AMC6821 datasheet(26/56 Pages) TI

26 / 56 page

www.ti.com

(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

Submit Documentation Feedback

Product Folder Link(s): AMC6821

AMC6821 bảng dữ liệu(PDF) 26 Page - Texas Instruments