 2003 Microchip Technology Inc.

DS21756B-page 21

TC642B/TC647B

FIGURE 5-7:

Output Drive Device Configurations.

TABLE 5-2:

FAN DRIVE DEVICE SELECTION TABLE (NOTE 2)

5.6

Bias Supply Bypassing and Noise

Filtering

should be bypassed with a 1.0

µF ceramic capacitor.

This capacitor will help supply the peak currents that

are required to drive the base/gate of the external fan

drive devices.

any noise on this pin can cause duty-cycle jittering. For

0.01 µF capacitor.

In order to keep fan noise off of the TC642B/TC647B

device ground, individual ground returns for the

TC642B/TC647B and the low side of the fan current

sense resistor should be used.

5.7

Design Example/Typical

Application

The system has been designed with the following

components and criteria.

System inlet air ambient temperature ranges from 0ºC

to 50ºC. At 20ºC and below, it is desired to have the

system cooling stay at a constant level. At 20ºC, the fan

should be run at 40% of its full fan speed. Full fan

speed should be reached when the ambient air is 40ºC.

The system has a surface mount, NTC-style thermistor

in a 1206 package. The thermistor is mounted on a

daughtercard, which is directly in the inlet air stream.

The thermistor is a NTC, 100 k

Ω @ 25ºC, Thermomet-

the thermistor is 4250. The system bias voltage to run

the fan controller is 5V and the fan voltage is 12V.

Device

Package

Max Vbe sat /

Vgs(V)

Min hfe

(V)

Fan Current

(mA)

Suggested

Rbase (

Ω)

MMBT2222A

SOT-23

1.2

50

40

150

800

MPS2222A

TO-92

1.2

50

40

150

800

MPS6602

TO-92

1.2

50

40

500

301

SI2302

SOT-23

2.5

NA

20

500

Note 1

MGSF1N02E

SOT-23

2.5

NA

20

500

Note 1

SI4410

SO-8

4.5

NA

30

1000

Note 1

SI2308

SOT-23

4.5

NA

60

500

Note 1

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

2: These drive devices are suggestions only. Fan currents listed are for individual fans.

GND

Fan Bias

FAN

a) Single Bipolar Transistor

GND

Fan Bias

b) N-Channel MOSFET

FAN