TC646
DS21446D-page 6 2002-2012 Microchip Technology Inc.
3.0 DETAILED DESCRIPTION
3.1 PWM
The PWM circuit consists of a ramp generator and
threshold detector. The frequency of the PWM is
determined by the value of the capacitor connected to
the C
F
input. A frequency of 30 Hz is recommended
(C
F
= 1 µF). The PWM is also the time base for the
Start-up Timer (see Section 3.3, “Start-Up Timer”). The
PWM voltage control range is 1.25V to 2.65V (typical)
for 0% to 100% output duty cycle.
3.2 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.
3.3 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 or auto-
shutdown mode. If the PWM frequency is 30 Hz
(C
F
= 1 µF), the resulting start-up time will be approxi-
mately one second. If a fan fault is detected, the Diag-
nostic Timer is triggered once, followed by the Start-up
Timer. If the fault persists, the device is shut down (see
Section 3.5, “F
AULT Output”).
3.4 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
auto-shutdown or 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 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,
“Typical Applications”, for more details.
3.5 FAULT Output
The TC646 detects faults in two ways:
First, pulses appearing at SENSE due to the PWM
turning on are blanked, with the remaining pulses being
filtered by a missing pulse detector. If consecutive
pulses are not detected for thirty-two PWM cycles
(1Sec if C
F
= 1 µF), the Diagnostic Timer is activated
and V
OUT
is driven high continuously for three PWM
cycles (100 msec if C
F
= 1 µF). If a pulse is not
detected within this window, the Start-up Timer is trig-
gered (see Section 3.3, “Start-up Timer”). This should
clear a transient fault condition. If the missing pulse
detector times out again, the PWM is stopped and
FAULT
goes low. When FAULT is activated due to this
condition, the device is latched in shutdown mode and
will remain off indefinitely. Therefore, the TC646 is pre-
vented from attempting to drive a fan under cata-
strophic fault conditions.
One of two things will restore operation: Cycling power
off and then on again or pulling V
IN
below V
SHDN
and
releasing it to a level above V
REL
. When one of these
two conditions is satisfied, the normal start-up cycle is
triggered and operation will resume if the fault has been
cleared.
The second condition by which the TC646 asserts 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 or some other
corrective action.
However, in this case, the fan will continue to run even
when FAULT
is asserted. If the system is allowed to
continue operation, and the temperature (and thus V
IN
)
falls, the FAULT
output will become inactive when V
IN
< V
OTF
.
3.6 Auto-Shutdown Mode
If the voltage on V
IN
becomes less than the voltage on
V
AS
, the fan is automatically shut off (auto-shutdown
mode). The TC646 exits auto-shutdown mode when
the voltage on V
IN
becomes higher than the voltage on
V
AS
by V
HAS
(the auto-shutdown Hysteresis Voltage
(see Figure 3-1)). The Start-up Timer is triggered and
normal operation is resumed upon exiting auto-shut-
down mode. The FAULT
output is unconditionally
inactive in auto-shutdown mode.
