MAX6501CMP075【PDF 6/8 页】Low-Cost, +2.7V to +5.5V, Micropower Tem

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Low-Cost, +2.7V to +5.5V, Micropower

Temperature Switches in SOT23 and TO-220

_______________________________________________________________________________________

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 SOT23-5 package may be small

enough to fit underneath a socketed μP, allowing the

device to monitor the μP’s temperature directly. The

TO-220 package can monitor the temperature of a heat

sink directly, and presents the lower thermal resistance

of the two packages. Use the monitor’s output to reset

the μP, assert an interrupt, or trigger an external alarm.

Accurate temperature monitoring depends on the thermal

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 tem-

perature monitor ensure that heat transfers quickly and

reliably.

The rise in die temperature due to self-heating is given

by the following formula:

= P

DISSIPATION

where P

DISSIPATION

is the power dissipated by the

MAX6501–MAX6504, and

is the package’s thermal

resistance.

The typical thermal resistance is 140°C/W for the

SOT23-5 package and 75°C/W for the TO-220 pack-

age. 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 additional 0.3mW of

power is dissipated within the IC. This corresponds to a

0.042°C shift in the die temperature in the SOT23-5.

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, multiple

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 monitored

device cause no permanent damage. The first tempera-

ture 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 output asserts when a

wide variety of destructive fault conditions occur, includ-

ing latchups, short circuits, and cooling-system failures.

MAX6502

+5V

TOVER

GND

HYST

FAN

HEAT

Figure 3. Overtemperature Fan Control

MAX6501

+3.3V

GND

HYST

GND

SHUTDOWN

HEAT

PULL-UP

100k

TOVER

INT

RESET

Figure 2. Microprocessor Alarm/Reset

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