TC646 Просмотр технического описания (PDF) - Microchip Technology

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TC646

PWM Fan Speed Controller with Auto-Shutdown and FanSense™ Technology

Microchip Technology 

TC646

A base-current limiting resistor is required with bipolar

transistors. This is shown in Figure 5-6.

VDD

Fan

VOH = 80% VDD

RBASE

+ VRBASEVB–+E(SAT)–

+

VRSENSE

–

Q1

RSENSE

FIGURE 5-6:

RBASE.

GND

Circuit For Determining

The correct value for this resistor can be determined as

follows:

VOH

VRSENSE

VRBASE

IBASE

= VRSENSE + VBE(SAT) + VRBASE

= IFAN x RSENSE

= RBASE x 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

RBASE = VOH - VBE(SAT) - VRSENSE

IBASE

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.

VDD

VDD

VDD

Fan

Fan

Fan

VOUT

RBASE

RBASE

VOUT

Q1

RSENSE

Q1

Q2

VOUT

RSENSE

Q1

RSENSE

GND

a) Single Bipolar Transistor

GND

b) Darlington Transistor Pair

FIGURE 5-7:

Output Drive Transistor Circuit Topologies.

GND

c) N-Channel MOSFET

DS21446C-page 14

 2002 Microchip Technology Inc.

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