TC642B_13 datasheet(20/36 Pages) MICROCHIP

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TC642B/TC647B

DS21756C-page 20

 2002-2013 Microchip Technology Inc.

FIGURE 5-5:

Fan Current During a Locked Rotor Condition.

5.5

Output Drive Device Selection

The TC642B/TC647B is designed to drive an external

NPN transistor or N-channel MOSFET as the fan

speed modulating element. These two arrangements

are shown in Figure 5-7. For lower current fans, NPN

transistors are a very economical choice for the fan

drive device. It is recommended that, for higher current

fans (300 mA and above), MOSFETs be used as the

fan drive device. Table 5-2 provides some possible part

numbers for use as the fan drive element.

When using a NPN transistor as the fan drive element,

a base current-limiting resistor must be used, as is

shown in Figure 5-7.

When using MOSFETs as the fan drive element, it is

very easy to turn the MOSFETs on and off at very high

rates. Because the gate capacitances of these small

MOSFETs are very low, the TC642B/TC647B can

charge and discharge them very quickly, leading to

very fast edges. Of key concern is the turn-off edge of

the MOSFET. Since the fan motor winding is essentially

an inductor, once the MOSFET is turned off, the current

that was flowing through the motor wants to continue to

flow. If the fan does not have internal clamp diodes

around the windings of the motor, there is no path for

this current to flow through, and the voltage at the drain

of the MOSFET may rise until the drain-to-source rating

of the MOSFET is exceeded. This will most likely cause

the MOSFET to go into avalanche mode. Since there is

very little energy in this occurrence, it will probably not

fail the device, but it would be a long-term reliability

issue.

The following is recommended:

• Ask how the fan is designed. If the fan has clamp

diodes internally, this problem will not be seen. If

the fan does not have internal clamp diodes, it is a

good idea to put one externally (Figure 5-6). Put-

ting a resistor between VOUT and the gate of the

MOSFET will also help slow down the turn-off and

limit this condition.

FIGURE 5-6:

Clamp Diode For Fan Turn-

Off.

GND

RSENSE

VOUT

Q1: N-Channel MOSFET

FAN

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