TC647
DS21447C-page 12
2002 Microchip Technology Inc.
FIGURE 5-5:
SENSE Waveforms.
Table 5-1 lists the recommended values of RSENSE 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-
sense resistor is approximately 450 mV in amplitude.
TABLE 5-1:
RSENSE VS. FAN CURRENT
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
and 5-9. The VOUT 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
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
VDS(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 VOUT voltage must be high enough to suf-
ficiently drive the gate of the MOSFET to minimize the
RDS(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.
Nominal Fan Current (mA)
RSENSE ()
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
1
Ch1
100mV
Tek Run: 10.0kS/s Sample
Ch2
100mV
M5.00ms
Ch1
142mV
GND
[
T
]
T
Waveform @ Sense Resistor
90mV
50mV
GND
Waveform @ Sense Pin
2