TC646 datasheet(14/28 Pages) MICROCHIP

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TC646

DS21446C-page 14

 2002 Microchip Technology Inc.

A base-current limiting resistor is required with bipolar

transistors. This is shown in Figure 5-6.

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

chosen 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. 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 in the case of fan

operation from +12V.

FIGURE 5-7:

Output Drive Transistor Circuit Topologies.

GND

VDD

RSENSE

RBASE

VOH = 80% VDD

+ V

RBASE

–

VBE

(SAT)

–

SENSE

–

Fan

RBASE =

VOH - VBE(SAT) - VRSENSE

IBASE

GND

VDD

RSENSE

RBASE

VOUT

Fan

a) Single Bipolar Transistor

GND

VDD

RSENSE

VOUT

c) N-Channel MOSFET

GND

VDD

RSENSE

RBASE

Fan

b) Darlington Transistor Pair

Fan

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