温度传感器毕业论文中英文资料外文翻译文献

中英文资料外文翻译文献

英文文献原文

Temperature Sensor ICs Simplify Designs

When you set out to select a temperature sensor, you are no longer limited to either an analog

output or a digital output device. There is now a broad selection of sensor types, one of which

should match your system's needs.

Until recently, all the temperature sensors on the market provided analog outputs.

Thermistors, RTDs, and thermocouples were followed by another analog-output device, the

silicon temperature sensor. In most applications, unfortunately, these analog-output devices

require a comparator, an ADC, or an amplifier at their output to make them useful.

Thus, when higher levels of integration became feasible, temperature sensors with digital

interfaces became available. These ICs are sold in a variety of forms, from simple devices that

signal when a specific temperature has been exceeded to those that report both remote and local

temperatures while providing warnings at programmed temperature settings. The choice now isn't

simply between analog-output and digital-output sensors; there is a broad range of sensor types

from which to choose.

Classes of Temperature Sensors

Four temperature-sensor types are illustrated in Figure 1. An ideal analog sensor provides an

output voltage that is a perfectly linear function of temperature (A). In the digital I/O class of

sensor (B), temperature data in the form of multiple 1s and 0s are passed to the microcontroller,

often via a serial bus. Along the same bus, data are sent to the temperature sensor from the

microcontroller, usually to set the temperature limit at which the alert pin's digital output will trip.

Alert interrupts the microcontroller when the temperature limit has been exceeded. This type of

device can also provide fan control.

Figure 1. Sensor and IC manufacturers currently offer four classes of temperature sensors.

OUT

versus temperature curve is for an IC whose digital output switches when a specific temperature

has been exceeded. In this case, the

than a comparator and a voltage reference. Other types of

form of the delay time after the part has been strobed, or in the form of the frequency or the period

of a square wave, which will be discussed later.

The system monitor (D) is the most complex IC of the four. In addition to the functions

provided by the digital I/O type, this type of device commonly monitors the system supply

voltages, providing an alarm when voltages rise above or sink below limits set via the I/O bus. Fan

monitoring and/or control is sometimes included in this type of IC. In some cases, this class of

device is used to determine whether or not a fan is working. More complex versions control the

fan as a function of one or more measured temperatures. The system monitor sensor is not

discussed here but is briefly mentioned to give a complete picture of the types of temperature

sensors available.

Analog-Output Temperature Sensors

Thermistors and silicon temperature sensors are widely used forms of analog-output

temperature sensors. Figure 2 clearly shows that when a linear relationship between voltage and

temperature is needed, a silicon temperature sensor is a far better choice than a thermistor. Over a

narrow temperature range, however, thermistors can provide reasonable linearity and good

sensitivity. Many circuits originally constructed with thermistors have over time been updated

using silicon temperature sensors.

Figure 2. The linearity of thermistors and silicon temperature sensors, two popular analog-output

temperature detectors, is contrasted sharply.

Silicon temperature sensors come with different output scales and offsets. Some, for example,

are available with output

transfer

functions that are proportional to K, others to °

C or °

F. Some of

the °

C parts provide an offset so that negative temperatures can be monitored using a single-ended

supply.

In most applications, the output of these devices is fed into a comparator or a

n A/D converter

to convert the temperature data into a digital format. Despite the need for these additional devices,

thermistors and silicon temperature sensors continue to enjoy popularity due to low cost and

convenience of use in many situations.

Digital I/O Temperature Sensors

About five years ago, a new type of temperature sensor was introduced. These devices

include a digital interface that permits communication with a microcontroller. The interface is

usually an I?

C or SMBus serial bus, but other serial interfaces such as SPI are common. In

addition to reporting temperature readings to the microcontroller, the interface also receives

instructions from the microcontroller. Those instructions are often temperature limits, which, if

exceeded, activate a digital signal on the temperature sensor IC that interrupts the microcontroller.

The microcontroller is then able to adjust fan speed or back off the speed of a microprocessor, for

example, to keep temperature under control.

This type of device is available with a wide variety of features, among them, remote

temperature sensing. To enable remote sensing, most high-performance CPUs include an on-chip

transistor that provides a voltage analog of the temperature. (Only one of the transistor's two p-n

junctions is used.)

Figure 3

shows a remote CPU being monitored using this technique. Other

applications utilize a discrete transistor to perform the same function.

Figure 3. A user-programmable temperature sensor monitors the temperature of a remote CPU's

on-chip p-n junction.

