TC649 Microchip, TC649 Datasheet - Page 11

• Manufacturer Part Number: TC649
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

FIGURE 5-3:
Per Section 1.0, “Electrical Characteristics”, the leak-
age current at the V
conservative to design for a divider current, I
100 µA. If V
EQUATION
We can further specify R
the divider voltage is equal to our desired V
yields the following:
EQUATION
Solving for the relationship between R
in:
EQUATION
In the case of this example, R
this relationship back into the V
yields the resistor values:
In this case, the standard values of 34.8 k
15.4 k
would be more than adequate.
2002 Microchip Technology Inc.
I
R
DIV
1
are very close to the calculated values and
R
= R
1
= 1e
DD
+ R
2
x
= 5.0V then:
– 4
2
V
R
R
I
AS
DIV
=
A
2
1
V
DD
= 15.3 k , and
= 34.7 k
=
V
AS
V
GND
1e
DD
5.0V
AS
- V
R
V
–4
V
pin is no more than 1 µA. It is
1
AS
R
R 2
R 1
1
5.0V
AS
A
DD
+ R
1
and R
+ R
Circuit.
V
x R
= R
= 50,000 = 50 k
1
2
AS
= (2.27) R
2
I
2
AS
IN
, therefore
2
2
x
by the condition that
equation above
1
5 -1.53
and R
2
1.53
. Substituting
AS
2
results
DIV
. This
and
, of
5.3
One boundary condition which may impact the
selection of the minimum fan speed is the irregular
activation of the Diagnostic Timer due to the TC649
“missing” fan commutation pulses at low speeds. This
is a natural consequence of low PWM duty cycles (typ-
ically 25% or less). Recall that the SENSE function
detects commutation of the fan as disturbances in the
current through R
the fan is energized (i.e., V
cycles, the V
may be rotating normally, but the commutation events
are occurring during the PWM’s off-time.
The phase relationship between the fan’s commutation
and the PWM edges tends to “walk around” as the
system operates. At certain points, the TC649 may fail
to capture a pulse within the 32-cycle missing pulse
detector window. If this happens, the 3-cycle
Diagnostic Timer will be activated, the V
be active continuously for three cycles and, if the fan is
operating normally, a pulse will be detected. If all is
well, the system will return to normal operation. There
is no harm in this behavior, but it may be audible to the
user as the fan accelerates briefly when the Diagnostic
Timer fires. For this reason, it is recommended that V
be set no lower than 1.8V.
5.4
The FanSense network, comprised of R
C
the fan motor (FanSense™ technology). This network
can be thought of as a differentiator and threshold
detector. The function of R
current into a voltage. C
voltage signal and provide a ground-referenced input to
the SENSE pin. Designing a proper SENSE network is
simply a matter of scaling R
essary amount of gain (i.e., the current-to-voltage con-
version
recommended for C
larger sense resistors, and higher value capacitors are
bulkier and more expensive. Using a 0.1 µF capacitor
results in reasonable values for R
illustrates a typical SENSE network. Figure 5-5 shows
the waveforms observed using a typical SENSE net-
work.
SENSE
, allows the TC649 to detect commutation of
Operations at Low Duty Cycle
(R
ratio).
FanSense
SENSE
OUT
output is “off” most of the time. The fan
A
SENSE
and C
SENSE
0.1 µF
™
SENSE
. These can only occur when
Network
OUT
SENSE
SENSE
. Smaller values require
SENSE
serves to AC-couple this
is “on”). At very low duty
ceramic
)
is to convert the fan
to provide the nec-
SENSE
DS21449C-page 11
TC649
OUT
capacitor
. Figure 5-4
SENSE
output will
and
AS
is