TC649_13 Datasheet, PDF (12/28 Pages) Microchip Technology – PWM Fan Speed Controller with Auto-Shutdown and FanSense™ Technology

TC649
V_DD
Fan
V_OUT
SENSE
R_BASE
C_SENSE
(0.1 µF Typ.)
Q1
R_SENSE
FIGURE 5-4:
GND
SENSE Network.
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
may not be 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 val-
ues listed, use the higher resistor value. The end result
of employing 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 TC649 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q₁ in Figures 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 5-6. When high current
gain is needed to drive larger fans, two transistors may
be used in a Darlington configuration. These circuit
topologies are shown in Figure 5-6: (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 TC649’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
imperative that the pass transistor be fully saturated
when “on”.
DS21449D-page 12
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