TC647_13 Datasheet, PDF (12/28 Pages) Microchip Technology – PWM Fan Speed Controller with FanSense™ Technology

TC647
Tek Run: 10.0kS/s Sample
[
T
]
Waveform @ Sense Resistor
1
GND
Waveform @ Sense Pin
T
90mV
50mV
2
GND
Ch1 100mV Ch2 100mV
M5.00ms Ch1
142mV
FIGURE 5-5:
SENSE Waveforms.
Table 5-1 lists the recommended values of R_SENSE
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: R_SENSE VS. FAN CURRENT
Nominal Fan Current (mA)
R_SENSE ()
50
9.1
100
4.7
150
3.0
200
2.4
250
2.0
300
1.8
350
1.5
400
1.3
450
1.2
500
1.0
5.5 Output Drive Transistor Selection
The TC647 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q₁ in Figures 3-1, 5-1, 5-4, 5-6, 5-7, 5-8
and 5-9. The V_OUT pin has a minimum source current
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
Q₁ are: (1) the breakdown voltage (V_(BR)CEO or
V_DS(MOSFET)) must be large enough to withstand the
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_OUT voltage must be high enough to suf-
ficiently drive the gate of the MOSFET to minimize the
R_DS(on) of the device; (4) rated fan current draw must
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.
DS21447D-page 12
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