Microchip MCP3201 Handleiding

Microchip Niet gecategoriseerd MCP3201

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DS21816A - Page 1
© 2003 Microchip Technology, Inc.
ANALOG DESIGN NOTE ADN001
Keeping Power Hungry Circuits Under
Thermal Control
By Bonnie C. Baker, Microchip Technology Inc.
Introduction
Projectors, large power supplies, datacom switches and routers,
pose an interesting heat dissipation problem. These applications
consume enough power to prompt a designer to cool off the
electronics with a fan. If the appropriate airflow across the
electronics is equal to or less than six to seven cubic feet per
minute (CFM), a good choice of fan would be the DC brushless
fan.
The fan speed of a DC brushless fan can be driven and controlled
by the electronics in a discrete solution, a microprocessor circuit
or a stand-alone fan controller IC. A discrete solution can be
highly customized but can be real-estate hungry. Although this
solution is a low cost alternative, it is challenging to implement
“smart” features, such as predictive fan failure or false fan failure
alarm rejection. Additionally, the hardware troubleshooting phase
for this system can be intensive as the feature set increases.
If you have a multiple fan application, the best circuit to use is
a microcontroller-based system. With the microcontroller, all
the fans and temperatures of the various environments can be
economically controlled with this one chip solution and a few
external components. The “smart” features that are difficult to
implement with discrete solutions are easily executed with the
microcontroller. The firmware of the microcontroller can be used
to set threshold temperatures and fan diagnostics for an array of
fans. Since the complexity of this system goes beyond the control
of one fan, the firmware overhead and firmware debugging can
be an issue.
For a one-fan circuit, the stand-alone fan controller IC is the
better choice. The stand-alone IC has fault detect circuitry
that can notify the system when the fan has failed, so that the
power consuming part of the system can be shutdown. The
stand-alone IC fan fault detection capability rejects glitches,
ensuring that false alarms are filtered. It can economically be
used to sense remote temperature with a NTC thermistor or
with the internal temperature sensor on-chip. As an added
benefit, the stand-alone IC can be used to detect the fan
faults of a two-wire fan, which is more economical than its
three-wire counterpart.
Regardless of the circuit option that is used, there are
three primary design issues to be considered in fan control
circuits, once the proper location of the fan is determined.
These three design issues are: fan excitation, temperature
monitoring and fan noise.
The circuit in Figure 1 illustrates how a two-wire fan can
be driven with a stand-alone IC. In this circuit, the TC647B
performs the task of varying the fan speed based on the
temperature that is sensed from the NTC thermistor. The
TC647B is also able to sense fan operation, enabling it to
indicate when a fan fault has occurred.
The speed of a brushless DC fan can be controlled by either
varying the voltage applied to it linearly or by pulse width
modulating (PWM) the voltage. The TC647B shown in Figure 1,
drives the base of transistor Q1 with a PWM waveform, which
in turn drives the voltage that is applied to the fan.
FAULT
SENSE
NTC
R1
R2
R3
R4GND
Fan Fault
Shutdown
Shutdown
(Optional)
Q1
+12V
+5V
VDD
VIN
VMIN
VOUT
RBASE
RSENSE
CSENSE
CF
1 µF
CF
TC647B
Fan
CB
1 µF
CB
0.01 µF
CB
0.01 µF
18
6
7
5
4
2
3
Figure 1. A two-wire fan can easily be driven and controlled by a thermistor-connected, TC647B
DS21816A - Page 2 © 2003 Microchip Technology, Inc.
By varying the pulse width of the PWM waveform, the speed
of the fan can be increased or decreased. The pulse width
modulation method of fan speed control is more efficient than
the linear regulation method.
The voltage across R
SENSE and the voltage at the SENSE
pin during PWM mode operation are shown in Figure 2. The
voltage at the sense resistor has both DC and AC content. The
AC content is generated by the commutation of the current
in the fan motor windings. These voltage transients across
RSENSE are coupled through CSENSE to the SENSE pin of the
TC647B. This removes the DC content of the sense resistor
voltage. There is an internal resistor, 10 k to ground, on the Ω
SENSE pin. The SENSE pin senses voltage pulses, which
communicate fan operation to the TC647B. If pulses are not
detected by the SENSE pin for one second, a fault condition is
indicated by the TC647B.
The temperature can easily be measured with an economic
solution, such as a thermistor. The thermistor is fast, small,
requires a two-wire interface and has a wide range of outputs.
As an added benefit, the layout flexibility is enhanced by being
able to place the thermistor remote from the TC647B. Although
thermistors are non-linear, they can be linearized over a
smaller temperature range (±25°C) with the circuits shown
in Figure 3. This linearization and level shifting is done using
standard, 1% resistors.
Although temperature proportional fan speed control and
fan fault detection for two-wire fans can be implemented in
a discrete circuit or the microcontroller version, it requires a
degree of attention from the designer. The TC647B is a switch
mode two-wire brushless DC fan speed controller. Pulse Width
Modulation (PWM) is used to control the speed of the fan in
relation to the thermistor temperature. Minimum fan speed is
set by a simple resistor divider on V
MIN. An integrated Start-up
Timer ensures reliable motor start-up at turn-on, coming out
of shutdown mode or following a transient fault with auto-fan
restart capability.
Figure 2. The fan response (across R
SENSE) to the PWM signal
at VOUT, is shown in the bottom trace. The capacitively
coupled signal to the SENSE pin of the TC647B is
shown in the top trace.
The TC647B also uses Microchip’s FanSense
™ technology,
which improves system reliability. All of these features included
in a single chip, gives the designer a leg up in a single fan
implementation.
A) Parallel resistor (2 terminal):
B) Series resistor (3 terminal):
C) Series / parallel resistor (3 terminal):
Best suited for gain dependent circuit
Best suited for voltage divider circuit
Voltage divider circuit with a voltage
level shift
RNTC R1RNTC =RNTC x R1
(RNTC + R1)
VOUT =VDD x R1
(RNTC + R1)
VOUT =VDD x R2
(RNTCIIR1 + R2)
RNTC
VDD
VOUT
R1
R2
RNTC
VDD
V
OUT
R1
Figure 3. A thermistor can be linearized over 50°C with a
standard resistor (A and B), as well as level shifted
(C) to match the input requirements of the TC647B.
For more information, please visit www.microchip.com
The Microchip name and logo are registered trademarks of Microchip Technology
Inc. in the U.S.A. and other countries. FanSense is a trademark of Microchip
Technology Inc. in the U.S.A. © 2003 Microchip Technology Inc.

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: MCP3201

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