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| 型号： | TC647VUATR |
| 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： | 2/28页 |
| 文件大小： | 478K |
| 代理商： | TC647VUATR |
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TC647
DS21447C-page 10
2002 Microchip Technology Inc.
5.1
Temperature Sensor Design
The temperature signal connected to VIN must output a
voltage in the range of 1.25V to 2.65V (typical) for 0%
to 100% of the temperature range of interest. The
circuit in Figure 5-2 illustrates a convenient way to
provide this signal.
FIGURE 5-2:
Temperature Sensing
Circuit.
Figure 5-2 illustrates a simple temperature dependent
voltage divider circuit. RT1 is a conventional 100 k @
25°C NTC thermistor, while R1 and R2 are standard
resistors. The supply voltage, VDD, is divided between
R2 and the parallel combination of RT1 and R1 (for con-
venience, the parallel combination of RT1 and R1 will
be referred to as RTEMP). The resistance of the ther-
mistor at various temperatures is obtained from the
manufacturer’s specifications. Thermistors are often
referred to in terms of their resistance at 25°C. Gener-
ally, the thermistor shown in Figure 5-2 is a non-linear
device with a negative temperature coefficient (also
linearize the thermistor temperature response and R2
is used to produce a positive temperature coefficient at
the VIN node. As an added benefit, this configuration
produces an output voltage delta of 1.4V, which is well
within the range of the VC(SPAN) specification of the
TC647. A 100 k
NTC thermistor is selected for this
application in order to keep IDIV at a minimum.
For the voltage range at VIN to be equal to 1.25V to
2.65V, the temperature range of this configuration is
0°C to 50°C. If a different temperature range is required
from this circuit, R1 should be chosen to equal the
resistance value of the thermistor at the center of this
new temperature range. It is suggested that a maxi-
mum temperature range of 50°C be used with this cir-
cuit due to thermistor linearity limitations. With this
change, R2 is adjusted according to the following
equations:
EQUATION
These two equations facilitate solving for the two
unknown variables, R1 and R2. More information about
Thermistors may be obtained from AN679, “Tempera-
ture Sensing Technologies”, and AN685, “Thermistors
in Single Supply Temperature Sensing Circuits”, which
can be downloaded from Microchip’s website at
www.microchip.com.
5.2
Minimum Fan Speed
A voltage divider on VMIN sets the minimum PWM duty
cycle and, thus, the minimum fan speed. As with the
VIN input, 1.25V to 2.65V corresponds to 0% to 100%
duty cycle. Assuming that fan speed is linearly related
to duty cycle, the minimum speed voltage is given by
the equation:
EQUATION
For example, if 2500 RPM equates to 100% fan speed,
and a minimum speed of 1000 RPM is desired, then
the VMIN voltage is:
EQUATION
The VMIN voltage may be set using a simple resistor
divider as shown in Figure 5-3. Per Section 1.0,
“Electrical Characteristics”, the leakage current at the
VMIN pin is no more than 1 A. It would be very
conservative to design for a divider current, IDIV, of
100 A. If VDD = 5.0V then;
EQUATION
R2 = 23.2 k
R1 = 100 k
RT1
NTC
Thermistor
100 k
@ 25C
IDIV
VIN
VDD
VDD x R2
RTEMP (T1) + R2
= V(T1)
RTEMP (T2) + R2
= V(T2)
VDD x R2
Where T1 and T2 are the chosen temperatures and
RTEMP is the parallel combination of the thermistor
and R1.
Minimum Speed
Full Speed
VMIN =
x (1.4V) + 1.25V
1000
2500
VMIN =
x (1.4V) + 1.25V = 1.81V
R1 + R2
IDIV = 1e–4A =
, therefore
5.0V
R1 + R2 =
= 50,000
= 50 k
1e–4A
5.0V
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