LM92

SNIS110D – MARCH 2000 – REVISED MARCH 2013

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APPLICATION HINTS

The temperature response graph in Figure 16 depicts a typical application designed to meet ACPI requirements.

In this type of application, the temperature scale is given an arbitrary value of "granularity", or the window within

which temperature notification events should occur. The LM92 can be programmed to the window size chosen by

the designer, and will issue interrupts to the processor whenever the window limits have been crossed. The

internal flags permit quick determination of whether the temperature is rising or falling.

The T_CRIT limit would typically use its separate output to activate hardware shutdown circuitry separate from

the processor. This is done because it is expected that if temperature has gotten this high that the processor may

not be responding. The separate circuitry can then shut down the system, usually by shutting down the power

supply.

Note that the INT and T_CRIT_A outputs are separate, but can be wire-ORed together. Alternatively the

T_CRIT_A can be diode ORed to the INT line in such a way that a T_CRIT_A event activates the INT line, but an

INT event does not activate the T_CRIT_A line. This may be useful in the event that it is desirable to notify both

the processor and separate T_CRIT_A shutdown circuitry of a critical temperature alarm at the same time

(maybe the processor is still working and can coordinate a graceful shutdown with the separate shutdown

circuit).

To implement ACPI compatible sensing it is necessary to sense whenever the temperature goes outside the

window, issue an interrupt, service the interrupt, and reprogram the window according to the desired granularity

of the temperature scale. The reprogrammed window will now have the current temperature inside it, ready to

issue an interrupt whenever the temperature deviates from the current window.

To understand this graph, assume that at the left hand side the system is at some nominal temperature. For the

temperature is rising. The system then programs the temperature limits to a value higher by an amount equal to

the desired granularity. Note that in Event Interrupt Mode, reprogramming the limits has caused a second,

known, interrupt to be issued since temperature has been returned within the window. In Comparator Interrupt

Mode, the LM92 simply stops issuing interrupts.

The 2nd event is another identical rise in temperature. The 3rd event is typical of a drop in temperature. This is

one of the conditions that demonstrates the power of the LM92, as the user receives notification that a lower limit

is exceeded in such a way that temperature is dropping.

The Critical Alarm Event activates the separate T_CRIT_A output. Typically, this would feed circuitry separate

from the processor on the assumption that if the system reached this temperature, the processor might not be

responding.

16

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