TC647 datasheet(13/28 Pages) MICROCHIP

13 / 28 page

 2002 Microchip Technology Inc.

DS21447C-page 13

TC647

FIGURE 5-6:

Circuit For Determining

RBASE.

The correct value for this resistor can be determined as

follows:

VOH

= VRSENSE + VBE(SAT) + VRBASE

VRSENSE

= IFAN x RSENSE

VRBASE

= RBASE x IBASE

IBASE

= IFAN / hFE

VOH is specified as 80% of VDD in Section 1.0,

“Electrical Characteristics”; VBE(SAT) is given in the cho-

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

RBASE.

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,

zener diode D1 offsets the -12V power supply voltage,

holding transistor Q1 off when VOUT is low. When VOUT is

high, the voltage at the anode of D1 increases by VOUT

causing Q1 to turn on. Operation is otherwise the same as

the case of fan operation from +12V.

TABLE 5-2:

TRANSISTORS AND MOSFETS FOR Q1 (VDD = 5V)

GND

VDD

RSENSE

RBASE

VOH = 80% VDD

+ V

RBASE

–

+ V

BE(SAT)

–

SENSE

–

Fan

RBASE =

VOH - VBE(SAT) - VRSENSE

IBASE

Device

Package

Max. VBE(sat)/VGS

(V)

Min. HFE

VCEO/VDS

(V)

Fan Current

(mA)

Suggested

RBASE (Ω)

MMBT2222A

SOT-23

1.2

150

800

MPS2222A

TO-92

1.2

150

800

MPS6602

TO-92

1.2

500

301

SI2302

SOT-23

2.5

500

Note 1

MGSF1N02E

SOT-23

2.5

500

Note 1

SI4410

SO-8

4.5

1000

Note 1

SI2308

SOT-23

4.5

500

Note 1

Note 1: A series gate resistor may be used in order to control the MOSFET turn-on and turn-off times.

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