lm96163cisdx-1 National Semiconductor Corporation, lm96163cisdx-1 Datasheet - Page 35

• Manufacturer Part Number: lm96163cisdx-1
• Description: Remote Diode Digital Temperature Sensor With Integrated Fan Control And Trutherm Bjt Transistor Beta Compensation Technology
• Manufacturer: National Semiconductor Corporation

PWM
Freq
[4:0]
The following table is true only when the 22.5 kHz PWM frequency with high resolution duty cycle is selected by setting bit 4 (PHR)
of the Enhanced Configuration register (0x45), clearing bit 3 (PWCKSL) of the PWM and RPM Configuration register (0x4A) and
setting PWM Frequency (0x4D) register to 0x08.
3.1.1 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:
3.2 LUT FAN CONTROL
The LM96163 fan control uses a temperature to duty cycle
look-up table (LUT) that has 12 indexes. High resolution duty
cycle (0.392%) is available when the PWM frequency is set
to 22.5 kHz. In addition ramp rate control is available to
acoustically smooth the duty cycle transition between LUT
steps.
Shown in Figure 7(a) is an example of the 12-point LUT tem-
perature to PWM transfer function that can be realized without
smoothing enabled. The table is comprised of twelve Duty-
Cycle and Temperature set-point pairs. Notice that the tran-
sitions between one index of the LUT to the next happen
instantaneously. If the PWM levels are set far enough apart
this can be acoustically very disturbing. The typical acoustical
threshold of change in duty cycle is 2%. Figure 7(b) has an
overlaid curve (solid line) showing what occurs at the transi-
tions when smoothing is enabled. The dashed lines shown in
Figure 7(b) are there to point out that multiple slopes can be
4D
8
Resolution,
0.392
Step
%
(a) Without smoothing
for 100%
4C [7:0]
Value
(Hex)
PWM
FF
FIGURE 7. Fan Control Transfer Function Example
4C [7:0] for
about 75%
Value
PWM
(Hex)
BF
30041034
4C [7:0]
for 50%
Value
(Hex)
PWM
80
35
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%.
realized easily. At the transitions the duty cycle increments in
LSb (0.39% for the case shown) steps. In the example shown
in Figure 7(b) the first pair is set for a duty-cycle of 31.25%
and a temperature of 0°C. For temperatures less than 0°C the
duty cycle is set to 0. When the temperature is greater than 0
°C but is less than 91 °C the duty cycle will remain at 31.25%.
The next pair is set at 37.5% and 91°C. Once the temperature
exceeds 91°C the duty cycle on the PWM output will gradually
transition from 31.25% to 37.5% in 0.39% steps at the pro-
grammed time interval. The LUT comparison temperature
resolution is programmable to either 1 °C or 0.5 °C. For the
curves of Figure 7 the comparison resolution is set to 0.5 °C
that is why the actual duty cycle transitions happen 0.5 °C
higher than the actual LUT entry. The duty cycle transition
time interval is programmable and is shown in the table titled
PWM Smoothing Time Intervals. Care should be taken so that
the LUT PWM and Temperature values are setup in ascend-
ing weight.
Clock, kHz
360 kHz
Internal
Freq at
22.50
PWM
(b) With smoothing
Not Available
Clock, Hz
Internal
Freq at
1.4 kHz
PWM
75% is Selected
Cycle, % When
Approximately
Actual Duty
74.902
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30041035