 2002 Microchip Technology Inc.

DS21444C-page 13

TC642

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 is shown in

otherwise consistent with the case of fan operation

from +12V.

FIGURE 5-7:

Output Drive Transistor Circuit Topologies.

FIGURE 5-8:

Powering the Fan From a -12V Supply.

GND

VDD

Fan

a) Single Bipolar Transistor

GND

c) N-Channel MOSFET

GND

Fan

b) Darlington Transistor Pair

Fan

GND

+5V

TC642

Fan

2.2 k

Ω

2.2

Ω

10 k

Ω

12.0V

Zener

-12V

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