 2002 Microchip Technology Inc.

DS21449C-page 13

TC649

Table 5-2 gives examples of some commonly available

transistors and MOSFETs. This table should be used

as a guide only since there are many transistors and

MOSFETs which will work just as well as those listed.

The critical issues when choosing a device to use as

(MOSFET)) must be large enough to withstand the

highest voltage applied to the fan (Note: This will occur

when the fan is off); (2) 5 mA of base drive current must

be enough to saturate the transistor when conducting

the full fan current (transistor must have sufficient

ficiently drive the gate of the MOSFET to minimize the

be within the transistor's/MOSFET's current handling

capability; and (5) power dissipation must be kept

within the limits of the chosen device.

FIGURE 5-6:

Output Drive Transistor Circuit Topologies.

TABLE 5-2:

Device

Package

(V)

(V)

Fan Current

(mA)

Suggested

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.

GND

VOUT

Fan

a) Single Bipolar Transistor

GND

VDD

C) N-Channel MOSFET

GND

Fan

b) Darlington Transistor Pair

Fan