© 2002 Microchip Technology Inc.

DS21688B-page 5

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.

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 a DC voltage present on the THRESH-

OLD pin. If the converted frequency-to-voltage value

from the fan’s pulses falls below the THRESHOLD

voltage, a FAULT signal is asserted through the ALERT

pin (active LOW).

In a 3.0V system, the external fan alert level on the

THRESHOLD pin can be designed from 0.0V (stalled

fan) and up to 2.4V (for 13,000 RPM) to cover most of

the common fan speeds. This failure detection system

works with linear controlled 2-wire fans and eliminates

the need for 3-wire fans. The TC670 can work with 3-

wire fans as well 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 output 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

connected to the low side of the fan to the ground return

leg of the fan. During normal fan operation, commuta-

tion occurs as each pole of the fan is energized. This

causes brief interruptions in the fan current, seen as

pulses across the sense resistor.

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 across the

nated with the internal 50k

Ω input resistance at the

SENSE pin of the TC670. When positive going fan

pulses at the SENSE input are greater than 124mV

(typ) the TC670 latches in those voltage spikes. This

124mV (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

124mV

50k

Ω

SENSE

ALERT

CLEAR

Logic

Frequency to

Voltage

GND

Bandgap

Oscillator

From

Microcontroller

5

6

+12V

ALERT

LED

DC

FAN

DC

FAN

SENSE

ALERT

2

4

3

1

0.1 µF

CLEAR

THRESHOLD

GND

V

TH RESHO LD

V

DD

R

2

R

2

R

3

+

----------------------







=

Note: This typical application circuit uses an LED to indicate that a

fan failure has occurred.