TC648
DS21448D-page 6 2001-2012 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
pin. A frequency of 30 Hz is recommended for
most applications (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
approximately one second.
3.4 Over-Temperature Fault (OTF)
Output
OTF is asserted when the PWM control voltage applied
to V
IN
becomes greater than that needed to drive 100%
duty cycle (see Section 1.0, “Electrical Characteris-
tics”). 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 subtle fault
exists (such as fan bearing failure or an airflow obstruc-
tion). This output may be treated as a “System Over-
heat” warning and used to trigger system shutdown or
some other corrective action. OTF
will become inactive
when V
IN
< V
OTF
.
3.5 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 TC648 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 V
AS
input should be grounded if auto-
shutdown mode is not used.
3.6 Shutdown Mode (Reset)
If an unconditional shutdown and/or device reset is
desired, the TC648 may be placed in shutdown mode
by forcing V
IN
to a logic low (i.e., V
IN
< V
SHDN
) (see
Figure 3-1). In this mode, all functions cease and the
OTF
output is unconditionally inactive. The TC648
should not be shut down unless all heat producing
activity in the system is at a negligible level. The TC648
exits shutdown mode when V
IN
becomes greater than
V
REL
, the release voltage.
Entering shutdown mode also performs a complete
device reset. Shutdown mode resets the TC648 into its
power-up state. OTF
is unconditionally inactive in shut-
down mode. Upon exiting shutdown mode (V
IN
>
V
REL
), the Start-up Timer will be triggered and normal
operation will resume, assuming V
IN
> V
AS
+ V
HAS
If V
IN
is not greater than (V
AS
+ V
HAS
) upon exiting
shutdown mode, the fan will not be restarted. To ensure
that a complete reset takes place, the user’s circuitry
must ensure that V
IN
> (V
AS
+ V
HAS
) when the device
is released from shutdown mode. A recommended
algorithm for management of the TC648 by a host
microcontroller or other external circuitry is given in
Section 5.0, “Typical Applications”. 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
thresh-
old. 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.
Note: If V
IN
< V
AS
when the device exits shutdown
mode, the fan will not restart as it will be in auto-shut-
down mode.
CAUTION: Shutdown mode is unconditional. That is,
the fan will remain off as long as the V
IN
pin is being
held low or V
IN
< V
AS
+ V
HAS
.
