TC649_13 datasheet(14/28 Pages) MICROCHIP

14 / 28 page

TC649

DS21449D-page 14

 2001-2012 Microchip Technology Inc.

A base-current limiting resistor is required with bipolar

transistors (Figure 5-7). 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 benefit from the fan being powered

from a negative supply to keep motor noise out of the

positive supply rails. This can be accomplished as

shown in Figure 5-8, with zener diode D1 offsetting 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 in the case of

fan operation from +12V.

FIGURE 5-7:

Circuit For Determining

RBASE.

FIGURE 5-8:

Powering the Fan from a -12V Supply.

RBASE =

VOH - VBE(SAT) - VRSENSE

IBASE

GND

VDD

RSENSE

RBASE

VOH = 80% VDD

+ V

RBASE

–

VBE

(SAT)

–

SENSE

–

Fan

GND

+5V

-12V

Q1*

VDD

VOUT

TC649

FAN

R2*

2.2 k

R3*

2.2

R4*

10 k

12.0V

Zener

Note: * Value depends on the specific application and is shown for example only.

See Section 5.0, "Typical Applications", for more details.

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