TC642BEPATR datasheet(14/34 Pages) MICROCHIP

14 / 34 page

TC642B/TC647B

DS21756B-page 14

 2003 Microchip Technology Inc.

FIGURE 4-5:

Clamp Diode for Fan.

4.7

FAULT Output

The FAULT output is an open-drain, active-low output.

For the TC642B and TC647B devices, the FAULT out-

put indicates when a fan fault condition has occurred.

For the TC642B device, the FAULT output also indi-

cates when an over-temperature (OTF) condition has

occurred.

For the TC642B device, an over-temperature condition

is indicated (FAULT output is pulled low) when the VIN

input reaches the VOTF threshold voltage (the VOTF

threshold voltage is typically 20 mV higher than the

VCMAX threshold and has 80 mV of hysteresis). This

indicates that maximum cooling capacity has been

reached (the fan is at full speed) and that an overheat-

ing situation can occur. When the voltage at the VIN

input falls below the VOTF threshold voltage by the hys-

teresis value (VOTF-HYS), the FAULT output returns to

the high-state (a pull-up resistor is needed on the

FAULT output).

A fan fault condition is indicated when fan current

pulses are no longer detected at the SENSE pin.

Pulses at the SENSE pin indicate that the fan is

spinning and conducting current.

If pulses are not detected at the SENSE pin for 32 PWM

cycles, the 3-cycle diagnostic timer is fired. This means

that the VOUT output is high for 3 PWM cycles. If pulses

are detected in this 3-cycle period, then normal PWM

operation is resumed and no fan fault is indicated. If no

pulses are detected in the 3-cycle period, the start-up

timer is activated and the VOUT output is driven high for

32 PWM cycles. If pulses are detected during this time-

frame, normal PWM operation is resumed. If no pulses

are detected during this time frame, a fan fault condition

exists and the FAULT output is pulled low.

During a fan fault condition, the FAULT output will

remain low until the fault condition has been removed.

During this time, the VOUT output is driven high contin-

uously to attempt to restart the fan, and the SENSE pin

is monitored for fan pulses. If a minimum of 16 pulses

are detected at the SENSE input over a 32 cycle time

period (one second for CF = 1.0 µF), the fan fault con-

dition no longer exists. The FAULT output is then

released and the VOUT output returns to normal PWM

operation, as dictated by the VIN and VMIN inputs.

If the VMIN voltage is pulled below the VSHDN level dur-

ing a fan fault condition, the FAULT output will be

released and the VOUT output will be shutdown

(VOUT = 0V). If the VMIN voltage then increases above

the VREL threshold, the device will go through the

normal start-up routine.

If, during a fan fault condition, the voltage at the VIN pin

drops below the VMIN voltage level, the TC642B/

TC647B device will continue to hold the FAULT line low

and drive the VOUT output to 100% duty cycle. If the fan

fault condition is then removed, the FAULT output will

be released and the VOUT output will be driven to the

duty cycle that is being commanded by the VMIN input.

The sink current capability of the FAULT output is listed

in the “Electrical Characteristics” table of Section 1.0.

4.8

Sensing Fan Operation (SENSE)

The SENSE input is an analog input used to monitor

the fan’s operation. It does this by sensing fan current

pulses, which represent fan rotation. When a fan

rotates, commutation of the fan current occurs as the

fan poles pass the armatures of the motor. The commu-

tation of the fan current makes the current waveshape

appear as pulses. There are two typical current wave-

forms of brushless DC fan motors. These are shown in

Figures 4-6 and 4-7.

FIGURE 4-6:

Fan Current With DC Offset

And Positive Commutation Current.

GND

RSENSE

VOUT

Q1: N-Channel MOSFET

FAN

Clamp Diode

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