Datasheet

 2002-2013 Microchip Technology Inc. DS21737B-page 25
TC664/TC665
MOSFETs are very low, the TC664/TC665 devices can
charge and discharge them very quickly leading to very
fast edges. Of key concern is the turn-off edge of the
MOSFET. Since the fan motor winding is essentially an
inductor, when the MOSFET is turned off, the current
that was flowing through the motor wants to continue to
flow. If the fan does not have internal clamp diodes
around the windings of the motor, there is no path for
this current to flow through and the voltage at the drain
of the MOSFET may rise until the drain to source rating
of the MOSFET is exceeded. This will most likely cause
the MOSFET to go into avalanche mode. Since there is
very little energy in this occurrence, it will probably not
fail the device, but it would be a long term reliability
issue. The following is recommended:
• Ask how the fan is designed. If the fan has clamp
diodes internally, you will not experience this
problem. If the fan does not have internal clamp
diodes, it is a good idea to put one externally
(Figure 7-8). You can also put a resistor between
V
OUT
and the gate of the MOSFET, which will help
slow down the turn-off and limit this condition.
FIGURE 7-7: Output Drive Device Configurations.
TABLE 7-2: FAN DRIVE DEVICE SELECTION TABLE (NOTE 2)
Device Package
Max Vbe sat /
Vgs(V)
Min hfe Vce/V
DS
Fan Current
(mA)
Suggested
Rbase (ohms)
MMBT2222A SOT-23 1.2 50 40 150 800
MPS2222A TO-92 1.2 50 40 150 800
MPS6602 TO-92 1.2 50 40 500 301
SI2302 SOT-23 2.5 NA 20 500 Note 1
MGSF1N02E SOT-23 2.5 NA 20 500 Note 1
SI4410 SO-8 4.5 NA 30 1000 Note 1
SI2308 SOT-23 4.5 NA 60 500 Note 1
Note 1: A series gate resistor may be used in order to control the MOSFET turn-on and turn-off times.
2: These drive devices are suggestions only. Fan currents listed are for individual fans.
Q
1
GND
V
DD
R
SENSE
R
BASE
V
OUT
FAN
a) Single Bipolar Transistor
Q
1
GND
V
DD
R
SENSE
V
OUT
b) N-Channel MOSFET
FAN