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參數(shù)資料
| 型號： | TC647VUATR |
| 元件分類： | 運動控制電子 |
| 英文描述： | BRUSHLESS DC MOTOR CONTROLLER, PDSO8 |
| 封裝： | PLASTIC, MSOP-8 |
| 文件頁數(shù)： | 3/28頁 |
| 文件大?。?/td> | 478K |
| 代理商： | TC647VUATR |
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2002 Microchip Technology Inc.
DS21447C-page 11
TC647
FIGURE 5-3:
VMIN Circuit.
We can further specify R1 and R2 by the condition that
the divider voltage is equal to our desired VMIN. This
yields the following equation:
EQUATION
Solving for the relationship between R1 and R2 results
in the following equation:
EQUATION
In this example, R1 = (1.762) R2. Substituting this rela-
tionship back into the previous equation yields the
resistor values:
R2 = 18.1 k
R1 = 31.9 k
In this case, the standard values of 31.6 k
and
18.2 k
are very close to the calculated values and
would be more than adequate.
5.3
Operations at Low Duty Cycle
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
RSENSE. These can only occur when the fan is ener-
gized (i.e., VOUT is “on”). At very low duty cycles, the
VOUT output is “off” most of the time. The fan may be
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 VOUT output will
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 VMIN be set no lower than 1.8V.
5.4
FanSense Network
(RSENSE and CSENSE)
The FanSense network, comprised of RSENSE and
CSENSE, allows the TC647 to detect commutation of
the fan motor (FanSense technology). This network
can be thought of as a differentiator and threshold
detector. The function of RSENSE is to convert the fan
current into a voltage. CSENSE serves to AC-couple this
voltage signal and provide a ground referenced input to
the SENSE pin. Designing a proper SENSE network is
simply a matter of scaling RSENSE to provide the
necessary amount of gain (i.e., the current-to-voltage
conversion ratio). A 0.1 F ceramic capacitor is recom-
mended for CSENSE. Smaller values require larger
sense resistors, and higher value capacitors are bulkier
and more expensive. Using a 0.1 F results in
typical SENSE network. Figure 5-5 shows the
waveforms observed using a typical SENSE network.
FIGURE 5-4:
SENSE Network.
R2
R1
GND
VDD
VMIN
IDIV
IIN
VDD x R2
R1 + R2
VMIN =
VDD - VMIN
VMIN
R1 = R2 x
Q1
GND
VDD
RSENSE
SENSE
RBASE
CSENSE
(0.1 F Typ.)
VOUT
FAN
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