TC649_13 datasheet(12/28 Pages) MICROCHIP

12 / 28 page

TC649

DS21449D-page 12

 2001-2012 Microchip Technology Inc.

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

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 val-

ues listed, use the higher resistor value. The end result

of employing 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 TC649 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 5-6. 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 5-6: (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 TC649’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

imperative that the pass transistor be fully saturated

when “on”.

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 ()

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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