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(TACH-DATARegister)

ACH-SETTINGRegister)

|Error|>0x000A

Yes

ACH-DATA

(TACH-SETTING)

IncreaseDCY

byoneSTEP

DecreaseDCY

byoneSTEP

MeasureRPMandadjustDCYonceeverysecond(TACH-FAST=0)

oronceevery250ms(T

=1).

ACH-FAST

AMC6821

SBAS386C – MAY 2006 – REVISED JULY 2007

The monitoring and adjustment is made once every second, or once every 250ms, as determined by the

TACH-FAST bit of Configuration Register 4 (bit 5, 0x04). Bits [STEP1:STEP0] of the DCY-RAMP Register define

the allowed amount of each adjustment. When the difference between the values of the TACH-DATA and TACH

Setting Registers are equal to or less than 0x000A, the adjustment finishes. 0x000A corresponds to about 1.8%

tolerance for 10,000RPMs, or 0.9% for 5000RPMs. This measurement architecture is illustrated in Figure 26.

In practice, the selected target speed must be not too low to operate the fan. When the TACH-MODE bit (bit 1 of

0x02) is cleared ('0'), the duty cycle of PWM-Out is forced to 30% when the calculated desired value of duty

cycle is less than 30%. Therefore, the TACH setting must be not greater than the value corresponding to the

RPM for 30% duty cycle. When TACH mode = '1', the TACH setting must not be greater than the value

corresponding to the allowed minimum RPM at which the fan runs properly.

Figure 26. RPM Fan DCY Loop

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