TC654(2002) View Datasheet(PDF) - Microchip Technology
Part Name
Description
Manufacturer
TC654 Datasheet PDF : 36 Pages
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MOSFETs are very low, the TC654/TC655 can charge
and discharge them very quickly, leading to very fast
edges. Of key concern is the turn-off edge of the MOS-
FET. Since the fan motor winding is essentially an
inductor, once 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
VDD
TC654/TC655
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
VOUT and the gate of the MOSFET, which will help
slow down the turn-off and limit this condition.
VDD
FAN
VOUT
RBASE
Q1
RSENSE
VOUT
FAN
Q1
RSENSE
FIGURE 7-7:
GND
a) Single Bipolar Transistor
Output Drive Device Configurations.
GND
b) N-Channel MOSFET
TABLE 7-2: FAN DRIVE DEVICE SELECTION TABLE (NOTE 2)
Device
Package
Max Vbe sat /
Vgs(V)
Min hfe
Vce/VDS
Fan Current Suggested
(mA)
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.
2002 Microchip Technology Inc.
DS21734A-page 25
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