TC647 Microchip, TC647 Datasheet - Page 12

• Manufacturer Part Number: TC647
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

TC647
FIGURE 5-5:
Table 5-1 lists the recommended values of R
based on the nominal operating current of the fan. Note
that the current draw specified by the fan manufacturer
may be a worst-case rating for near-stall conditions and
not the fan’s nominal operating current. The values in
Table 5-1 refer to actual average operating current. If
the fan current falls between two of the values listed,
use the higher resistor value. The end result of employ-
ing Table 5-1 is that the signal developed across the
sense resistor is approximately 450 mV in amplitude.
TABLE 5-1:
DS21447C-page 12
Nominal Fan Current (mA)
2
1
Ch1 100mV
100
150
200
250
300
350
400
450
500
50
[
Ch2
Tek Run: 10.0kS/s Sample
R
Waveform @ Sense Resistor
Waveform @ Sense Pin
SENSE
100mV
SENSE Waveforms.
T
T
VS. FAN CURRENT
M5.00ms
R
Ch1
SENSE
]
9.1
4.7
3.0
2.4
2.0
1.8
1.5
1.3
1.2
1.0
142mV
( )
SENSE
GND
90mV
50mV
GND
5.5
The TC647 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q
and 5-9. The V
of 5 mA and a minimum sink current of 1 mA. Bipolar
transistors or MOSFETs may be used as the power
switching element, as shown in Figure 5-7. When high
current gain is needed to drive larger fans, two transis-
tors may be used in a Darlington configuration. These
circuit topologies are shown in Figure 5-7: (a) shows a
single NPN transistor used as the switching element;
(b) illustrates the Darlington pair; and (c) shows an N-
channel MOSFET.
One major advantage of the TC647’s PWM control
scheme versus linear speed control is that the power
dissipation in the pass element is kept very low. Gener-
ally, low cost devices in very small packages, such as
TO-92 or SOT, can be used effectively. For fans with
nominal operating currents of no more than 200 mA, a
single transistor usually suffices. Above 200 mA, the
Darlington or MOSFET solution is recommended. For
the fan sensing function to work correctly, it is impera-
tive that the pass transistor be fully saturated when
“on”.
Table 5-2 gives examples of some commonly available
transistors and MOSFETs. This table should be used
as a guide only since there are many transistors and
MOSFETs which will work just as well as those listed.
The critical issues when choosing a device to use as
Q
V
highest voltage applied to the fan (Note: This will occur
when the fan is off); (2) 5 mA of base drive current must
be enough to saturate the transistor when conducting
the full fan current (transistor must have sufficient
gain); (3) the V
ficiently drive the gate of the MOSFET to minimize the
R
be within the transistor's/MOSFET's current handling
capability; and (5) power dissipation must be kept
within the limits of the chosen device.
A base-current limiting resistor is required with bipolar
transistors. This is shown in Figure 5-6.
DS
1
DS(on)
(MOSFET)) must be large enough to withstand the
are: (1) the breakdown voltage (V
of the device; (4) rated fan current draw must
Output Drive Transistor Selection
1
OUT
OUT
in Figures 3-1, 5-1, 5-4, 5-6, 5-7, 5-8
voltage must be high enough to suf-
pin has a minimum source current
2002 Microchip Technology Inc.
(BR)CEO
or