LM64

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SNAS207A – MAY 2004 – REVISED MARCH 2013

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

PWM Value

Closest PWM

Temperature

Calculated

Value

25

8.0

8

35

8.5

9

45

10.0

10

55

12.5

13

65

16.0

16

75

20.5

21

85

26.0

26

95

32.5

33

105

40.0

40

We can then program the Lookup Table with the temperature and Closest PWM Values required for the curve

required in our example.

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.

Copyright © 2004–2013, Texas Instruments Incorporated

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