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Using IC Temperature Sensors to Protect
Electronic Systems
Choosing the appropriate sensor and protection strategy to prevent overheating of an electronic system can save you money and simplify design.
Kerry Lacanette, National Semiconductor Corp.
All electronic systems, from portable consumer products to precision industrial
devices, are affected by extremes of heat and cold. If not protected, the
components in these systems can be damaged by temperatures that fall outside
the components' operating ranges.
Several strategies have evolved to ensure the maintenance of safe temperature
operating ranges for electronic systems. The best protection technique depends
on the nature of the system and its most sensitive components. A key element
of any technique, though, is the sensing device. And IC temperature sensors
bring to this application a number of advantages.
IC Temperature Sensors
Operation of IC temperature sensors is usually based on the behavior
of silicon PN junctions as a function of temperature. The most common approach
is to force a current through two PN junctions with different active areas.
The difference between the forward voltages on the two junctions is proportional
to absolute temperature:
VF1 -VF2 = kT/q1n j1/j2 (1)
Some IC temperature sensors simply produce an output signal (either voltage
or current) proportional to absolute temperature. For many applications,
though, an output proportional to ºC, ºF, or some other scale
is more practical. By starting with a differential voltage proportional
to absolute temperature and applying appropriate gains and level shifts,
an IC designer can develop an output signal with a convenient mapping of
temperature to voltage—for example, 10 mV/ºC. Such functions as comparators,
A/D converters (ADCs), event sequencers, and programmable registers can
be added to make the sensors more useful.
How do IC temperature sensors compare with other types of temperature sensors (e.g., RTDs, thermocouples, and thermistors) in system protection applications?
Their greatest advantages are integration (all the necessary signal conditioning
circuitry is on-chip), low cost, and ease of design. The last advantage
is due primarily to the level of integration—the system designer need not
worry about linearization, cold junction compensation, comparators, ADCs,
voltage references, or other such issues. The primary limitation of temperature
sensing ICs is that their temperature range is limited to that of typical
silicon ICs (usually -55ºC to 150ºC). This limitation is
not a concern in electronic system protection applications because electronic
systems are virtually always limited to narrower temperature ranges than
the sensors.
Simple Sensors for Simple Systems
IC temperature sensors can make it easy for a designer to add protection
functions to a system. An example is the 60 W audio power amplifier shown
in Figure 1.
The temperature of the power device depends on many factors,
primarily ambient temperature, air flow across the heat sink, heat sink
thermal resistance, load impedance, and output power. A fan can reduce amplifier
temperature, but it also generates audible noise. Operating the fan only
when the temperature is high enough to require forced-air cooling significantly
reduces noise output. If the system is well designed, the fan will not be
needed under normal operating conditions.
The sensor should be mounted in a location that provides good correlation
with die temperature. If the heat sink is mounted against one side of the
printed circuit board, mounting the sensor on the other side of the circuit
board works well. IC1 consists of a temperature sensor, a 1.25 V voltage
reference, and a dual comparator.
The temperature sensor's output voltage is:
VTEMP = 395 mV + (6.20 mV/°C) (2)
Resistors R1 and R2 set the comparators' thresholds. The open-collector
output drives the P-channel MOSFET. A 5ºC built-in hysteresis prevents
the fan's speed from alternating rapidly about the set point.
If desired, the second comparator on the sensor IC can be used by adding
a third resistor to the voltage divider to set a second threshold. A second
comparator output can perform additional functions (e.g., provide a signal
to turn off the amplifier if temperature continues to rise and reaches a
damaging level). If the amplifier IC contains a built-in overtemperature
shutdown function—as the amplifier shown in Figure 1 does—the second comparator
output can be used to perform another function, such as driving an LED to
warn of the imminent shutdown of the system. Integrating the sensor, voltage
reference, and two comparators on a single chip reduces the parts count
and the cost even in simple thermostat applications, such as this one.
Digital Communications for Microprocessor-Based Systems
A thermostat IC is well suited for applications that require "bang bang" control (on/ off control) of one or two functions, but some applications require more sophisticated communications between the sensor and the system.
In a microprocessor-based system, the system's microprocessor can use an
algorithm that chooses the actions that will best protect the system from
overheating. The microprocessor can read the system's temperature with a
conventional analog-output IC temperature sensor followed by an ADC or with
an IC that has a built-in ADC. This lets the microprocessor query the sensor
for temperature data. Integrating the sensor, voltage reference, ADC, bus
interface, and a programmable watchdog capability not only saves space but
simplifies the design process.
