TC649 Microchip, TC649 Datasheet - Page 12

• Manufacturer Part Number: TC649
• Description: PWM Fan Speed Controller
• Manufacturer: Microchip

TC649
FIGURE 5-4:
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
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.
DS21449C-page 12
2
1
Ch1 100mV
SENSE
V
OUT
[
Ch2
Tek Run: 10.0kS/s Sample
Waveform @ Sense Resistor
(0.1 µF Typ.)
Waveform @ Sense Pin
C
100mV
SENSE Network.
SENSE Waveforms.
R
SENSE
BASE
T
T
M5.00ms
V
GND
Fan
DD
Ch1
]
R
Q 1
SENSE
142mV
SENSE
GND
90mV
50mV
GND
TABLE 5-1:
5.5
The TC649 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q
The V
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”.
Nominal Fan Current (mA)
OUT
Output Drive Transistor Selection
pin has a minimum source current of 5 mA
1
100
150
200
250
300
350
400
450
500
50
in Figures 5-1, 5-4, 5-6, 5-7, 5-8 and 5-9.
R
SENSE
2002 Microchip Technology Inc.
VS. FAN CURRENT
R
SENSE
9.1
4.7
3.0
2.4
2.0
1.8
1.5
1.3
1.2
1.0
( )