AMC6821SDBQG4 Datasheet, PDF (19/56 Pages) Texas Instruments – Intelligent Temperature Monitor and PWM Fan Controller

AMC6821
www.ti.com
SBAS386C – MAY 2006 – REVISED JULY 2007
Some fans (such as the JMC™ four-wire fan) are powered directly by dc power, instead of being powered by the
PWM. In this case, the TACH mode must be set to '1'. When TACH-MODE = 1, the PWM-Out pin is not forced
ON; instead, the status is controlled completely by the DCY register, just as in normal operation. Setting
TACH-MODE to '1' also disables the internal correction circuit because no extra duty cycle is applied. Setting the
TACH mode to '1' allows TACH reading continuously, regardless of the status of the PWM-Out pin.
The selection of the TACH mode affects the RPM monitoring and control. When the TACH-MODE bit is equal to
'1', the duty cycle of the PWM-Out pin is always determined by the calculated value; the TACH data are always
updated at every RPM monitoring. However, when the TACH-MODE bit is equal to '0', in the Software-RPM
Control mode the PWM-Out pin is forced to 30% if the calculated duty cycle is less than 30%; in other modes,
the PWM-Out pin is forced to 0% and the TACH data are not updated if the calculated duty cycle is less than 7%.
FAN RPM Out-of-Range Detection
The larger value of the TACH data corresponds to a slower speed. When the TACH data are larger than the
TACH-Low-Limit, the fan runs at a speed below the predefined minimum RPM, and the FANS bit in Status
Register 1 is set to '1'. Note that no FANS (fan-slow) detections are made during spin-up. The FANS bit is
cleared ('0') only after reading this register and reasserted ('1') in the next monitoring if a fan-slow is detected.
After spin-up, FANS is set ('1') even if the TACH data are less than the TACH-Low-Limit until the register is read.
When the TACH data are less than the TACH-High-Limit, the fan runs at a speed above the predefined
maximum RPM, and the RPM-ALARM bit in Status Register 1 is set ('1'). Note that the RPM-ALARM bit is
cleared when reading the register. Once cleared, this bit is not reasserted in the next monitoring cycle even if the
condition persists. This bit may be reasserted only if the RPM drops below the allowed maximum speed.
When FANS = 1 or RPM-ALARM = 1, there is a fan-out-of-range interrupt and FAN-ORN is generated if the
FANIE bit in Configuration Register 1 is set ('1'). This interrupt makes the SMBALERT pin go low.
RPM Alarm (TACH is less than the high limit)
FANS (TACH is greater than the low limit)
(RPM out-of-range)
FANIE
(FAN-TACH interrupt enabled)
FAN-ORN Interrupt
Figure 18. RPM Out-of-Range Detection
Copyright © 2006–2007, Texas Instruments Incorporated
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