A high-performance personal computer is a good example of a system that
uses this technique. The latest generation of high-speed microprocessors
enables computer manufacturers to offer systems with unparalleled levels
of performance. But the performance gains are accompanied by increases in
power consumption, which can result in damaging heat buildup in the computer
chassis. Many systems are reliable when operated under normal conditions,
but when a fan fails or a ventilation path is blocked, damage can occur
quickly if corrective action is not taken.
A digital-output temperature sensor (see Figure 2) that can be mounted close
to the microprocessor or other significant heat sources constantly converts
temperature to 9-bit words—8 magnitude bits and 1 sign bit—that provide
a resolution of 1/2ºC/LSB. At any time, the microprocessor can query
the sensor over a two-wire, I2C-compatible digital interface for the current
temperature. The microprocessor can also program thermostat trip points
for the sensor. When a trip point temperature is exceeded, a separate open-drain
output changes state, indicating that the system is overheating.
The thermostat output might drive an interrupt processor, as shown in Figure
2. This allows the system to operate normally unless the temperature rises
to a potentially damaging level. Only then does the microprocessor have
to read the temperature and take action to protect the system.
The digital temperature sensor that is shown in Figure 2 has three address
pins that can be connected to either the positive supply or the ground to
allow as many as eight sensors (each with a unique address) to reside on
the same bus. The sensors can be located at various potential trouble spots
within the chassis.
The critical sensor is usually the one located close to the microprocessor.
Not only is the microprocessor the component most likely to get too hot,
it is also the most expensive component in the system to replace if it is
damaged. If the microprocessor is socketed, placing the sensor on the circuit
board under the microprocessor (between the microprocessor and the circuit
board) gives good correlation to microprocessor temperature.
Multiple Sensing for Mission-Critical Systems
Computer makers are moving to a new level of system monitoring with the advent of devices that not only sense temperature but also constantly
check other aspects of system health so that problems can be fixed before
they cause unscheduled downtime. An example of such a system is the circuit
shown in Figure 3.
Like the sensor IC in Figure 2, this device
monitors temperature and communicates with the host via a two-wire digital
interface. It also communicates over the ISA bus, for ease of integration
into personal computer motherboards. An additional logic input allows the
IC to accept inputs from other temperature sensors with open-drain comparator
outputs, like the sensors in Figures 1 and 2. Thus, several satellite sensors
can be located in appropriate places within the chassis, and all their outputs
can feed the main sensor IC.
System downtime can be reduced dramatically if problems are identified and
located when or before they occur. The sensing device in Figure 3 has three
inputs that accept tachometer outputs from cooling fans. The device monitors
those inputs constantly, and if a fan's speed drops below a host-programmed
threshold (typically 10% below nominal speed), the device notifies the system.
The system can then inform the user (or system administrator, if the CPU
is on a network) that a fan appears to be failing, and the problem fan can
be replaced before it fails. Because the repair can take place before failure
occurs, it can be scheduled for noncritical times to avoid interruption
of critical work.
In addition to sensing temperature and fan speed, the IC in Figure 3 has
a multichannel, 8-bit ADC that can be used to monitor analog-output temperature
sensors, power supply voltages, or any other important quantities. An internal
4.086 V reference sets the ADC's sensitivity to 16 mV/LSB. Five of the ADC's
input channels accept positive input voltages, and the other two channels
have internal inverting amplifiers that allow those inputs to accept negative
input voltages. In addition, the amplifiers' gains can be set using external
resistors, allowing the sensitivities of the two negative inputs to be adjusted.
All the ADC inputs, the fan speed counters, and the internal temperature
sensor are continually monitored and compared against host-programmable
limit thresholds. If any threshold is exceeded, the IC sends an interrupt
to the host, which can then interrogate the sensor, identify the problem,
and take corrective action.
Note that not all personal computers need this level of sensor sophistication.
Many systems are adequately protected by an approach like the one shown
in Figure 2. The functionality of a device such as National Semiconductor's
LM78 in Figure 3, however, is often needed in systems that cannot afford
to be shut down for repairs.
Summary
Knowing the right sensor and protection scheme to prevent thermal damage
of an electronic system can save money and design effort. Silicon IC temperature
sensors are well suited for these applications because their high degree
of integration simplifies the design task and reduces system component count.
The needs of simpler applications are generally satisfied by thermostat-style
sensors, and microprocessor-based systems are better served by sensors with
digital interfaces. Mission-critical computer systems need the additional
protection and monitoring capability that a full system watchdog can provide.
Kerry Lacanette is Applications Manager of the Analog Division, National Semiconductor Corp., 2900 Semiconductor Dr., Santa Clara, CA 95052; 520-751-3769, fax 520-751-2379.
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