TC665 Microchip Technology Inc., TC665 Datasheet - Page 26

• Manufacturer Part Number: TC665
• Description: Smbus Pwm Fan Speed Controllers With Fan Fault Detection
• Manufacturer: Microchip Technology Inc.

TC664/TC665
FIGURE 7-8:
off.
7.6
The bias supply (V
should be bypassed with a 1 µF ceramic capacitor. This
capacitor will help supply the peak currents that are
required to drive the base/gate of the external fan drive
devices.
As the V
any noise on this pin can cause duty cycle jittering. For
this reason, the V
0.01 µF capacitor.
In order to keep fan noise off of the TC664/TC665
device ground, individual ground returns for the TC664/
TC665 and the low side of the fan current sense resis-
tor should be used.
7.7
There are many different fan designs available in the
marketplace today. The motor designs can vary and,
along with it, the number of current pulses in one fan
revolution. In order to correctly measure and commu-
nicate the fan speed, the TC664/TC665 devices must
be programmed for the proper number of fan current
pulses per revolution. This is done by setting the FPPR
bit in the Configuration Register to the proper values
(see Section 6.3 for settings). A fan's current pulses
per revolution can be determined in the following
manner.
DS21737A-page 26
IN
Bias Supply Bypassing and Noise
Filtering
Determining Current Pulses Per
Revolution of Fans
pin controls the duty cycle in a linear fashion,
V
OUT
Q
1
IN
DD
- N-Channel MOSFET
pin should be bypassed with a
) for the TC664/TC665 devices
Clamp Diode For Fan Turn-
GND
FAN
R
Q
SENSE
1
The first piece of information required is the fan's full
speed RPM rating. The fan RPM rating can then be
converted to give the time for one revolution using the
following equation:
EQUATION
The fan current can now be monitored over this time
period. The number of pulses occurring in this time
period is the fan's "Current Pulses per Rotation" rating
which is needed in order to accurately read fan RPM.
Example: The full speed fan RPM rating is 8200 RPM.
From this, the time for one fan revolution is calculated
to be 7.3 msec, using the previously discussed equa-
tion. Using a current probe, the fan current can be mon-
itored as the fan is operating at full speed. Figure 7-9
shows the fan current pulses for this example. The
7.44 msec window, marked by the cursors, is very near
the 7.3 msec calculated above and is within the toler-
ance of the fan ratings. Four current pulses occur within
this 7.44 msec time frame. Given this information,
FPPR (bits 2-1<01>) in the Configuration Register
should be set to '10' to indicate 4 current pulses per
revolution.
FIGURE 7-9:
Revolution Fan.
7.8
During the PWM mode of operation, some fans will
generate an extra current pulse. This pulse occurs
when the external drive device is turned on and is, in
most cases, caused by the fan's electronics that control
the fan motor. This pulse does not represent true fan
current and needs to be blanked out. This is particularly
important for detecting a fan in a locked rotor condition.
Time for one revolution (msec.)
How to Eliminate False Current
Pulse Sensing
Four Current Pulses Per
2002 Microchip Technology Inc.
=
60
----------------------- -
Fan RPM
1000