TC670

DS21688D-page 6

 2001-2012 Microchip Technology Inc.

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.

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.

FIGURE 4-2:

Typical Application Circuit.

4.1

SENSE Input

As shown in Figure 4-2, the SENSE input (Pin 6) is

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

nated with the internal 50 k

 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.

THRESHOLD

124 mV

50 k



SENSE

ALERT

CLEAR

Logic

Frequency-to-

Voltage

GND

Bandgap

Oscillator

Note: This typical application circuit uses a LED to

indicate that a fan failure has occurred.

5

6

+12V

ALERT

LED

DC

FAN

DC

FAN

SENSE

ALERT

2

4

3

1

+5V

0.1 µF

CLEAR

THRESHOLD

GND