Datasheet

TC642B/TC647B
DS21756C-page 14  2002-2013 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 V
IN
input reaches the V
OTF
threshold voltage (the V
OTF
threshold voltage is typically 20 mV higher than the
V
CMAX
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 V
IN
input falls below the V
OTF
threshold voltage by the hys-
teresis value (V
OTF-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 V
OUT
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 V
OUT
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 V
OUT
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 C
F
= 1.0 F), the fan fault con-
dition no longer exists. The FAULT
output is then
released and the V
OUT
output returns to normal PWM
operation, as dictated by the V
IN
and V
MIN
inputs.
If the V
MIN
voltage is pulled below the V
SHDN
level dur-
ing a fan fault condition, the FAULT
output will be
released and the V
OUT
output will be shutdown
(V
OUT
= 0V). If the V
MIN
voltage then increases above
the V
REL
threshold, the device will go through the
normal start-up routine.
If, during a fan fault condition, the voltage at the V
IN
pin
drops below the V
MIN
voltage level, the TC642B/
TC647B device will continue to hold the FAULT
line low
and drive the V
OUT
output to 100% duty cycle. If the fan
fault condition is then removed, the FAULT
output will
be released and the V
OUT
output will be driven to the
duty cycle that is being commanded by the V
MIN
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.
Q
1
GND
R
SENSE
V
OUT
Q
1
: N-Channel MOSFET
FAN
Clamp Diode