 2003 Microchip Technology Inc.

DS21688C-page 7

TC670

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

be a worst-case rating and not the fan’s nominal oper-

ating current. 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

A 0.1 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 2.0, “Typical Performance

Curves”, the relationship between the threshold volt-

age 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 cor-

rectly, 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 2.5 mA (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 sink current of this pin is 25 mA, it is

recommended that the current sinking into the ALERT

output does not exceed 20 mA.

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

being 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 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