TC646VOA Microchip Technology, TC646VOA Datasheet - Page 12

• Manufacturer Part Number: TC646VOA
• Description: IC PWM FAN SPD CNTRLR SD 8-SOIC
• Manufacturer: Microchip Technology
• Type: Controller - PWM Fanr

Specifications of TC646VOA

• Applications: Fan Controller, Brushless (BLDC)
• Number Of Outputs: 1
• Voltage - Supply: 3 V ~ 5.5 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Motor Type: DC Brushless
• No. Of Outputs: 1
• Output Current: 5mA
• Supply Voltage Range: 3V To 5.5V
• Driver Case Style: SOIC
• No. Of Pins: 8
• Operating Temperature Range: 0°C To +85°C
• Product: Fan / Motor Controllers / Drivers
• Operating Supply Voltage: 3 V to 5.5 V
• Supply Current: 1 mA
• Mounting Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Voltage - Load: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

TC646
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:
EQUATION
Solving for the relationship between R
in:
EQUATION
In the case of this example, R
Substituting this relationship back into the original
equation yields the resistor values:
In this case, the standard values of 34.8 k
15.4 k are very close to the calculated values and
would be more than adequate.
DS21446C-page 12
R
R
R
2
1
1
= 15.3 k , and
= 34.7 k
I
= R
DIV
R
2
DD
1
= 1e
x
+ R
= 5.0V then…
V
I
AS
DIV
V
–4
2
DD
A =
=
=
V
V
GND
AS
AS
- V
DD
V
R
1e
V
DD
1
pin is no more than 1 µA. It is
AS
5.0V
AS
R
R
R
–4
1
+ R
2
1
1
and R
A
x R
5.0V
+ R
= R
CIRCUIT
V
2
2
AS
1
= 50,000 = 50k
2
2
= (2.27) R
I
2
IN
x
, therefore
by the condition that
5 - 1.53
1
1.53
and R
2
.
AS
and
. This
2
results
DIV
, of
5.3
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
gized (i.e., V
V
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
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
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
necessary amount of gain (i.e., the current-to-voltage
conversion ratio). A 0.1 µF ceramic capacitor is
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.
OUT
SENSE
SENSE
output is “off” most of the time. The fan may be
. These can only occur when the fan is ener-
, allows the TC646 to detect commutation of
Operations at Low Duty Cycle
FanSense
(R
SENSE
OUT
is “on”). At very low duty cycles, the
and C
SENSE
™
SENSE
Network
™
2002 Microchip Technology Inc.
SENSE
SENSE
. Smaller values require
technology). This network
serves to AC-couple this
SENSE
)
is to convert the fan
SENSE
to provide the
OUT
. Figure 5-4
SENSE
output will
and
AS