TC649B datasheet(20/36 Pages) MICROCHIP

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TC646B/TC648B/TC649B

DS21755B-page 20

 2003 Microchip Technology Inc.

TABLE 5-1:

FAN CURRENT VS. RSENSE

The values listed in Table 5-1 are for fans that have the

fan current waveshape shown in Figure 4-7. With this

waveshape, the average fan current is closer to the

peak value, which requires the resistor value to be

higher. When using a fan that has the fan current wave-

shape shown in Figure 4-6, the resistor value can often

be decreased since the current peaks are higher than

the average and it is the AC portion of the voltage that

gets coupled to the SENSE pin.

The key point when selecting an RSENSE value is to try

to minimize the value in order to minimize the power

dissipation in the resistor. In order to do this, it is critical

to know the waveshape of the fan current and not just

the average value.

Figure 5-4 shows some typical waveforms for the fan

current and the voltage at the SENSE pin.

FIGURE 5-4:

Typical Fan Current and

SENSE Pin Waveforms.

Another important factor to consider when selecting the

RSENSE value is the fan current value during a locked-

rotor condition. When a fan is in a locked-rotor condi-

tion (fan blades are stopped even though power is

being applied to the fan), the fan current can increase

dramatically (often 2.5 to 3.0 times the normal operat-

ing fan current). This will effect the power rating of the

RSENSE resistor selected.

When selecting the fan for the application, the current

draw of the fan during a locked-rotor condition should

be considered. Especially if multiple fans are being

used in the application.

There are two main types of fan designs when looking

at fan current draw during a locked-rotor condition.

The first is a fan that will simply draw high DC currents

when put into a locked-rotor condition. Many older fans

were designed this way. An example of this is a fan that

draws an average current of 100 mA during normal

operation. In a locked-rotor condition, this fan will draw

250 mA of average current. For this design, the

RSENSE power rating must be sized to handle the

250 mA condition. The fan bias supply must also take

this into account.

The second style design, which represents many of the

newer fan designs today, acts to limit the current in a

locked-rotor condition by going into a pulse mode of

operation. An example of the fan current waveshape

for this style fan is shown in Figure 5-5. The fan repre-

sented in Figure 5-5 is a Panasonic®, 12V, 220 mA fan.

During the on-time of the waveform, the fan current is

peaking up to 550 mA. Due to the pulse mode opera-

tion, the actual RMS current of the fan is very near the

220 mA rating. Because of this, the power rating for the

RSENSE resistor does not have to be oversized for this

application.

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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