AMC6821-Q1 Datasheet, PDF (26/54 Pages) Texas Instruments – INTELLIGENT TEMPERATURE MONITOR AND PWM FAN CONTROLLER

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.
Measure RPM and adjust DCY once every second (TACH-FAST = 0)
or once every 250ms (TACH-FAST = 1).
(TACH-DATA Register) - TACH-SETTING Register)
No
|Error| > 0x000A
Yes
(TACH-DATA) >
(TACH-SETTING)
Yes
Increase DCY
by one STEP
No
Decrease DCY
by one STEP
Figure 26. RPM Fan DCY Loop
26
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