LM96063 datasheet(34/46 Pages) TI2

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100

105

TEMPERATURE (°C)

6.25

12.5

18.75

31.25

37.5

43.75

56.25

62.5

68.75

81.25

87.5

93.75

100

110

12-point LUT

Entry

PWM Output

Transition

100

105

TEMPERATURE (°C)

6.25

12.5

18.75

31.25

37.5

43.75

56.25

62.5

68.75

81.25

87.5

93.75

100

110

Ramp rate control

between transitions

has 0.39% step size

Multiple slopes

easily realizable

with 12-point LUT

100

_(%)

for

Value

PWM

Value

PWM

DutyCycle

LM96063

SNIS149B – NOVEMBER 2011 – REVISED MARCH 2013

www.ti.com

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 LM96063 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 12 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 13 has an overlaid curve (solid line) showing what occurs at the transitions when smoothing is

enabled. The dashed lines shown in Figure 13 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 13the 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

Fan Control Transfer Function Example 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.

Fan Control Transfer Function Example

Figure 12. Without smoothing

Figure 13. With smoothing

Also included is programmable hysteresis that is not described by the curves of Fan Control Transfer Function

Example. 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.

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