Rainbow Electronics LM85 - Manual (Page 18)

Functional Description
(Continued)
In Auto Fan Mode, when the temperature for a zone is above the Temperature Limit (Registers 67-69h) and below its Absolute
Temperature Limit (Registers 6A-6Ch), the speed of a fan assigned to that zone is determined as follows.
When the temperature reaches the Fan Temp Limit for a zone, the PWM output assigned to that zone will be Fan PWM Minimum.
Between Fan Temp Limit and (Fan Temp Limit + Range), the PWM duty cycle will increase linearly according to the temperature
as shown in the figure below. The PWM duty cycle will be 100% at (Fan Temp Limit + Range).
Example for PWM1 assigned to Zone 1:
– Zone 1 Fan Temp Limit (Register 67h) is set to 50˚C (32h).
– Range (Register 5Fh) is set to 8˚C (6xh).
– Fan 1 PWM Minimum (Register 64h) is set to 50% (32h).
In this case, the PWM1 duty cycle will be 50% at 50˚C.
Since (Zone 1 Fan Temp Limit) + (Zone 1 Range) = 50˚C + 8˚C = 58˚C, the fan will run at 100% duty cycle when the temperature
of the Zone 1 sensor reaches 58˚C.
Since the midpoint of the fan control range is 54˚C, and the median duty cycle is 75% (Halfway between the PWM Minimum and
100%), PWM1 duty cycle would be 75% at 54˚C.
Above (Zone 1 Fan Temp Limit) + (Zone 1 Range), the duty cycle will be 100%.
PWM frequency bits [3:0]
The PWM frequency bits [3:0] determine the PWM frequency for the fan.
PWM Frequency Selection (Default = 011 = 30.04 Hz)
TABLE 6. Register Setting vs PWM Frequency
Freq [2:0] PWM Frequency (Exact frequencies vary by
manufacturer)
National Actual PWM Frequency
000 10 Hz 10.01 Hz
001 15 Hz 15.02 Hz
010 23 Hz 23.14 Hz
011 30 Hz 30.04 Hz
100 38 Hz 38.16 Hz
101 47 Hz 47.06 Hz
110 62 Hz 61.38 Hz
111 94 Hz 94.12 Hz
Range Selection RAN [3:0]
RAN [3:0] Range (˚C)
0000 2
0001 2.5
0010 3.33
20035306
FIGURE 1. Fan Activity above Fan Temp Limit
LM85
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