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TC652/TC653
3.2
Start-Up Timer
3.6
Shutdown (SHDN)
To ensure reliable fan start-up, the Start-up Timer turns
PWM high for about 2 seconds whenever the fan is
started from the off state. This occurs at power-up and
when coming out of Shutdown mode.
The fan can be unconditionally shutdown by pulling low
the SHDN pin. During shutdown, FAULT output is high
and PWM output is low. This is ideal for notebook
computers and other portable applications when you
need to change batteries and must not have the fan
3.3
SENSE Input
running at that time. Thermal monitoring and T OVER are
(FanSense? Technology)
The SENSE input, Pin 5, is connected to a low value
still in operation during shutdown. I DD shutdown current
is around 50 μ A.
current sensing resistor in the ground return leg of the
fan circuit through the capacitor. During normal fan
3.7
Auto-Shutdown Mode
operation, commutation occurs as each pole of the fan
is energized. This causes brief interruptions in the fan
current, seen as pulses across the sense resistor. If the
device is not in Shutdown mode, and pulses are not
appearing at the SENSE input, a FAULT exists. The
short, rapid change in fan current (high di/dt) causes a
corresponding dv/dt across the sense resistor, R SENSE .
The waveform on R SENSE is differentiated and con-
verted to a logic-level pulse-train by C SENSE and the
internal signal processing circuitry. The presence and
frequency of this pulse-train is a direct indication of fan
The TC653 has auto-shutdown. If the temperature is
below the factory set point at minimum speed (T L ),
PWM is low and the fan is automatically shut off (Auto-
shutdown mode). This feature is ideal for notebook
computers and other portable applications that need to
conserve as much battery power as possible and thus
run a fan when it is only absolutely needed. The TC653
will continue to be active so as to monitor temperature
for T OVER . The TC653 exits Auto-shutdown mode
when the temperature rises above the factory set point
(T 1 ).
operation.
3.8
Temperature Selection Guide
3.4
FAULT
(Minimum Fan Speed/Full Speed)
This pin goes low to indicate a fan FAULT condition.
Pulses appearing at SENSE pin due to the PWM turn-
ing on are blanked and the remaining pulses are
filtered by a Missing Pulse Detector. If consecutive
pulses are not detected for 32 PWM cycles (about 2
sec), the PWM is Low and FAULT goes low. FAULT can
be disabled by momentarily toggling SHDN or V DD pin,
or cycling system power. FAULT remains high during
Shutdown mode.
The five temperature regions defined by the six
thresholds are defined in the TC652/TC653 by means
of factory trimming. Once a T L and T H are set, the T 1 –
T 4 thresholds are automatically equally spaced
between T L and T H .
3.5
Over-Temperature Alert (T OVER )
This pin goes low when the T H set point is exceeded
by 10°C (typical). This indicates that the fan is at
maximum drive, and the potential exists for system
overheating: either heat dissipation in the system has
gone beyond the cooling system's design limits, or
some FAULT exists such as fan bearing failure or an
airflow obstruction. This output may be treated as a
“System Overheat” warning and used to trigger system
shutdown, or bring other fans to full speed in the
system. The fan will continue to run at 100% speed
while T OVER is asserted. Built-in hysteresis prevents
T OVER from “chattering” when measured temperature
is at or near the T H + 10°C trip point. As temperature
falls through the T H + 10°C trip point, hysteresis
maintains the T OVER output low until measured
temperature is 5°C above the trip point setting.
DS21453B-page 6
2002 Microchip Technology Inc.
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