TB067 Datasheet, PDF (1/6 Pages) Microchip Technology – Temperature Sensor Strategies Keep System Thermal Problems in Check

TB067
Temperature Sensor Strategies Keep System Thermal
Problems in Check
Author: Microchip Technology Inc.
INTRODUCTION
Various concerns must be addressed when dealing
with temperature sensing applications, which fall into
two broad categories: precision temperature measure-
ment and system thermal management. Precision
temperature measurement applications (such as those
encountered in the process control industry) require
highly accurate temperature sensors. Applications of
this type frequently use a thermocouple (or higher
precision sensor) in concert with low offset/low drift
analog interface circuits to acquire and condition the
temperature signal with minimum error. Sensor
absolute accuracy and linearity are primary
considerations; sensor cost is a secondary issue.
System thermal management schemes, on the other
hand, regulate the creation (and/or disposal) of heat
within the system. Examples of system thermal man-
agement applications are CPU over-temperature
detectors in personal computers and thermal shutdown
circuits in power systems. The temperature sensor
must be accurate enough to detect when temperature
is approaching the upper boundry of the “safe” operat-
ing limit. Typically, full scale sensor measurement
errors of 2°C to 5°C are adequate for such applications.
The primary concerns in sensor selection are sensor
cost, size, and ease of interface to the processor.
OVER-TEMPERATURE SHUTDOWN
Heat sinking the high power dissipation devices in a
system is the simplest thermal management strategy.
In this case, only the system must be monitored for
catastrophic over-temperature conditions due to a
malfunction or excessive external ambient tempera-
ture. Temperature is sensed within the system
enclosure (internal system ambient) or at the heat sink
itself. When measured temperature exceeds a preset
limit, a fault is indicated and the system shuts down.
Figure 1 illustrates a simple power supply over-temper-
ature shutdown circuit using Microchip’s TC622
temperature switch. This device integrates a solid-state
temperature sensor and user-programmable limit
detector in a single TO-220 package, allowing direct
attachment to the heat sink surface. The device
operates from power supply voltages between 4.5V
and 18.0V, and has both high true and low true logic
outputs (OUT, OUT). These outputs are driven active
when measured temperature equals (or exceeds) the
temperature setting determined by the value of external
programming resistor, R_SET. In the example of
Figure 1, the TC622 outputs are driven active when the
heatsink temperature equals the trip point temperature
set by R_SET. When this occurs, the crowbar circuit is
activated, causing the supply output to fold back to
zero. The TC622 outputs remain active until the heat-
sink temperature falls a minimum of 2°C (built-in
hysteresis) below the trip point temperature, at which
time the device again allows normal supply operation.
Heat Sink
V_CC
Output Device
V_OUT
Output
Device
TC622
RSET
Circuit Board
TC622 Heat Sink Mounting
TC622
SET
V_DD
OVERTEMP
OUT
GND
OUT
Power
Good
Signal
Crowbar
Circuit
FIGURE 1:
Heat Sink Surface
TC622 Power Supply Over Temperature Shutdown
 2003 Microchip Technology Inc.
DS91067A-page 1