AMC6821 Datasheet, PDF (24/54 Pages) Burr-Brown (TI) – Analog Monitor and Control Circuit

AMC6821
SBAS386C – MAY 2006 – REVISED JULY 2007
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Fan Spin-Up
The PWM duty cycle controls the cooling fan speed. To spin up a fan from a stopped state or under-speed
status, the spin-up process is applied to overcome the fan inertia. During the first third of spin-up, the duty cycle
of the PWM gradually increases from 33.3% to 100%, and then maintains at 100% through the rest of the
process. At the end of the spin-up process, the duty cycle is adjusted to 33.3%. After starting, the fan speed is
controlled normally. The spin-up process is shown in Figure 24. The bits [STIME2:STIME0] (bits 2:0 of 0x20)
define the spin-up time, from 0.2 seconds to 8 seconds, as shown in Table 13. Fan speed is monitored
immediately after the spin-up process.
Spin-up is disabled by setting the FSPD bit of the Fan Characteristics Register to '1'. If disabled, the spin-up
process is not applied when the fan stops or an RPM is detected below the minimum speed. The TACH low limit
register defines the minimum speed. After power-on or reset, the FSPD bit is cleared and the spin-up is always
performed, regardless of the state of the FANS bit (bit 1 of 0x02).
Note that no FANS (fan-slow) detections are performed during spin-up. This bit is cleared ('0') only after reading
it, and reasserts '1' in the next monitoring if a fan-slow condition is detected. After spin-up, FANS is set ('1') even
if the TACH data are less than the TACH low limit until the flag is read.
DCY
100%
STIME2
0
0
0
0
1
1
1
1
33.3%
Normal
Control
0
T_SPIN-UP/3 T_SPIN-UP
Figure 24. Spin-Up Process
STIME1
0
0
1
1
0
0
1
1
Table 13. Spin-Up Time
STIME0
0
1
0
1
0
1
0
1
SPIN-UP TIME (seconds)
0.2
0.4
0.6
0.8
1
2 (default)
4
8
24
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