Datasheet

90 95 100 105
TEMPERATURE (°C)
0
6.25
12.5
18.75
25
31.25
37.5
43.75
50
56.25
62.5
68.75
75
81.25
87.5
93.75
100
PWM DUTY CYCLE
(%)
85 110
12-point LUT
Entry
PWM Output
Transition
90 95 100 105
TEMPERATURE (°C)
0
6.25
12.5
18.75
25
31.25
37.5
43.75
50
56.25
62.5
68.75
75
81.25
87.5
93.75
100
PWM DUTY CYCLE
(%)
85 110
Ramp rate control
between transitions
has 0.39% step size
Multiple slopes
easily realizable
with 12-point LUT
LM96163
www.ti.com SNAS433D –JUNE 2008–REVISED MAY 2013
(a) Without smoothing (b) With smoothing
Figure 14. Fan Control Transfer Function Example
Also included is programmable hysteresis that is not described by the curves of Figure 14. The hysteresis takes
effect as temperature is decreasing and moves all the temperature set-points down by the programmed amount.
For the example shown here if the hysteresis is set to 1°C and if the temperature is decreasing from 96.5°C the
duty cycle will remain at 68.75% and will not transition to 62.5% until the temperature drops below 95.5°C.
If at any time the TCRIT output were to activate the PWM duty cycle will be instantaneously forced to 100% thus
forcing the fans to full on.
Table 8. PWM Smoothing Time Intervals
Time Interval 0-100% DC Time
(seconds)
w/ 6.25% resolution w/ 0.39% resolution
(seconds) Seconds
0.182 2.913 43.7
0.091 1.456 21.6
0.046 0.728 10.9
0.023 0.364 5.45
The Table 8 table describes the programmable time interval preventing abrupt changes in the PWM output duty
cycle and thus preventing abrupt acoustical noise changes as well. The threshold of acoustically detecting fan
noise transition is at about a 2% duty cycle change. The table describes the time intervals that can be
programmed and the total amount of time it will take for the PWM output to change from 0% to 100% for each
time interval. For example if the time interval for each step is set to 0.091 seconds the time it will take to make a
0 to 100% duty cycle change will be 21.6 seconds when the duty cycle resolution is set to 0.39% or 1.46
seconds when the resolution is 6.25%. One setting will apply to all LUT transitions.
COMPUTING RPM OF THE FAN FROM THE TACH COUNT
The Tach Count Registers 46
HEX
and 47
HEX
count the number of periods of the 90 kHz tachometer clock in the
LM96163 for the tachometer input from the fan assuming a 2 pulse per revolution fan tachometer, such as the
fans supplied with the Intel boxed processors. The RPM of the fan can be computed from the Tach Count
Registers 46
HEX
and 47
HEX
. This can best be shown through an example.
Example:
Given: the fan used has a tachometer output with 2 per revolution.
Let:
Register 46 (LSB) is BF
HEX
= Decimal (11 x 16) + 15 = 191 and
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