2001-2012 Microchip Technology Inc. DS21447D-page 5
TC647
3.0 DETAILED DESCRIPTION
3.1 PWM
The PWM circuit consists of a ramp generator and
threshold detector. The frequency of the PWM is deter-
mined 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 an
asymmetric complimentary 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 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
persists, the device is shut down (see Section 3.6,
“FAULT
Output”).
3.4 Shutdown Control (Optional)
If V
MIN
(Pin 3) is pulled below V
SHDN
, the TC647 will go
into shutdown mode. This can be accomplished by
driving V
MIN
with an open-drain logic signal or using an
external transistor, as shown in Figure 3-1. All functions
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
/
V
REL
threshold. 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.
3.5 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
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.
3.6 FAULT Output
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 32 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 triggered (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.
The TC647 may be configured to continuously attempt
fan restarts, if so desired.
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.
Note: At this point, action must be taken to restart
the fan by momentarily pulling V
MIN
below
V
SHDN
, or cycling system power. In either
case, the fan cannot remain disabled due
to a fault condition as severe system dam-
age could result. If the fan cannot be
restarted, the system should be shut down.
