TC646 datasheet(13/28 Pages) MICROCHIP

13 / 28 page

 2002 Microchip Technology Inc.

DS21446C-page 13

TC646

FIGURE 5-4:

SENSE Network.

FIGURE 5-5:

SENSE Waveforms.

Table 5-1 lists the recommended values of 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

not 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 TC646 is designed to drive an external transistor

or MOSFET for modulating power to the fan. This is

shown as Q1 in Figures 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 transistors may

be used in a Darlington configuration. These 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 TC646’s PWM control

scheme versus linear speed control is that the power

dissipation in the pass element is kept very low.

Generally, 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 imperative 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.

GND

VDD

RSENSE

SENSE

RBASE

CSENSE

(0.1

µF Typ.)

VOUT

Fan

Ch1 100mV

Tek Run: 10.0kS/s Sample

Ch2

100mV

M5.00ms Ch1

142mV

GND

[

]

Waveform @ Sense Resistor

90mV

50mV

GND

Waveform @ Sense Pin

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

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