TC665 Microchip Technology Inc., TC665 Datasheet - Page 24

• Manufacturer Part Number: TC665
• Description: Smbus Pwm Fan Speed Controllers With Fan Fault Detection
• Manufacturer: Microchip Technology Inc.

TC664/TC665
7.4
The network comprised of R
the TC664/TC665 devices to detect commutation of the
fan motor. R
age. C
SENSE pin. The goal of the SENSE network is to pro-
vide a voltage pulse to the SENSE pin that has a mini-
mum amplitude of 120 mV. This will ensure that the
current pulse caused by the fan commutation is recog-
nized by the TC664/TC665 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 purpose 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.
DS21737A-page 24
SENSE
Note:
SENSE
. Smaller values will require larger sense resis-
SENSE
FanSense Network (R
C
V
SENSE
SENSE
OUT
SENSE
SENSE
AC couples this voltage signal to the
See Table 7-1 for R
, the operating fan current should be
)
(0.1 µF typical)
converts the fan current into a volt-
will change with the current rating
SENSE
C
Typical Sense Network.
715
R
SENSE
ISO
SENSE
. Figure 7-5 illustrates a
SENSE
FAN
SENSE
and C
SENSE
according to the
R
SENSE
SENSE
value.
&
allows
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 TC664/TC665 devices are 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 current fans, NPN transistors are a very econom-
ical choice for the fan drive device. It is recommended
that, for higher current fans (500 mA and above), MOS-
FETs be used as the fan drive device. Table 7-2 pro-
vides 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)