Datasheet

 2001-2012 Microchip Technology Inc. DS21448D-page 7
TC648
FIGURE 3-1: TC648 Nominal Operation.
4.0 SYSTEM BEHAVIOR
The flowcharts describing the TC648’s behavioral
algorithms 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 shut-
down or auto-shutdown 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) Branch to Normal Operation.
(4) End.
4.2 Normal Operation
Normal Operation is an endless loop which may only
be exited by entering shutdown or auto-shutdown
mode. The loop can be thought of as executing at the
frequency of the oscillator and PWM.
(1) Drive V
OUT
to a duty cycle proportional to V
IN
on a
cycle by cycle basis.
(2) If an over-temperature fault occurs, (V
IN
> V
OTF
),
activate OTF
; release OTF when V
IN
< V
OTF
.
(3) Is the TC648 in shutdown or auto-shutdown
mode?
If so.....
a. V
OUT
duty cycle goes to zero.
b. OTF
is disabled.
c. Exit the loop and wait for V
IN
> (V
AS
+ V
HAS
),
then execute Power-up sequence.
(4) End.
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