TC649B datasheet(14/36 Pages) MICROCHIP

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TC646B/TC648B/TC649B

DS21755B-page 14

 2003 Microchip Technology Inc.

For the TC646B and TC648B devices, an over-temper-

ature condition is indicated 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 overheating situation can occur.

When the voltage at the VIN input falls below the VOTF

threshold voltage by the hysteresis value (VOTF-HYS),

the FAULT/OTF output will return to the high state (a

pull-up resistor is needed on the FAULT/OTF output).

For the TC646B/TC649B devices, 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, nor-

mal PWM operation is resumed and no fan fault is indi-

cated. 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-

period, 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. Therefore, The FAULT output is

released and the VOUT output returns to normal PWM

operation, as dictated by the VIN and VAS inputs.

If the VIN voltage is pulled below the VSHDN level during

a fan fault condition, the FAULT output will be released

and the VOUT output will be shutdown (VOUT = 0V). If

the VIN voltage then increases above the VREL thresh-

old and is above the VAS voltage, 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 VAS voltage level, the TC646B/

TC649B 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 TC646B/TC649B device will enter

auto-shutdown mode until the VIN voltage is brought

above the VAS voltage by the auto-shutdown hysteresis

value (VHAS). The TC646B/TC649B device will then

resume normal PWM mode operation.

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 (the TC648B device does not incor-

porate the fan sensing feature). It does this by sensing

fan current pulses that represent fan rotation. When a

fan rotates, commutation of the fan current occurs as

the fan poles pass the armatures of the motor. The

commutation of the fan current makes the current

waveshape appear as pulses. There are two typical

current waveforms of brushless DC fan motors,

illustrated in Figures 4-6 and 4-7.

FIGURE 4-6:

Fan Current With DC Offset

And Positive Commutation Current.

FIGURE 4-7:

Fan Current With

Commutation Pulses To Zero.

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