MAX6501–MAX6504
Low-Cost, +2.7V to +5.5V, Micropower
Temperature Switches in SOT23
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Applications Information
Thermal Considerations
The MAX6501–MAX6504 supply current is typically
30µA. When used to drive high-impedance loads, the
devices dissipate negligible power. Therefore, the die
temperature is essentially the same as the package
temperature. The key to accurate temperature monitor-
ing is good thermal contact between the MAX6501–
MAX6504 package and the device being monitored. In
some applications, the SOT23 package may be small
enough to fit underneath a socketed µP, allowing the
device to monitor the µP’s temperature directly. Use the
monitor’s output to reset the µP, assert an interrupt, or
trigger an external alarm.
Accurate temperature monitoring depends on the ther-
mal resistance between the device being monitored
and the MAX6501–MAX6504 die. Heat flows in and out
of plastic packages, primarily through the leads. Pin 2
of the SOT23-5 package provides the lowest thermal
resistance to the die. Short, wide copper traces leading
to the temperature monitor ensure that heat transfers
quickly and reliably.
The rise in die temperature due to self-heating is given
by the following formula:
ΔT
J
= P
DISSIPATION
x θ
JA
where P
DISSIPATION
is the power dissipated by the
MAX6501–MAX6504, and θ
JA
is the package’s thermal
resistance.
The typical thermal resistance is 140°C/W for the
SOT23 package. To limit the effects of self-heating,
minimize the output currents. For example, if the
MAX6501 or MAX6503 sink 1mA, the output voltage is
guaranteed to be less than 0.3V. Therefore, an addi-
tional 0.3mW of power is dissipated within the IC. This
corresponds to a 0.042°C shift in the die temperature in
the SOT23.
Temperature-Window Alarm
The MAX6501–MAX6504 temperature switch outputs
assert when the die temperature is outside the factory-
programmed range. Combining the outputs of two
devices creates an over/undertemperature alarm. The
MAX6501/MAX6503 and the MAX6502/MAX6504 are
designed to form two complementary pairs, each con-
taining one cold trip-point output and one hot trip-point
output. The assertion of either output alerts the system to
an out-of-range temperature. The MAX6502/MAX6504
push/pull output stages can be ORed to produce a ther-
mal out-of-range alarm. More favorably, a MAX6501/
MAX6503 can be directly wire-ORed with a single exter-
nal resistor to accomplish the same task (Figure 4).
The temperature window alarms shown in Figure 4 can
be used to accurately determine when a device’s tem-
perature falls out of the -5°C to +75°C range. The ther-
mal-overrange signal can be used to assert a thermal
shutdown, power-up, recalibration, or other temperature-
dependent function.
Low-Cost, Fail-Safe
Temperature Monitor
In high-performance/high-reliability applications, multi-
ple temperature monitoring is important. The high-level
integration and low cost of the MAX6501–MAX6504
facilitate the use of multiple temperature monitors to in-
crease system reliability. Figure 5’s application uses
two MAX6502s with different temperature thresholds to
ensure that fault conditions that can overheat the moni-
tored device cause no permanent damage. The first
temperature monitor activates the fan when the die
temperature exceeds +45°C. The second MAX6502
triggers a system shutdown if the die temperature
reaches +75°C. The second temperature monitor’s out-
put asserts when a wide variety of destructive fault con-
ditions occur, including latchups, short circuits, and
cooling-system failures.
