Datasheet

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ValuePWM
DutyCycle
LM96163
SNAS433D –JUNE 2008–REVISED MAY 2013 www.ti.com
NOTE
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.
Actual Duty
PWM PWM PWM PWM PWM
PWM Cycle, %
Step Value Value Value Freq at Freq at
Freq When
Resolution, 4C [7:0] 4C [7:0] for 4C [7:0] 360 kHz 1.4 kHz
4D Approximately
% for 100% about 75% for 50% Internal Internal
[4:0] 75% is
(Hex) (Hex) (Hex) Clock, kHz Clock, Hz
Selected
8 0.392 FF BF 80 22.50 Not Available 74.902
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:
(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%.
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 14 (a) is an example of the 12-point LUT temperature 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 transitions 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 14 (b) has an overlaid curve (solid line) showing what occurs at the transitions when
smoothing is enabled. The dashed lines shown in Figure 14 (b) are there to point out that multiple slopes can be
realized easily. At the transitions the duty cycle increments in LSb (0.39% for the case shown) steps. In the
example shown in Figure 14 (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 programmed time interval. The LUT comparison temperature resolution is programmable to either 1 °C or 0.5
°C. For the curves of Figure 14 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 Table 8. Care should be taken so that the LUT PWM and
Temperature values are setup in ascending weight.
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