TC642
DS21444D-page 12 2001-2012 Microchip Technology Inc.
Table 5-1 lists recommended values for 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 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.
TABLE 5-1: R
SENSE
VS. FAN CURRENT
5.5 Output Drive Transistor Selection
The TC642 is designed to drive an external transistor
or MOSFET for modulating power to the fan. This is
shown as Q
1
in Figures 3-1, 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 5-7. When high
current gain is needed to drive larger fans, two transis-
tors may be used in a Darlington configuration. Three
possible 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 TC642’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
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.
A base-current limiting resistor is required with bipolar
transistors (Figure 5-6).
FIGURE 5-6: Circuit For Determining
R
BASE
.
The correct value for this resistor can be determined as
follows:
V
OH =
V
R
SENSE
+ V
BE
(SAT)
+ V
R
BASE
V
R
SENSE
= I
FAN
x R
SENSE
V
R
BASE
= R
BASE
x I
BASE
I
BASE
= I
FAN
/ h
FE
V
OH
is specified as 80% of V
DD
in Section 1.0, “Electri-
cal Characteristics”; V
BE
(SAT)
is given in the chosen
transistor’s data sheet. It is now possible to solve for
R
BASE
.
EQUATION
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
Q
1
GND
V
DD
R
SENS
R
BASE
V
OH
= 80% V
DD
+
V
R
BASE
–
+
V
BE
(SAT)
–
+
V
R
SENSE
–
Fan
R
BASE
=
V
OH
- V
BE
(SAT)
- V
R
SENSE
I
BASE
