 2002 Microchip Technology Inc.

DS21447C-page 13

TC647

FIGURE 5-6:

Circuit For Determining

The correct value for this resistor can be determined as

follows:

sen transistor data sheet. It is now possible to solve for

EQUATION

Some applications require the fan to be powered from the

negative 12V supply to keep motor noise out of the

positive voltage power supplies. As shown in Figure 5-8,

the case of fan operation from +12V.

TABLE 5-2:

Q1

GND

+ V

RBASE

–

+ V

BE(SAT)

–

+

SENSE

–

Fan

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.