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DS20001734C-page 25

TC654/TC655

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

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

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

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.

GND

FAN

a) Single Bipolar Transistor

GND

b) N-Channel MOSFET

FAN