LM85 National Semiconductor, LM85 Datasheet - Page 18

• Manufacturer Part Number: LM85
• Description: Hardware Monitor with Integrated Fan Control
• Manufacturer: National Semiconductor

www.national.com
Freq [2:0]
000
001
010
011
100
101
110
111
Functional Description
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:
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)
Range Selection RAN [3:0]
– 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).
PWM Frequency (Exact frequencies vary by
manufacturer)
10 Hz
15 Hz
23 Hz
30 Hz
38 Hz
47 Hz
62 Hz
94 Hz
RAN [3:0]
0000
0001
0010
TABLE 6. Register Setting vs PWM Frequency
FIGURE 1. Fan Activity above Fan Temp Limit
(Continued)
18
Range (˚C)
2
2.5
3.33
National Actual PWM Frequency
10.01 Hz
15.02 Hz
23.14 Hz
30.04 Hz
38.16 Hz
47.06 Hz
61.38 Hz
94.12 Hz
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