Design Feature: August 15, 1996

In the collective unconscious of component engineers, a truly rugged system contains stable, well-behaved components and devices that operate at comfortable, benign temperatures. Contrast this system with its real-world equivalent. Parameter shifts and other unwelcome influences can cause temperatures to rise to dangerous levels. Consider, for example, a Class-AB power-output stage, which normally operates with a cool and moderate quiescent current. Parameter shifts in the stage's bias network can transmogrify the totem-pole structure into a current-gobbling Class-A stage. If the structure is bipolar, the specters of thermal runaway, smoke, and flames could develop. Temperature-sensing or -switching functions in a number of ICs can help prevent such scenarios either by monitoring the stage temperature or by initiating preventive or corrective action.

System designers might define "rugged" as "immune to catastrophic failure." Temperature-sensor and -switch ICs can help a system live up to this definition in one of three ways. First, the devices can provide a constant visual indication of the temperatures in crucial parts of the system and give early warning of developing problems. A temperature-switch IC can also shut down faulty circuitry before a condition becomes catastrophic. Finally, a temperature sensor or switch can provide feedback information in a closed-loop system to remedy the temperature-rise problem without shutting down any circuitry.

If you're a free-spirited designer who considers buying packaged functions a cop-out, you could always design your own temperature-sensing circuitry. A forward-biased diode junction, for example, has a fairly predictable TC of approximately 2 mV/°C; however, diodes exhibit much device-to-device variation. Thermistors are another option, but you need either linearizing circuitry or a software look-up table to straighten their bow-shaped curves. Platinum resistance-temperature detectors (RTDs) are linear, but you need a lot of amplifying circuitry. Thermocouples work well, but keeping that cold junction in a bucket of ice is annoying. If you don't want to spend a lot of time designing, debugging, tweaking, and calibrating, you should consider using one of the temperature-management ICs discussed here.

Table 1 lists temperature-management ICs, ranging from purely analog, voltage-vs-temperature devices to mixed-signal VLSI chips containing logic and ADCs. Most of the ICs rely on a bandgap reference with a known TC to provide temperature information. If you need only to sense temperature and provide an analog readout, the simplest and least expensive devices are adequate. If you need to monitor more than temperature or if you want to effect a direct interface to a digital system, more complex ICs are more appropriate.

A couple of "thermometer" ICs from National Semiconductor provide an analog voltage output that's a linear function of temperature in degrees Celsius over the range -40 to +125°C. The LM50 operates from a 4.5 to 10V single supply and consumes only 130-µA quiescent current. This low consumption produces only a 0.2°C rise from self-heating. The LM60 uses a 2.7 to 10V supply and draws only 110 µA for a 0.1°C self-heating temperature rise. Both parts have a dc offset voltage to provide for negative temperatures. Their respective outputs in millivolts are 10T+500 and 6.25T+424, where T is temperature in degrees Celsius.

Figure 1 shows a typical application of the National thermometer ICs. The circuit is essentially a thermostat, with high and low trip points that you can set by choosing appropriate resistor values. Positive feedback is beneficial in this configuration. Dispensing with the R₃-R₂ positive-feedback network configures the circuit for only one trip point. In this case, the comparator "chatters" if the temperature hovers at the trip value. The positive feedback, usually responsible for causing instability, stops system uncertainty in this case. Both the LM50 and LM60 come in a tiny, three-pin SOT-23 case, shipped as 250 or 3000 units in tape-and-reel format.

A family of linear temperature sensors from Analog Devices has programmable power shutdown for power conservation in battery-operated systems. The TMP35/36/37 operate from 2.7 to 5.5V supplies and drain only 50 µA in operating mode and 0.5 µA in shutdown mode. The three devices produce an analog output, which is linearly proportional to temperature in degrees Celsius. Their respective operating-temperature ranges are 10 to 125°C, -40 to +125°C, and 5 to 100°C.

