TC670 View Datasheet(PDF) - Microchip Technology
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TC670 Datasheet PDF : 14 Pages
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TC670
4.0 DETAILED DESCRIPTION
The TC670 is an integrated fan speed sensor that
predicts/detects fan failure, consequently preventing
thermal damage to systems with cooling fans. When
the fan speed falls below a user-programmed threshold
level, the TC670 asserts an ALERT signal. This
threshold is set with an external resistor divider
network.
VDD
THRESHOLD
Logic
CLEAR
ALERT
GND
Frequency-to-
Voltage
50 kΩ
SENSE
Bandgap Oscillator 124 mV
FIGURE 4-1:
TC670 Block Diagram.
As shown in Figure 4-1, the TC670 senses the fan
pulses and internally converts those pulses from a
frequency into an analog voltage. This voltage is then
compared with the DC voltage at the THRESHOLD pin.
If the converted frequency-to-voltage value from the
fan's pulses falls below the threshold voltage, the
ALERT output is pulled low.
In a 3.0V system, the external fan alert level on the
THRESHOLD pin can be designed from 0.0V (stalled
fan) to 2.4V (for 13,000 RPM) to cover most of the com-
mon fan speeds. This failure detection system works
with linear-controlled 2-wire fans and eliminates the
need for 3-wire fans. The TC670 can also work with
3-wire fans either by using the SENSE circuit or by
directly sensing the RPM output from the 3rd wire.
A CLEAR pin is provided to allow the user to reset the
ALERT pin status back to a high state. This clear option
also allows the flexibility of connecting the ALERT out-
put of the TC670 with other alert/fault interrupts in the
system without having a risk of a system shutdown due
to false fan fault condition.
+5V
4 VDD
ALERT
LED
0.1 µF
3 CLEAR
R3
ALERT 5
R4
+12V
DC
1
THRESHOLD
SENSE
6
CSENSE FFAANN
R2
GND
RSENSE
2
Note: This typical application circuit uses a LED to
indicate that a fan failure has occurred.
FIGURE 4-2:
Typical Application Circuit.
4.1 SENSE Input
As shown in Figure 4-2, the SENSE input (Pin 6) is
connected to the sense resistor (RSENSE) through a
capacitor (CSENSE). The low value current sensing
resistor (RSENSE) is connected between the ground
return leg of the fan and the fan bias ground. During
normal fan operation, commutation occurs as each
pole of the fan is energized. This causes the fan current
to be an AC waveform with fast falling edges.
These short, rapid changes in fan current cause a
corresponding dV/dt voltage across the sense resistor,
as well as a corresponding dI/dt current through the
sense capacitor. The current through CSENSE is termi-
nated with the internal 50 kΩ input resistance at the
SENSE pin of the TC670. When positive-going fan
pulses at the SENSE input are greater than 124 mV
(typ.), the TC670 latches-in those voltage spikes. This
124 mV (typ.) SENSE input built-in threshold reduces
false triggering errors caused by extraneous noise
pulses associated with a running fan. The presence
and frequency of these pulses is a direct indication of
fan operation and fan speed.
DS21688C-page 6
2003 Microchip Technology Inc.
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