fabricated on the same silicon chip as the CPU, as shown in Fig. 7. The advantage of fabricating the temperature sensor on the CPU chip is to make the temperature measurement accurate enough and to minimize the time delay due to heat conduction so as to prevent overheating. The CPU can be protected from overheating by controlling a cooling fan or by slowing down the clock speed. Interfacing devices for this purpose are commercially available. For example, the MAX6656 (Dallas Semiconductor) device can detect temperatures at three locations, such as the CPU, the battery, and the circuit board, and the output can be used to control a cooling fan. To control the clock frequency, a specially designed frequency generator can be used. For example, the AV9155 (Integrated Circuit Systems) device allows for a gradual transition between frequencies, so that it obeys the CPU’s cycle-to-cycle timing specifications.

FUTURE

It is 25 years since convenient IC temperature sensors were introduced for scientific and industrial temperature measurements. In medicine, the application of this type of sensor is in its infancy. There are many applications where

TMP05

TMP05

TMP05

Output

Start pulse

#1

#2

#N

GND

(a)

#1

#2

#N

T1 T2

Celsius temperature = 406 – (731 × (T1/T2 ))

(b)

Figure 6. (a) The connecting scheme for a daisy chain operation of a serial-digital- output-type temperature sensor, and (b) the output waveform. The temperature using the Celsius scale at each sensor can be determined from the ratio of the duration of the highest and lowest points in each cycle.