Rainbow Electronics LM41 - Manual (Page 24)

2.0 Register Set
(Continued)
Bit Type Description
1-0 RO CR0 and CR1 (Conversion Rate bits 0 and 1) These bits control the conversion rate of the LM41 for
more details see Table Conversion Rate Control and desciption below.
7-2 RO Reserved. Will always read "0".
Conversion Rate Control
LowPwr [CR1:CR0] Typical Conversion Rate (ms)
0 00 Fastest*: continuous
1 00 91
0 01 91
1 01 364
0 10 182 (default)
1 10 728
0 11 364
1 11 1456
*Fastest: 7.5ms(remote) + 7.5msec (local) + 5x1.42msec (voltage) = 22.1 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, when using a Super I/O host on a motherboard 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 LM41. If attentions are enabled the conversion rate and scan rate will be
equal.
3.0 Application Hints
The LM41 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 LM41’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
LM41 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 LM41’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 LM41’s temperature. The LM41 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 LM41 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.
20070315
FIGURE 7. 90 nm Pentium 4 Temperature vs LM41
Temperature Reading
LM41
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