LM92

SNIS110D – MARCH 2000 – REVISED MARCH 2013

www.ti.com

FUNCTIONAL DESCRIPTION

The LM92 temperature sensor incorporates a band-gap type temperature sensor, 13-bit ADC, and a digital

comparator with user-programmable upper and lower limit values. The comparator activates either the INT line

for temperatures outside the TLOW and THIGH window, or the T_CRIT_A line for temperatures which exceed

T_CRIT. The lines are programmable for mode and polarity.

TEMPERATURE COMPARISON

LM92 provides a window comparison against a lower (TLOW) and upper (THIGH) trip point. A second upper trip

point (T_CRIT) functions as a critical alarm shutdown. Figure 7 depicts the comparison function as well as the

modes of operation.

Status Bits

The internal Status bits operate as follows:

“True”:

Temperature above a THIGH or T_CRIT is “true” for those respective bits. A “true” for TLOW is

temperature below TLOW.

“False”:

Assuming temperature has previously crossed above THIGH or T_CRIT, then the temperature must

drop below the points corresponding THYST (THIGH − THYST or T_CRIT − THYST) in order for the condition to be

false. For TLOW, assuming temperature has previously crossed below TLOW, a “false” occurs when temperature

goes above TLOW + THYST.

The Status bits are not affected by reads or any other actions, and always represent the state of temperature vs.

setpoints.

Hardwire Outputs

The T_CRIT_A hardwire output mirrors the T_CRIT_A flag, when the flag is true, the T_CRIT_A output is

asserted at all times regardless of mode. Reading the LM92 has no effect on the T_CRIT_A output, although the

internal conversion is restarted.

The behavior of the INT hardwire output is as follows:

Comparator Interrupt Mode (Default):

User reading part resets output until next measurement completes. If

condition is still true, output is set again at end of next conversion cycle. For example, if a user never reads the

part, and temperature goes below TLOW then INT becomes active. It would stay that way until temperature goes

above TLOW + THYST. However if the user reads the part, the output would be reset. At the end of the next

conversion cycle, if the condition is true, it is set again. If not, it remains reset.

Event Interrupt Mode:

User reading part resets output until next condition "event" occurs (in other words,

output is only set once for a true condition, if reset by a read, it remains reset until the next triggering threshold

has been crossed). Conversely, if a user never read the part, the output would stay set indefinitely after the first

event that set the output. An “event” for Event Interrupt Mode is defined as:

1. Transitioning upward across a setpoint, or

2. Transitioning downward across a setpoint's corresponding hysteresis (after having exceeded that setpoint).

For example, if a user never read the part, and temperature went below TLOW then INT would become active. It

would stay that way forever if a user never read the part.

However if the user read the part, the output would be reset. Even if the condition is true, it will remain reset. The

temperature must cross above TLOW + THYST to set the output again.

In either mode, reading any register in the LM92 restarts the conversion. This allows a designer to know exactly

when the LM92 begins a comparison. This prevents unnecessary Interrupts just after reprogramming setpoints.

Typically, system Interrupt inputs are masked prior to reprogramming trip points. By doing a read just after

resetting trip points, but prior to unmasking, unexpected Interrupts are prevented.

Avoid programming setpoints so close that their hysteresis values overlap. An example would be that with a

THYST value of 2 °C then setting THIGH and TLOW to within 4 °C of each other will violate this restriction. To be

more specific, with THYST set to 2 °C assume THIGH set to 64 °C. If TLOW is set equal to, or higher than 60 °C this

restriction is violated.

8

Submit Documentation Feedback

Copyright © 2000–2013, Texas Instruments Incorporated

Product Folder Links: LM92