LM64 datasheet(31/42 Pages) TI2

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8.1.1.1 Computing Duty Cycles for a Given Frequency

Select a PWM Frequency from the first column corresponding to the desired actual frequency in columns 6 or 7.

Note the PWM Value for 100% Duty Cycle.

Find the Duty Cycle by taking the PWM Value of Register 4C and computing:

100

_(%)

for

Value

PWM

Value

PWM

DutyCycle

(1)

Example: For a PWM Frequency of 24, a PWM Value at 100% = 48 and PWM Value actual = 28, then the Duty

Cycle is (28/48) × 100% = 58.3%.

8.1.2 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.

8.1.3 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

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LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

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