Main Power Supply

LM89

SMBus

Master

MCU/

GPU/

ASIC/

FPGA

2.2nF*

Core Voltage

D+

D-

SMBData

SMBCLK

Shutdown Control

3.3V derived

from Aux.

Supply

*Note: 2.2nF capacitor must be placed as close as possible to D+ and D- pins of the LM89.

T_CRIT_A

ALERT

LM89

SNIS128D – AUGUST 2002 – REVISED JUNE 2014

www.ti.com

10 Application and Implementation

10.1 Application Information

The LM89 can be applied easily in the same way as other integrated-circuit temperature sensors, and its remote

diode sensing capability allows it to be used in new ways as well. It can be soldered to a printed circuit board,

and because the path of best thermal conductivity is between the die and the pins, its temperature will effectively

be that of the printed circuit board lands and traces soldered to the LM89's pins. This presumes that the ambient

air temperature is almost the same as the surface temperature of the printed circuit board; if the air temperature

is much higher or lower than the surface temperature, the actual temperature of the LM89 die will be at an

intermediate temperature between the surface and air temperatures. Again, the primary thermal conduction path

is through the leads, so the circuit board temperature will contribute to the die temperature much more strongly

than will the air temperature.

To measure temperature external to the LM89's die, use a remote diode. This diode can be located on the die of

a target IC, allowing measurement of the IC's temperature, independent of the LM89's temperature.

10.2 Typical Application

The LM89 has been optimized to measure the remote thermal diode of a 0.13 micron Pentium 4, a Mobile

Pentium 4 Processor-M processor or other embedded thermal diodes that have similar characteristics. A discrete

diode can also be used to sense the temperature of external objects or ambient air. Remember that a discrete

diode's temperature will be affected, and often dominated, by the temperature of its leads. Most silicon diodes do

not lend themselves well to this application. It is recommended that a MMBT3904 transistor base emitter junction

be used with the collector tied to the base (diode-connected).

An LM89 with a diode-connected MMBT3904 will have a typical -1°C offset.

10.2.1 Design Requirements

10.2.1.1 Diode Nonideality

10.2.1.1.1

Diode Nonideality Factor Effect On Accuracy

24

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