LM64 datasheet(26/29 Pages) NSC

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3.0 Application Notes (Continued)

3.2 USE OF THE LOOKUP TABLE FOR NON-LINEAR

PWM VALUES VS TEMPERATURE

The Lookup Table, Registers 50 through 5F, can be used to

create a non-linear PWM vs Temperature curve that could be

used to reduce the acoustic noise from processor fan due to

linear or step transfer functions. An example is given below:

EXAMPLE:

In a particular system it was found that the best acoustic fan

noise performance was found to occur when the PWM vs

Temperature transfer function curve was parabolic in shape.

From 25˚C to 105˚C the fan is to go from 20% to 100%.

Since there are 8 steps to the Lookup Table we will break up

the Temperature range into 8 separate temperatures. For the

80˚C over 8-steps = 10˚C per step. This takes care of the

x-axis.

For the PWM Value, we first select the PWM Frequency. In

this example we will make the PWM Frequency (Register

4C) 20.

For 100% Duty Cycle then, the PWM value is 40. For 20%

the minimum is 40 x (0.2) = 8.

We can then arrange the PWM, Temperature pairs in a

parabolic fashion in the form of y = 0.005

• (x −25)

2 +8

Temperature

PWM Value

Calculated

Closest PWM

Value

8.0

8.5

10.0

12.5

16.0

20.5

26.0

32.5

105

40.0

We can then program the Lookup Table with the temperature

and Closest PWM Values required for the curve required in

our example.

3.3 NON-IDEALITY FACTOR AND TEMPERATURE

ACCURACY

The LM64 can be applied to remote diode sensing in the

same way as other integrated-circuit temperature sensors. 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 its pins. This

presumes that the ambient air temperature is nearly 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 LM64 die

will be an intermediate temperature between the surface and

air temperatures. Again, the primary thermal conduction path

is through the leads, so the circuit board surface temperature

will contribute to the die temperature much more than the air

temperature.

To measure the temperature external to the die use a remote

diode. This diode can be located on the die of the target IC,

such as a CPU processor chip, allowing measurement of the

IC’s temperature, independent of the LM64’s temperature.

The LM64 has been optimized for use with a MMBT3904

diode-connected transistor.

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 diode-connected

MMBT3904 transistor be used. The base of the transistor is

connected to the collector and becomes the anode. The

emitter is the cathode.

3.3.1 Diode Non_Ideality

When a transistor is connected to a diode the following

relationship holds for V

be, T, and IF:

where

•

q = 1.6x10

−19 Coulombs (the electron charge)

•

T = Absolute Temperature in Kelvin

•

k = 1.38x10

−23 joules/K (Boltzmann’s constant)

•

η is the non-ideality factor of the manufacturing process

used to make the thermal diode

•

s = Saturation Current and is process dependent

•

f = Forward Current through the base emitter junction

•

be = Base Emitter Voltage Drop

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

eliminated, yielding the following equation

In the above equation,

η and I

s are dependent upon the

process that was used in the fabrication of the particular

diode. By forcing two currents with a very controlled ratio (N)

and measuring the resulting voltage difference, it is possible

to eliminate the I

s term. Solving for the forward voltage

difference yields the relationship:

The non-ideality factor,

η, is the only other parameter not

accounted for and depends on the diode that is used for

measurement. Since

∆V

be is proportional to both

η and T, the

variations in

η cannot be distinguished from variations in

temperature. Since the temperature sensor does not control

the non-ideality factor, it will directly add to the inaccuracy of

the sensor.

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