TC670

DS21688B-page 6

© 2002 Microchip Technology Inc.

The design of the proper input SENSE circuitry is a

amount of gain and proper selection of the sensing

capacitor. The following table (Table 4-1) lists some

nominal operating current of the fan. Please note that

the current draw specified by the fan manufacturer may

not be the fan’s nominal operating current, but a worst-

case rating. If the fan current falls between two of the

values listed, it is recommended that the higher value

resistor is used.

TABLE 4-1:

RECOMMENDED VALUES

A0.1

µF ceramic capacitor is recommended for

sense resistors whereas larger capacitors are more

expensive and occupy more board space.

4.2

THRESHOLD Input

The voltage at the THRESHOLD input sets the

equivalent minimum allowable fan speed for the appli-

cation. As shown in Section 3.0 typical performance

curves, the relationship between the THRESHOLD

voltage and minimum fan speed is also power supply

and temperature dependant.

All the values for the THRESHOLD voltage that are

shown in these graphs represent typical numbers and

might not be optimized for all fans in all applications. To

ensure accurate fan speed monitoring of a specific fan

in a specific application, the user must perform a one-

time correlation check with the prototype.

There are two techniques that can be used to calibrate

the system. One approach is to find the fan’s full scale

capability and mathematically estimate the minimum

acceptable speed of the fan. A second technique is to

identify the fan’s minimum speed and calibrate the

THRESHOLD voltage accordingly.

4.2.1

THRESHOLD CALIBRATION USING

FAN’S FULL SCALE SPEED

The fan should first be run at full speed. At full speed

the THRESHOLD voltage level should be adjusted until

the ALERT output is asserted. With this full scale value

of the THRESHOLD voltage, the value can be scaled

down to the Fan Fault speed as a percentage of the full

speed. For example, if the fan full speed THRESHOLD

voltage is 1.5V, then the Fan Fault THRESHOLD

voltage at 30% of full speed would be 30% x 1.5V =

0.45V.

4.2.2

THRESHOLD CALIBRATION USING

FAN’S MINIMUM ALLOWABLE

SPEED ESTIMATE

For a more exact Fan Fault trip point, the user can run

the fan at its minimum allowed speed. At this speed, the

THRESHOLD voltage can be adjusted until the ALERT

output is asserted.

4.3

CLEAR Input

The CLEAR input allows the user to reset the ALERT

pin to a high status. This is an active HIGH input.

Consequently, as long as CLEAR is HIGH, ALERT will

always be HIGH as well. To allow ALERT to operate

correctly CLEAR must be held LOW. This feature can

be implemented so that false Fan Fault conditions do

not initiate system shutdown.

4.4

ALERT Output

The ALERT output is an open drain output capable of

sinking 2.5mA (typ). The ALERT output is asserted

whenever the detected fan speed equals or falls below

the equivalent voltage set at the THRESHOLD pin. The

ALERT output is only deactivated once the CLEAR pin

is brought to a HIGH state. Although the absolute

maximum output current of this pin is 25mA, it is

recommended that this current sinking into the ALERT

Output does not exceed 20mA.

4.5

To assure proper operation of the TC670 in a noisy

(Pin 4) must be decoupled with a 0.1

µF capacitor as

shown in Figure 4-1. This capacitor should be located

promptly terminated to the ground plane. A Ceramic

capacitor is recommended.

4.6

Ground Terminal, GND

The GND pin (Pin 2) of the TC670 should be connected

directly to the analog ground plane of the circuit board.

Care should be taken in circuit layout to keep this pin

away from switching signals, such as the fan excitation

signals in order to avoid false signals on the SENSE

pin.

Nominal Fan Current

(mA)

100

4.7

200

2.4

300

1.8

400

1.3

500

1.0

600

0.8