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Fan Spin-Up

Normal

Control

DCY

100%

SPIN-UP

33.3%

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

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

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.

Figure 24. Spin-Up Process

Table 13. Spin-Up Time

STIME2

STIME1

STIME0

SPIN-UP TIME (seconds)

0.2

0.4

0.6

0.8

2 (default)

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