Another class of nondigital temperature-management ICs includes temperature switches, or thermostats. With these devices, you set either one or two temperature trip points. The one-point ICs usually contain a certain amount of built-in hysteresis to prevent chattering at the trip point. In the two-point units, you can set both the high and low temperature thresholds. The TC622 and 624 temperature switches from Telcom Semiconductor are members of a single-trip-point family. The devices provide a logic-one (and its complement) output when the temperature exceeds the setpoint you select with an external resistor. Figure 2 shows a typical output-vs-temperature characteristic. The devices have 2°C of temperature hysteresis to prevent chattering. The TC622 operates from a 4.5 to 18V supply, and the TC624 operates from 2.7 to 4.5V.

One way that you could use the output signal from the TC622 or the 624 is in a closed-loop system to turn a fan on and off. However, the 2°C hysteresis might prove inadequate in such a system. For a closed-loop system, a temperature switch that allows you to program two setpoints is better. The TC623 from Telcom Semiconductor provides two resistor-programmable setpoints and a control latch.Figure 3 shows a typical output-vs-temperature characteristic. Both the high and low setpoints have 2°C built-in hysteresis to prevent chattering. The control output goes high with the high-limit output and goes low with the low-limit output. For the setpoints of the TC622, 624, and 623, the external trip resistor is 0.5997T^2.1312, where T is the desired trip temperature in degrees Kelvin.

Two temperature setpoints also come with National's LM55/65 and LM56/66 low-power thermostats. The LM55 and LM56 have one output that goes low when the temperature reaches the high setpoint and goes high when the temperature reaches the low setpoint. The LM56 and 66 have two digital outputs, corresponding to each setpoint. The LM55 and LM65 offer resistor-programmable setpoints; the LM56 and LM66 are factory-programmed versions. Figure 4 shows an application using the LM56 in a closed-loop cooling system. When the temperature rises above the low-threshold preset value, the fan turns on. If the temperature exceeds the high-threshold preset value, the system clock slows by a factor of two.

Another recent thermostatic IC is the TMP12 airflow and temperature sensor from Analog Devices. This device provides a linear output proportional to temperature over a -40 to +100°C range and includes an internal 100(ohm) heater resistor to provide power-IC emulation. You can use resistors to program its temperature setpoints; in addition, the thermal hysteresis at the setpoints is programmable. The AD22105 is also a thermostatic IC, with one resistor-programmable setpoint that has 4°C preset hysteresis.

Unitrode's UC3730, a thermal-monitor IC, is another nondigital device. The UC3730 contains a temperature transducer, a precision reference, a temperature comparator, and an alarm buffer. The outputs include an analog voltage proportional to the die temperature, an alarm-delay output that activates when temperature exceeds the set threshold, and another alarm output whose activation you can delay by using an RC network at the alarm-delay output. A reference output can supply currents as high as 250 mA. By setting this reference-output current, you can set the die temperature to a desired value and use the IC as an airflow sensor.

Digital sensors abound

The TMP03 (open-collector) and TMP04 (CMOS/TTL-compatible) digital thermometers from Analog Devices use mark-space ratio modulation to provide an output signal a microprocessor can directly read. Figure 5 shows a block diagram and a typical output waveform. The output is a 35-Hz square wave with a duty cycle that is a function of temperature. Temperature in degrees Celsius is 235-400t_H/t_L, where t_H and t_L are the high and low times, respectively, of the square wave. The resolution of the temperature reading is a function of the size of the counter you use to measure the high and low periods. A 12-bit counter operating at 94 kHz yields a 0.284°C resolution; a 14-bit counter operating at 376 kHz gives a 0.071°C resolution.

The TMP03 and TMP04 undergo laser-trimming for accuracy and linearity. However, you can further improve performance by performing additional calibration. To achieve a single-point, room-temperature calibration, measure the IC's output and the temperature. You then modify the offset constant (235 in the above expression) by an amount equal to the actual temperature minus the IC-indicated temperature. A more complicated two-point calibration, in which you measure the output at two temperatures, allows you to also tweak the slope term (400 in the above expression).

