Datasheet

TC646
DS21446D-page 8  2002-2012 Microchip Technology Inc.
4.0 SYSTEM BEHAVIOR
The flowcharts describing the TC646’s behavioral
algorithm are shown in Figure 4-1. They can be
summarized as follows:
4.1 Power-Up
(1) Assuming the device is not being held in auto-shut-
down mode (V
IN
> V
AS
)..........
(2) Turn V
OUT
output on for 32 cycles of the PWM
clock. This ensures that the fan will start from a
dead stop.
(3) During this Start-up Timer, if a fan pulse is
detected, branch to Normal Operation; if none are
received…
(4) Activate the 32-cycle Start-up Timer one more time
and look for fan pulse; if a fan pulse is detected,
proceed to Normal Operation; if none are
received…
(5) Proceed to Fan Fault.
(6) End.
4.2 Normal Operation
“Normal Operation” is an endless loop which may only
be exited by entering shutdown mode, auto-shutdown
mode or Fan Fault. The loop can be thought of as
executing at the frequency of the oscillator and PWM.
(1) Reset the missing pulse detector.
(2) Is the TC646 in shutdown or auto-shutdown
mode?
If so...
a. V
OUT
duty cycle goes to zero.
b. FAULT
is disabled.
c. Exit the loop and wait for V
IN
> (V
AS
+ V
HAS
) to
resume operation.
(3) If an over-temperature fault occurs (V
IN
> V
OTF
),
activate FAULT
; release FAULT when
V
IN
< V
OTF
.
(4) Drive V
OUT
to a duty cycle proportional to V
IN
on
a cycle by cycle basis.
(5) If a fan pulse is detected, branch back to the
start of the loop (1).
(6) If the missing pulse detector times out …
(7) Activate the 3-cycle Diagnostic Timer and look
for pulses; if a fan pulse is detected, branch
back to the start of the loop (1); if none are
received…
(8) Activate the 32-cycle Start-up Timer and look for
pulses; if a fan pulse is detected, branch back to
the start of the loop (1); if none are received…
(9) Quit Normal Operation and go to Fan Fault.
(10) End.
4.3 Fan Fault
Fan fault is an infinite loop wherein the TC646 is
latched in shutdown mode. This mode can only be
released by a reset (i.e., V
IN
being brought below
V
SHDN
, then above (V
AS
+ V
HAS
), or by power-cycling).
(1) While in this state, FAULT
is latched on (low) and
the V
OUT
output is disabled.
(2) A reset sequence applied to the V
IN
pin will exit the
loop to Power-up.
(3) End.