Bit

Type

Description

7-2

RO

Reserved. Will always read "0".

Conversion Rate Control

LowPwr

[CR1:CR0]

Typical Conversion Rate (ms)

0

00

Fastest*: continuous

100

91

001

91

1

01

364

0

10

182 (default)

1

10

728

0

11

364

1

11

1456

*Fastest: 2x7.5ms(remote) + 7.5msec (local) + 5x1.42msec (voltage) = 29.6 ms total

The sensor conversion rate is controlled by this register as well as the Low Power Bit of Device Control Register. This register is

not defined by the SensorPath specification. Therefore, on a motherboard when using a Super I/O host this register must be

modified during BIOS run time. The conversion rate is dependent on system physical requirements and limitations. The thermal

response time of the MSOP package is one such requirement. Most systems will function properly with the default settings. The

master scan rate is related to the conversion rate of the LM40. If attentions are enabled the conversion rate and scan rate will be

equal.

3.0 Application Hints

The LM40 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 LM40’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 of the

LM40 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 LM40’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 LM40’s temperature. The LM40 has been

optimized to measure the remote diode of a 90 nm Pentium

4 processor as shown in Figure 7. 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 2N3904 transistor

base emitter junction be used with the collector tied to the

base.

A diode connected 2N3904 approximates the junction avail-

able on a Pentium microprocessor for temperature measure-

ment. Therefore, the LM40 can sense the temperature of this

diode effectively. Although, an offset will be observed. The

temperature reading will be offset by approximately −4.5˚C,

therefore a correction factor of +4.5˚C should be added to all

temperature readings when using a 2N3904 transistor.

20068415

FIGURE 7. 90 nm Pentium 4 Temperature vs LM40

Temperature Reading

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