AMC6821 Datasheet, PDF (21/47 Pages) Burr-Brown (TI) – Analog Monitor and Control Circuit

AMC6821
www.ti.com
SBAS386A – MAY 2006 – REVISED MAY 2006
Fan Spin-Up
The PWM duty cycle controls the cooling fan speed. To spin-up a fan from a stop 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 15. 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 6. 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.
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 15. Spin-Up Process
STIME1
0
0
1
1
0
0
1
1
Table 6. 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
4
8
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