TC646
V
DD
5.3
Operations at Low Duty Cycle
R
1
I
DIV
I
IN
V
AS
R
2
One boundary condition which may impact the selec-
tion of the minimum fan speed is the irregular activation
of the Diagnostic Timer due to the TC646 “missing” fan
commutation pulses at low speeds. This is a natural
consequence of low PWM duty cycles (typically 25% or
less). Recall that the SENSE function detects commu-
tation of the fan as disturbances in the current through
R
SENSE
. These can only occur when the fan is ener-
gized (i.e., V
OUT
is “on”). At very low duty cycles, the
V
OUT
output is “off” most of the time. The fan 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 TC646 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
OUT
output will
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
AS
be set no lower than 1.8V.
GND
FIGURE 5-3:
V
AS
CIRCUIT
Per Section 1.0, “Electrical Characteristics”, the leak-
age current at the V
AS
pin is no more than 1 µA. It is
conservative to design for a divider current, I
DIV
, of
100 µA. If V
DD
= 5.0V then…
EQUATION
I
DIV
= 1e
–4
A =
R
1
+ R
2
=
5.0V
R
1
+ R
2
5.0V
1e A
–4
, therefore
= 50,000Ω = 50kΩ
5.4
FanSense
™
Network
(R
SENSE
and C
SENSE
)
We can further specify R
1
and R
2
by the condition that
the divider voltage is equal to our desired V
AS
. This
yields:
EQUATION
V
AS
=
V
DD
x R
2
R
1
+ R
2
Solving for the relationship between R
1
and R
2
results
in:
EQUATION
R
1
= R
2
x
V
DD
- V
AS
V
AS
= R
2
x
5 - 1.53
1.53
The FanSense network, comprised of R
SENSE
and
C
SENSE
, allows the TC646 to detect commutation of
the fan motor (FanSense
™
technology). This network
can be thought of as a differentiator and threshold
detector. The function of R
SENSE
is to convert the fan
current into a voltage. C
SENSE
serves to AC-couple this
voltage signal and provide a ground-referenced input to
the SENSE pin. Designing a proper SENSE network is
simply a matter of scaling R
SENSE
to provide the
necessary amount of gain (i.e., the current-to-voltage
conversion ratio). A 0.1 µF ceramic capacitor is
recommended for C
SENSE
. Smaller values require
larger sense resistors, and higher value capacitors are
bulkier and more expensive. Using a 0.1 µF capacitor
results in reasonable values for R
SENSE
. Figure 5-4
illustrates a typical SENSE network. Figure 5-5 shows
the waveforms observed using a typical SENSE net-
work.
In the case of this example, R
1
= (2.27) R
2
.
Substituting this relationship back into the original
equation yields the resistor values:
R
2
= 15.3 kΩ, and
R
1
= 34.7 kΩ
In this case, the standard values of 34.8 kΩ and
15.4 kΩ are very close to the calculated values and
would be more than adequate.
DS21446C-page 12
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
