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TC642VPA Scheda tecnica(PDF) 10 Page - Microchip Technology

Il numero della parte TC642VPA
Spiegazioni elettronici  PWM Fan Speed Controller with FanSense??Technology
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Produttore elettronici  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC642VPA Scheda tecnica(HTML) 10 Page - Microchip Technology

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TC642
DS21444C-page 10
 2002 Microchip Technology Inc.
FIGURE 5-2:
Temperature Sensing
Circuit.
Generally, the thermistor shown in Figure 5-2 is a non-
linear device with a negative temperature coefficient
(also called an NTC thermistor). In Figure 5-2, R1 is
used to linearize the thermistor temperature response,
while 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 TC642. A 100 k
Ω 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. With this change, R2 is
adjusted according to the formulas below. It is
suggested that a maximum temperature range of 50°C
be used with this circuit due to thermistor linearity
limitations.
The following two equations permit solving for the two
unknown variables, R1 and R2. More information
regarding thermistors can be found in AN679, “Temper-
ature Sensing Technologies”, and AN685, “Thermistors
in Single Supply Temperature Sensing Circuits”, which
can be downloaded from Microchip’s web site at:
www.microchip.com.
EQUATION
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 typically 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 µA. It would be very
conservative to design for a divider current, IDIV, of
100 µA. If VDD = 5.0V then;
EQUATION
FIGURE 5-3:
VIN Circuit.
R2 = 23.2 kΩ
R1 = 100 kΩ
NTC Thermistor
100 k
Ω @ 25˚C
RT1
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 (1.4) + 1.25V
1000
2500
VMIN =
x (1.4) + 1.25V = 1.81V
R1+ R2
IDIV = 100µA =
, therefore
5.0V
R1 + R2 =
= 50,000
Ω = 50kΩ
100µA
5.0V
R2
R1
GND
VDD
VMIN
IDIV
IIN


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