Datasheet

(TACH-DATARegister) T- ACH-SETTINGRegister)
|Error|>0x000A
No
Yes
No
Yes
(T )>ACH-DATA
(TACH-SETTING)
IncreaseDCY
byoneSTEP
DecreaseDCY
byoneSTEP
MeasureRPMandadjustDCYonceeverysecond(TACH-FAST=0)
oronceevery250ms(T =1).ACH-FAST
AMC6821-Q1
SBAS475 – JUNE 2009 ...................................................................................................................................................................................................... www.ti.com
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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