Another important feature found on some of these types of sensors (including the sensor

shown in Figure 3) is the ability to interrupt a microcontroller when the measured temperature

falls outside a range bounded by high and low limits. On other sensors, an interrupt is generated

when the measured temperature exceeds either a high or a low temperature threshold (i.e., not

both). For the sensor in Figure 3, those limits are transmitted to the temperature sensor via the

SMBus interface. If the temperature moves above or below the circumscribed range, the alert

signal interrupts the processor.

Pictured in

Figure 4

is a similar device. Instead of monitoring one p-n junction, however, it

monitors four junctions and its own internal temperature. Because Maxim's MAX1668 consumes

a small amount of power, its internal temperature is close to the ambient temperature. Measuring

the ambient temperature gives an indication as to whether or not the system fan is operating

properly.

Figure 4. A user- programmable temperature sensor monitors its own local temperature and the

temperatures of four remote p-n junctions.

Controlling a fan while monitoring remote temperature is the chief function of the IC shown

in

Figure 5

. Users of this part can choose between two different modes of fan control. In the

PWM mode, the microcontroller controls the fan speed as a function of the measured temperature

by changing the duty cycle of the signal sent to the fan. This permits the power consumption to be

far less than that of the

linear mode

of control that this part also provides. Because some fans emit

an audible sound at the frequency of the PWM signal controlling it, the linear mode can be

advantageous, but at the price of higher power consumption and additional circuitry. The added

power consumption is a small fraction of the power consumed by the entire system, though.

Figure 5. A fan controller/temperature sensor IC uses either a PWM- or linear-mode control

scheme.

This IC provides the alert signal that interrupts the microcontroller when the temperature

violates specified limits. A safety feature in the form of the signal called

version of

up while temperature is rising to a dangerous level, the alert signal would no longer be useful.

However, overt, which goes active once the temperature rises above a level set via the SMBus, is

typically used to control circuitry without the aid of the microcontroller. Thus, in this

high-temperature scenario with the microcontroller not functioning, overt could be used to shut

down the system power supplies directly, without the microcontroller, and prevent a potentially

catastrophic failure.

This digital I/O class of devices finds widespread use in servers, battery packs, and hard-disk

drives. Temperature is monitored in numerous locations to increase a server's reliability: at the

motherboard (which is essentially the ambient temperature inside the chassis), inside the CPU die,

and at other heat- generating components such as graphics accelerators and hard-disk drives.

Battery packs incorporate temperature sensors for safety reasons and to optimize charging profiles,

which maximizes battery life.

There are two good reasons for monitoring the temperature of a hard-disk drive, which

depends primarily on the speed of the spindle motor and the ambient temperature: The read errors

in a drive increase at temperature extremes, and a hard disk's MTBF is improved significantly

through temperature control. By measuring the temperature within the system, you can control

motor speed to optimize reliability and performance. The drive can also be shut down. In high-end

systems, alerts can be generated for the system administrator to indicate temperature extremes or

situations where data loss is possible.

Analog-Plus Temperature Sensors

generate a logic output derived from the measured temperature and are distinguished from digital

I/O sensors primarily because they output data on a single line, as opposed to a serial bus.

In the simplest instance of an analog-plus sensor, the logic output trips when a specific

temperature is exceeded. Some of these devices are tripped when temperature rises above a preset

threshold, others, when temperature drops below a threshold. Some of these sensors allow the

temperature threshold to be adjusted with a resistor, whereas others have fixed thresholds.

The devices shown in

Figure 6

are purchased with a specific internal temperature threshold.

The three circuits illustrate common uses for this type of device: providing a warning, shutting

down a piece of equipment, or turning on a fan.

Figure 6. ICs that signal when a temperature has been exceeded are well suited for

over/undertemperature alarms and simple on/off fan control.

When an actual temperature reading is needed, and a microcontroller is available, sensors that

transmit the reading on a single line can be useful. With the microcontroller's internal counter

measuring time, the signals from this type of temperature sensor are readily transformed to a

measure of temperature. The sensor in

Figure 7

outputs a square wave whose frequency is

proportional to the ambient temperature in Kelvin. The device in

Figure 8

is similar, but the

period of the square wave is proportional to the ambient temperature in kelvins.

Figure 7. A temperature sensor that transmits a square wave whose frequency is proportional to

the measured temperature in Kelvin forms part of a heater controller circuit.