TC642
DS21444D-page 6 2001-2012 Microchip Technology Inc.
The second condition by which the TC642 detects a
fault is when the PWM control voltage applied to V
IN
becomes greater than that needed to drive 100% duty
cycle (see Section 1.0, “Electrical Characteristics”).
This indicates that the fan is at maximum drive and the
potential exists for system overheating. Either heat dis-
sipation in the system has gone beyond the cooling
system’s design limits or some subtle fault exists (such
as fan bearing failure or an airflow obstruction). This
output may be treated as a system overheat warning
and be used to trigger system shutdown. However, in
this case, the fan will continue to run even when FAULT
is asserted. If a shutdown is desired, FAULT may be
connected to V
MIN
outside the device. This will latch the
TC642 in shutdown mode when any fault occurs.
3.3 V
OUT
Output
The V
OUT
pin is designed to drive a low cost transistor
or MOSFET as the low side power switching element
in the system. Various examples of driver circuits will
be shown throughout this data sheet. This output has
asymmetric complementary drive and is optimized for
driving NPN transistors or N-channel MOSFETs. Since
the system relies on PWM rather than linear control,
the power dissipation in the power switch is kept to a
minimum. Generally, very small devices (TO-92 or
SOT packages) will suffice.
3.4 Start-Up Timer
To ensure reliable fan start-up, the Start-up Timer turns
the V
OUT
output on for 32 cycles of the PWM whenever
the fan is started from the off state. This occurs at
power-up and when coming out of shutdown mode. If
the PWM frequency is 30 Hz (C
F
= 1 µF), the resulting
start-up time will be approximately one second. If a
fault is detected, the Diagnostic Timer is triggered
once, followed by the Start-up Timer. If the fault per-
sists, the device is shut down (see Section 3.2, “FAULT
Output”).
3.5 Shutdown Control (Optional)
If V
MIN
(Pin 3) is pulled below V
SHDN
, the TC642 will go
into shutdown mode. This can be accomplished by
driving V
MIN
with an open-drain logic signal or by using
an external transistor, as shown in Figure 3-1. All func-
tions are suspended until the voltage on V
MIN
becomes
higher than V
REL
(0.85V @ V
DD
= 5.0V). Pulling V
MIN
below V
SHDN
will always result in complete device
shutdown and reset. The FAULT
output is
unconditionally inactive in shutdown mode.
A small amount of hysteresis, typically one percent of
V
DD
(50 mV at V
DD
= 5.0V), is designed into the V
SHDN
and
V
REL
thresholds. The levels specified for V
SHDN
and V
REL
in Section 1.0, “Electrical Characteristics”,
include this hysteresis, plus adequate margin to
account for normal variations in the absolute value of
the threshold and hysteresis.
3.6 SENSE Input
(FanSense
™
Technology)
The SENSE input (Pin 5) is connected to a low value
current sensing resistor in the ground return leg of the
fan circuit. During normal fan 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 differen-
tiated and converted 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 operation (see Section 5.0, “Typ-
ical Applications”, for more details).
CAUTION: Shutdown mode is unconditional. That is,
the fan will not be activated regardless of the voltage
at V
IN
. The fan should not be shut down until all heat
producing activity in the system is at a negligible level.
