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AN767
DS00767A-page 1
© 2002 Microchip Technology, Inc.
Interfacing Microchip's Fan Speed Controllers to a SPI
™
Port
INTRODUCTION
Microchip's TC642, TC643, and TC646 are the world's first inte-
grated circuits dedicated for controlling and monitoring fan speed.
The new family of fan speed controllers modulate fan speed to
compensate for changes in system temperature. This means the
fan runs at full speed only when necessary, significantly extending
fan life. In addition to increased fan life, fan speed is controlled by
PWM control circuitry that is more efficient than conventional linear
techniques. The ICs provide other features such as fan current
limiting, minimum speed control, auto shutdown, and speed, fault,
and over-temperature indication.
These ICs can be used in a stand-alone configuration if interfaced
with other support/control ICs. This application note discusses
ways in which these devices can be controlled over a SPI
™
port.
Author: Paul Paglia,
Microchip Technology, Inc.
FIGURE 1: TC642/646 in standalone configuration.
SPI is a trademark of Motorola, Inc.
STANDARD IMPLEMENTATION
The TC642, TC643, and TC646 use two different methodologies to
control fan speed. The TC642/646 provide a standalone solution
whereas the TC643 requires an external drive signal and RPM
sense circuitry. The TC642/646 are designed to provide speed
control for a power supply cooling fan. The ICs are driven by an
analog voltage, usually provided by a local thermistor, that is
proportional to system temperature.The fan speed increases with
increasing input voltage. In such a configuration, these ICs provide
a self-contained, hardware-based solution whose only interface to
the digital world is via fan fault and overtemp logic flags (see
Figure 1).
Using a different approach, the TC643 was designed to interface
with an ASIC or microcontroller that receives temperature data
from one or several remote sensors. The ASIC or microcontroller
generates the PWM drive signal for the TC643 based upon the
system's thermal algorithm. The TC643 monitors fan speed and
provides an RPM output that can be monitored by the ASIC or
microcontroller (see Figure 2). By monitoring the RPM output, early
signs of fan degradation (bearing wear) can be detected, allowing
corrective action to be taken prior to a catastrophic failure. The
TC643 also has built-in current limit circuitry in case a catastrophic
event should take place.
Fan
To
Microcontroller
12V
TC642/646
FAULT
V
OUT
SENSE
GND
TC642/646
5V
5V
NTC
10kΩ
@
25°C
F
A
N
Power
Supply
Main System
Board
Cable
Harness
V
DD
V
IN
V
MIN
/V
AS
C
F
R1
R2
C3
0.01µF
R3
10kΩ
C1
1µF
R6
4.7Ω
2N2222A
C2
0.1µF
R5
1.1kΩ
12V @
100mA
+5V
R7
10kΩ
R4
10kΩ