The other temperature-management ICs offer true, serial digital outputs. National's LM75, for example, uses a 9-bit delta-sigma ADC to digitize the output of a bandgap temperature sensor. The I²C-compatible device contains registers with which the host system can program both a temperature-alarm threshold and a hysteresis level below that threshold. Default settings for those thresholds are 80 and 75°C, respectively. The overtemperature-shutdown output of the LM75 is normally active low; you can program the output for the opposite polarity. Figure 6 shows a simple fan-control application in default mode without the I²C interface. With the interface, you could change the temperature thresholds and eliminate the npn transistor by programming the output for active high.

Dallas Semiconductor's DS1621 is similar to National's LM75; the DS1621 also provides 9-bit temperature data and user-settable thermostatic setpoints. In the Dallas device, user settings are nonvolatile, so you can program the devices before their insertion into a system. You can obtain resolution higher than 9 bits by performing some coding tricks described in the data sheet. The company also offers a two-wire temperature sensor with 256 bytes of user-programmable EEPROM. The DS1624 has a command structure similar to that of the DS1621, so upgrading a design to accommodate temperature-compensation information, for example, is easy. Dallas has also upgraded its older DS1620 digital thermometer and thermostat to operate from 2.7 to 5.5V supplies.

National's LM78 universal watchdog chip contains an 8-bit delta-sigma ADC, a seven-input multiplexer, a bandgap temperature sensor, and two amplifiers to accommodate negative inputs. The LM78 also has three tachometer inputs with counters to determine fan speed. One way that you can use the multiplexer inputs is to monitor supplies of 12, 5, 3.3, 2.5, -5, and -12V. Through an ISA or I2C interface, you can program the chip to issue an alarm when any voltage, fan speed, or temperature goes beyond a preset limit.

Linear Technology's LTC1392 is also a watchdog chip. The device is a 10-bit ADC that monitors power supply, temperature, and differential input voltage. For measuring temperature, the output code of the ADC is linearly proportional to the temperature in degrees Celsius. The ability to measure rail-to-rail differential voltages is handy for measuring current. Figure 7 shows a typical application in which the LTC1392 provides measurements of temperature, the 5V supply voltage, and the load current to the µC.

Watchdog circuits, such as the LM78 and LTC1392, contribute to a system's robustness by alerting the user to potential problems. A power-supply line could be 30% high, for example, without provoking immediate breakdown. Without a monitoring function for voltage or temperature, this overvoltage could cause overheating, which, in turn, could cause a catastrophic failure. With a watchdog monitor, the user immediately notices both the overvoltage condition and the high temperature before a disaster could occur.

One of the more unusual temperature-management ICs is Dallas Semiconductor's DS1820 one-wire digital thermometer. This device is a multidrop temperature sensor with a 9-bit serial digital output. The DS1820 can derive power either from the data line in parasitic mode or from a separate supply pin. For multidrop applications, each DS1820 has a 48-bit silicon serial number stored in on-chip ROM. The device can work in conjunction with a DS2407 sensor/actuator chip.

Figure 8 shows the block diagram of a temperature-control demonstration board available from Dallas Semiconductor. The DS1820 supplies temperature information to the computer; you click on an arrow to set the trip points for a cooler (green LED) or a heater (red LED). The piezo element on the DS2407 senses motion of the demonstration board. Note that all the Dallas devices in this circuit operate with parasitic power from the data line.

Dallas Semiconductor complements the DS1820 thermometer with the DS1821 programmable digital thermostat. You can program high- and low-temperature setpoints in nonvolatile memory to achieve any desired value of hysteresis. Similar to the DS1820, the DS1821 uses one-wire (the same wire used for programming) communications to and from the data line. Unlike the DS1820, the DS1821 is not a multidrop device. The DS1821 one-wire bus is a system that has a single bus master and one slave; the DS1821 behaves as a slave.

Modern temperature-management ICs make designing rugged systems easy. The devices can provide early warning of impending problems. Their flexible programming functions allow you to preset limits for sounding alarms or shutting down the system. The circuits with digital outputs are convenient for computer-controlled, closed-loop cooling systems.

You can reach Senior Technical Editor Bill Travis at (617) 558-4471, fax (617) 558-4470, e-mail b.travis@cahners.com