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| 型号： | TC649VUATR |
| 元件分类： | 运动控制电子 |
| 英文描述： | BRUSHLESS DC MOTOR CONTROLLER, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 3/28页 |
| 文件大小： | 478K |
| 代理商： | TC649VUATR |
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2002 Microchip Technology Inc.
DS21449C-page 11
TC649
FIGURE 5-3:
VAS Circuit.
Per Section 1.0, “Electrical Characteristics”, the leak-
age current at the VAS pin is no more than 1 A. It is
conservative to design for a divider current, IDIV, of
100 A. If VDD = 5.0V then:
EQUATION
We can further specify R1 and R2 by the condition that
the divider voltage is equal to our desired VAS. This
yields the following:
EQUATION
Solving for the relationship between R1 and R2 results
in:
EQUATION
In the case of this example, R1 = (2.27) R2. Substituting
this relationship back into the VAS equation above
yields the resistor values:
R2 = 15.3 k, and
R1 = 34.7 k
In this case, the standard values of 34.8 k
and
15.4 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
selection of the minimum fan speed is the irregular
activation of the Diagnostic Timer due to the TC649
“missing” fan commutation pulses at low speeds. This
is a natural consequence of low PWM duty cycles (typ-
ically 25% or less). Recall that the SENSE function
detects commutation of the fan as disturbances in the
current through RSENSE. These can only occur when
the fan is energized (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 TC649 may fail
to capture a pulse within the 32-cycle missing pulse
detector window. If 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 accelerates briefly when the Diagnostic
Timer fires. For this reason, it is recommended that VAS
be set no lower than 1.8V.
5.4
FanSense Network
(RSENSE and CSENSE)
The FanSense network, comprised of RSENSE and
CSENSE, allows the TC649 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 nec-
essary amount of gain (i.e., the current-to-voltage con-
version ratio). A
0.1 F
ceramic capacitor is
recommended for CSENSE. Smaller values require
larger sense resistors, and higher value capacitors are
bulkier and more expensive. Using a 0.1 F capacitor
results in reasonable values for RSENSE. Figure 5-4
illustrates a typical SENSE network. Figure 5-5 shows
the waveforms observed using a typical SENSE net-
work.
R2
R1
GND
VDD
VAS
IDIV
IIN
R1 + R2
IDIV = 1e
– 4A
, therefore
5.0V
R1 + R2 =
= 50,000
= 50 k
1e–4A
5.0V
VAS =
VDD x R2
R1 + R2
R1 = R2 x
= R2 x
VDD - VAS
VAS
5 -1.53
1.53
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