TC646 View Datasheet(PDF) - Microchip Technology
Part Name
Description
MFG CO.
'TC646' PDF : 28 Pages View PDF
VDD
Fan
VOUT
RBASE
SENSE
CSENSE
(0.1 µF Typ.)
Q1
RSENSE
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 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-
ing Table 5-1 is that the signal developed across the
sense resistor is approximately 450 mV in amplitude.
2002 Microchip Technology Inc.
TC646
TABLE 5-1: RSENSE VS. FAN CURRENT
Nominal Fan Current (mA)
RSENSE (Ω)
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 TC646 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 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 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.
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
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.
DS21446C-page 13
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