2.7 MANUFACTURERS ID REGISTER

(Read Address FEh) The default value is 01h.

2.8 DIE REVISION CODE REGISTER

(Read Address FFh) The default value is A1h. This register will increment by 1 every time there is a revision to the die by National

Semiconductor.

3.0 Applications Hints

The LM95231 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 be-

cause 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

LM95231’s pins. This presumes that the ambient air tem-

perature 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 tempera-

ture of the LM95231 die will be at an intermediate tempera-

ture 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 tempera-

ture much more strongly than will the air temperature.

To measure temperature external to the LM95231’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 LM95231’s temperature. 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

an MMBT3904 transistor base emitter junction be used with

the collector tied to the base.

The LM95231’s TruTherm technology allows accurate sens-

ing of integrated thermal diodes, such as those found on

processors. With TruTherm technology turned off, the

LM95231 can measure a diode connected transistor such as

the MMBT3904.

The LM95231 has been optimized to measure the remote

thermal diode integrated in a Pentium 4 processor on 90nm

process or an MMBT3904 transistor. Using the Remote Di-

ode Model Select register either pair of remote inputs can be

assigned to be either a Pentium 4 processor on 90nm pro-

cess or an MMBT3904.

3.1 DIODE NON-IDEALITY

3.1.1 Diode Non-Ideality Factor Effect on Accuracy

When a transistor is connected as a diode, the following

relationship holds for variables V

(1)

where:

•

q = 1.6x10

•

T = Absolute Temperature in Kelvin

•

k = 1.38x10

•

η is the non-ideality factor of the process the diode is

manufactured on,

•

I

•

I

•

V

In the active region, the -1 term is negligible and may be

eliminated, yielding the following equation

(2)

In Equation (2),

η and I

that was used in the fabrication of the particular diode. By

forcing two currents with a very controlled ratio (I

measuring the resulting voltage difference, it is possible to

eliminate the I

ence yields the relationship:

(3)

Solving Equation (3) for temperature yields:

(4)

Equation (4) holds true when a diode connected transistor

such as the MMBT3904 is used. When this “diode” equation

is applied to an integrated diode such as a processor tran-

sistor with its collector tied to GND as shown in Figure 3 it

will yield a wide non-ideality spread. This wide non-ideality

spread is not due to true process variation but due to the fact

that Equation (4) is an approximation.

TruTherm technology uses the transistor equation, Equation

(5), which is a more accurate representation of the topology

of the thermal diode found in an FPGA or processor.

(5)

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