TC647 Microchip, TC647 Datasheet - Page 11

• Manufacturer Part Number: TC647
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

FIGURE 5-3:
We can further specify R
the divider voltage is equal to our desired V
yields the following equation:
EQUATION
Solving for the relationship between R
in the following equation:
EQUATION
In this example, R
tionship back into the previous equation yields the
resistor values:
In this case, the standard values of 31.6 k
18.2 k
would be more than adequate.
5.3
One boundary condition which may impact the selec-
tion of the minimum fan speed is the irregular activation
of the Diagnostic Timer due to the TC647 “missing” fan
commutation pulses at low speeds. This is a natural
consequence of low PWM duty cycles (typically 25% or
less). Recall that the SENSE function detects commu-
tation of the fan as disturbances in the current through
R
gized (i.e., V
SENSE
2002 Microchip Technology Inc.
. These can only occur when the fan is ener-
are very close to the calculated values and
Operations at Low Duty Cycle
OUT
R
I
DIV
V
1
MIN
= R
is “on”). At very low duty cycles, the
GND
1
V
DD
=
= (1.762) R
2
R
R
x
R 2
2
1
R 1
V
= 18.1 k
= 31.9 k
MIN
1
V
and R
V
DD
R
V
DD
1
Circuit.
V
MIN
- V
+ R
MIN
I
IN
x R
2
. Substituting this rela-
2
MIN
2
2
by the condition that
1
and R
MIN
2
results
. This
and
V
rotating normally, but the commutation events are
occurring during the PWM’s off-time.
The phase relationship between the fan’s commutation
and the PWM edges tends to “walk around” as the
system operates. At certain points, the TC647 may fail
to capture a pulse within the 32-cycle missing pulse
detector window. When this happens, the 3-cycle
Diagnostic Timer will be activated, the V
be active continuously for three cycles and, if the fan is
operating normally, a pulse will be detected. If all is
well, the system will return to normal operation. There
is no harm in this behavior, but it may be audible to the
user as the fan will accelerate briefly when the
Diagnostic Timer fires. For this reason, it is
recommended that V
5.4
The FanSense network, comprised of R
C
the fan motor (FanSense™ technology). This network
can be thought of as a differentiator and threshold
detector. The function of R
current into a voltage. C
voltage signal and provide a ground referenced input to
the SENSE pin. Designing a proper SENSE network is
simply a matter of scaling R
necessary amount of gain (i.e., the current-to-voltage
conversion ratio). A 0.1 µF ceramic capacitor is recom-
mended for C
sense resistors, and higher value capacitors are bulkier
and more expensive. Using a 0.1 µF results in
reasonable values for R
typical
waveforms observed using a typical SENSE network.
FIGURE 5-4:
OUT
SENSE
output is “off” most of the time. The fan may be
, allows the TC647 to detect commutation of
SENSE
SENSE network.
FanSense
(R
V
OUT
SENSE
SENSE
and C
(0.1 µF Typ.)
MIN
. Smaller values require larger
C
™
R
SENSE Network.
SENSE
SENSE
BASE
SENSE
Network
be set no lower than 1.8V.
SENSE
SENSE
Figure 5-5
serves to AC-couple this
. Figure 5-4 illustrates a
SENSE
)
GND
is to convert the fan
V
FAN
DD
DS21447C-page 11
TC647
R
Q 1
to provide the
OUT
SENSE
shows
SENSE
output will
and
the