TC654 Microchip Technology, TC654 Datasheet - Page 24

• Manufacturer Part Number: TC654
• Description: (TC654 / TC655) PWM Fan Speed Controllers With Fan Fault Detection
• Manufacturer: Microchip Technology

TC654/TC655
7.4
The network comprised of R
the TC654/TC655 devices to detect commutation of the
fan motor. R
age. C
SENSE pins (SENSE1 & SENSE2). The goal of the
SENSE network is to provide a voltage pulse to the
SENSE pin that has a minimum amplitude of 120 mV.
This will ensure that the current pulse caused by the
fan commutation is recognized by the TC654/TC655
device.
A 0.1 µF ceramic capacitor is recommended for
C
tors be used. Using a 0.1 µF capacitor results in rea-
sonable values for R
typical SENSE network.
FIGURE 7-5:
The value of R
of the fan. A key point is that the current rating of the
fan specified by the manufacturer may be a worst case
rating. The actual current drawn by the fan may be
lower than this rating. For the purposes of setting the
value for R
measured.
Table 7-1 shows values of R
nominal operating current of the fan. The fan currents
are average values. If the fan current falls between two
of the values listed, use the higher resistor value.
DS21734A-page 24
SENSE
SENSE1
SENSE2
Note:
V
SENSE
OUT
. Smaller values will require larger sense resis-
FanSense Network (R
C
See Table 7-1 for R
SENSE
SENSE
SENSE
SENSE
AC couples this voltage signal to the
, the operating fan current should be
)
converts the fan current into a volt-
(0.1 µF typical)
C
will change with the current rating
SENSE1
SENSE
Typical Sense Network.
(0.1 µF typical)
715
R
C
SENSE1
ISO1
SENSE2
SENSE
. Figure 7-5 illustrates a
SENSE
FAN
and R
1
R
SENSE1
and C
715
R
R
SENSE2
SENSE
ISO2
ISO2
according to the
R
SENSE
SENSE2
values.
&
FAN
allows
2
TABLE 7-1:
Figure 7-6 shows some typical waveforms for the fan
current and the voltage at the Sense pins.
FIGURE 7-6:
Sense Pin Waveforms.
7.5
The TC654/TC655 is designed to drive two external
NPN transistors or two external N-channel MOSFETs
as the fan speed modulating elements. These two
arrangements are shown in Figure 7-7. For lower cur-
rent fans, NPN transistors are a very economical
choice for the fan drive device. It is recommended that,
for higher current fans (500 mA and above), MOSFETs
be used as the fan drive device. Table 7-2 provides
some possible part numbers for use as the fan drive
element.
When using an NPN transistor as the fan drive ele-
ment, a base current limiting resistor must be used.
This is shown in Figure 7-7.
When using MOSFETs as the fan drive element, it is
very easy to turn the MOSFETs on and off at very high
rates. Because the gate capacitances of these small
Nominal Fan Current
Output Drive Device Selection
(mA)
100
150
200
250
300
350
400
450
500
50
R
SENSE
Typical Fan Current and
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
(ohm)