SNAS207B May 2004 – January 2024 LM64
PRODUCTION DATA
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 |
---|---|---|
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.