Datasheet

Normal Fan Speed Control
TACHSetting
Register
DCY
Adjustment
FanSpeed
Counter
+3V/+5V
TACH
PWM-Out
+V
TACHData
AMC6821
+
-
PWM
Control
ThePWMdutycycleincreasesifthe ACHdataisaboveT
thesettingvalue,decreasesiftheTACHdataisbelow
thesetting,anddoesnotchangeiftheTACHdatais
equaltothesetting(withatoleranceof0x000A).
NOTE:Thetachresistornetworkis
usedtolimittheTACHinputvoltageto5.5Vmax.
AMC6821-Q1
www.ti.com ...................................................................................................................................................................................................... SBAS475 – JUNE 2009
The fan speed is controlled by four different modes:
• software DCY control;
• software RPM control,
• auto remote temperature fan control;
• maximum fast-speed calculated control.
The Auto Temperature-Fan Control mode consists of auto remote temperature-fan control and maximum
fast-speed calculated control. It is an intelligent closed-loop control. In this mode, the fan speed is controlled
either by the remote temperature (Auto-Remote Temperature-Fan Control) or by maximum speed calculated for
internal and remote temperature. This control mode optimizes fan speed for a given temperature to intelligently
manage the system thermals/acoustics. The user writes the proper registers to define the linear feedback control
algorithm parameters. After programming, the AMC6821 runs stand-alone, even without the intervention of the
micro-controller. It ensures that if the controller or system locks up, the fan can still be controlled based on
temperature measurements, and the fan speed adjusted to correct any changes in system temperature.
Software-RPM works as a fan speed regulator to maintain the speed at a programmable target value. It is a
closed-loop mode and can run stand-alone as well. The Software-DCY mode is an open-loop mode; the PWM
output duty cycle changes to the target value immediately after the user writes the desired duty cycle to the
device registers.
Bits FDRC1 and FDRC0 in Configuration Register 1 determine the operation mode.
Software DCY Control Mode
When the bits [FDRC1:FDRC0] = [00], the fan works in the software DCY control mode. The host writes the
desired duty cycle value corresponding to the required RPM into the DCY register. The duty cycle changes to the
new value immediately after the writing. In this mode, if the TACH measurement is enabled (bit 2 of 0x01 = 1)
and the TACH-MODE bit (bit 1 of 0x01) is cleared ('0'), the duty cycle from the PWM-OUT pin is forced to 0%
when the value in the DCY register is less than 7%. However, if the TACH measurement is disabled (bit 2 of
0x01 is cleared) or the TACH mode is set ('1'), the DCY register always keeps the programmed value written by
the host and is not forced to '0' even when the programmed value is less than 7%.
Software-RPM Control Mode (Fan Speed Regulator)
This mode works as a fan speed regulator that maintains the speed at a programmable target value. It works
only when the TACH measurement is enabled (bit 2 of 0x02 = 1). When the bits [FDRC1:FDRC0] = [01], the fan
works in the software RPM control mode, as shown in Figure 25 . The host writes the proper value into the TACH
Setting Register to set the target fan speed. The actual fan speed is monitored by an on-chip fan speed counter,
and the result is stored in the TACH-DATA Register (refer to the Fan Speed Measurement section for more
details). The actual speed is compared with the setting value. If there is a difference, the duty cycle is adjusted.
Figure 25. Software RPM Control
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Product Folder Link(s): AMC6821-Q1