TC646
V
DD
TABLE 5-1:
50
100
150
R
SENSE
VS. FAN CURRENT
R
SENSE
(Ω)
9.1
4.7
3.0
2.4
2.0
1.8
1.5
1.3
1.2
1.0
Nominal Fan Current (mA)
Fan
200
250
300
R
BASE
V
OUT
SENSE
C
SENSE
(0.1
µF
Typ.)
R
SENSE
Q
1
350
400
450
500
5.5
Output Drive Transistor Selection
GND
FIGURE 5-4:
SENSE Network.
Tek Run: 10.0kS/s Sample
[
T
]
The TC646 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q
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 transistors 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.
GND
Waveform @ Sense Resistor
1
Waveform @ Sense Pin
T
2
90mV
50mV
GND
Ch1 100mV
Ch2
100mV
M5.00ms Ch1
142mV
FIGURE 5-5:
SENSE Waveforms.
One major advantage of the TC646’s PWM control
scheme versus linear speed control is that the power
dissipation in the pass element is kept very low.
Generally, 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”.
Table 5-2 gives examples of some commonly available
transistors and MOSFETs. This table should be used
MOSFETs which will work just as well as those listed.
The critical issues when choosing a device to use as
Q
1
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.
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.
2002 Microchip Technology Inc.
DS21446C-page 13
MICROCHIP [ MICROCHIP TECHNOLOGY ]
