2002-2012 Microchip Technology Inc. DS21446D-page 7
TC646
FIGURE 3-1: TC646 Nominal Operation.
3.7 Shutdown Mode (Reset)
If an unconditional shutdown and/or device reset is
desired, the TC646 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
FAULT
output is unconditionally inactive. The TC646
should not be shut down unless all heat producing
activity in the system is at a negligible level. The TC646
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 TC646 into its
power-up state. The Start-up and Fault Timers, and any
current faults, are cleared. FAULT
is unconditionally
inactive in shutdown mode. Upon exiting shutdown
mode (V
IN
> V
REL
), the Start-up Timer will be triggered
and normal operation will resume, assuming no fault
conditions exist and V
IN
> V
AS
+ V
HAS
If a fan fault has occurred and the device has latched
itself into shutdown mode, performing a reset will not
clear the fault unless 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. Consequently, there
is no way to establish that the fan fault has been
cleared. 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 TC646
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
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.
2.6V
V
AS
+
V
HAS
V
IN
t
RESET
V
REL
V
SHDN
V
AS
1.2V
TC646
Status
Normal
Operation
Auto-Shutdown
Mode
Normal
Operation
Shut-
Down
Normal
Operation
GND
Time
TEMP.
HI
LO
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
.
