TC642EPA datasheet(12/28 Pages) MICROCHIP

12 / 28 page

TC642

DS21444C-page 12

 2002 Microchip Technology Inc.

Table 5-1 lists recommended values for RSENSE based

on the nominal operating current of the fan. Note that

the current draw specified by the fan manufacturer may

be a worst-case rating for near-stall conditions and may

not be the fan’s nominal operating current. The values

in Table 5-1 refer to actual average operating current. If

the fan current falls between two of the values listed,

use the higher resistor value. The end result of employ-

ing Table 5-1 is that the signal developed across the

sense resistor is approximately 450 mV in amplitude.

TABLE 5-1:

RSENSE VS. FAN CURRENT

5.5

Output Drive Transistor Selection

The TC642 is designed to drive an external transistor

or MOSFET for modulating power to the fan. This is

shown as Q1 in Figures 3-1, 5-1, 5-4, 5-6, 5-7, 5-8

and 5-9. The VOUT pin has a minimum source current

of 5 mA and a minimum sink current of 1 mA. Bipolar

transistors or MOSFETs may be used as the power

switching element, as shown in Figure 5-7. When high

current gain is needed to drive larger fans, two transis-

tors may be used in a Darlington configuration. Three

possible circuit topologies are shown in Figure 5-7: (a)

shows a single NPN transistor used as the switching

element; (b) illustrates the Darlington pair; and (c)

shows an N-channel MOSFET.

One major advantage of the TC642’s PWM control

scheme versus linear speed control is that the power

dissipation in the pass element is kept very low. Gener-

ally, low cost devices in very small packages, such as

TO-92 or SOT, can be used effectively. For fans with

nominal operating currents of no more than 200 mA, a

single transistor usually suffices. Above 200 mA, the

Darlington or MOSFET solution is recommended. For

the fan sensing function to work correctly, it is impera-

tive that the pass transistor be fully saturated when

“on”.

Table 5-2 gives examples of some commonly available

transistors and MOSFETs. This table should be used

as a guide only since there are many transistors and

MOSFETs which will work just as well as those listed.

The critical issues when choosing a device to use as

Q1 are: (1) the breakdown voltage (V(BR)CEO or VDS

(MOSFET)) must be large enough to withstand the

highest voltage applied to the fan (Note: This will occur

when the fan is off); (2) 5 mA of base drive current must

be enough to saturate the transistor when conducting

the full fan current (transistor must have sufficient

gain); (3) the VOUT voltage must be high enough to suf-

ficiently drive the gate of the MOSFET to minimize the

RDS(on) of the device; (4) rated fan current draw must

be within the transistor's/MOSFET's current handling

capability; and (5) power dissipation must be kept

within the limits of the chosen device.

A base-current limiting resistor is required with bipolar

transistors (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, “Electri-

cal Characteristics”; VBE(SAT) is given in the chosen

transistor’s data sheet. It is now possible to solve for

RBASE.

EQUATION

Nominal Fan Current (mA)

RSENSE (Ω)

9.1

100

4.7

150

3.0

200

2.4

250

2.0

300

1.8

350

1.5

400

1.3

450

1.2

500

1.0

GND

VDD

RSENSE

RBASE

VOH = 80% VDD

+ V

RBASE

–

+ V

BE(SAT)

–

SENSE

–

Fan

RBASE =

VOH - VBE(SAT) - VRSENSE

IBASE

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