TC647 datasheet(11/28 Pages) MICROCHIP

11 / 28 page

 2002 Microchip Technology Inc.

DS21447C-page 11

TC647

FIGURE 5-3:

VMIN Circuit.

We can further specify R1 and R2 by the condition that

the divider voltage is equal to our desired VMIN. This

yields the following equation:

EQUATION

Solving for the relationship between R1 and R2 results

in the following equation:

EQUATION

In this example, R1 = (1.762) R2. Substituting this rela-

tionship back into the previous equation yields the

resistor values:

R2 = 18.1 kΩ

R1 = 31.9 kΩ

In this case, the standard values of 31.6 k

Ω and

18.2 k

Ω are very close to the calculated values and

would be more than adequate.

5.3

Operations at Low Duty Cycle

One boundary condition which may impact the selec-

tion of the minimum fan speed is the irregular activation

of the Diagnostic Timer due to the TC647 “missing” fan

commutation pulses at low speeds. This is a natural

consequence of low PWM duty cycles (typically 25% or

less). Recall that the SENSE function detects commu-

tation of the fan as disturbances in the current through

RSENSE. These can only occur when the fan is ener-

gized (i.e., VOUT is “on”). At very low duty cycles, the

VOUT output is “off” most of the time. The fan may be

rotating normally, but the commutation events are

occurring during the PWM’s off-time.

The phase relationship between the fan’s commutation

and the PWM edges tends to “walk around” as the

system operates. At certain points, the TC647 may fail

to capture a pulse within the 32-cycle missing pulse

detector window. When this happens, the 3-cycle

Diagnostic Timer will be activated, the VOUT output will

be active continuously for three cycles and, if the fan is

operating normally, a pulse will be detected. If all is

well, the system will return to normal operation. There

is no harm in this behavior, but it may be audible to the

user as the fan will accelerate briefly when the

Diagnostic Timer fires. For this reason, it is

recommended that VMIN be set no lower than 1.8V.

5.4

FanSense™ Network

(RSENSE and CSENSE)

The FanSense network, comprised of RSENSE and

CSENSE, allows the TC647 to detect commutation of

the fan motor (FanSense™ technology). This network

can be thought of as a differentiator and threshold

detector. The function of RSENSE is to convert the fan

current into a voltage. CSENSE serves to AC-couple this

voltage signal and provide a ground referenced input to

the SENSE pin. Designing a proper SENSE network is

simply a matter of scaling RSENSE to provide the

necessary amount of gain (i.e., the current-to-voltage

conversion ratio). A 0.1 µF ceramic capacitor is recom-

mended for CSENSE. Smaller values require larger

sense resistors, and higher value capacitors are bulkier

and more expensive. Using a 0.1 µF results in

reasonable values for RSENSE. Figure 5-4 illustrates a

typical SENSE network. Figure 5-5 shows the

waveforms observed using a typical SENSE network.

FIGURE 5-4:

SENSE Network.

GND

VDD

VMIN

IDIV

IIN

VDD x R2

R1 + R2

VMIN =

VDD - VMIN

VMIN

R1 = R2 x

GND

VDD

RSENSE

SENSE

RBASE

CSENSE

(0.1 µF Typ.)

VOUT

FAN

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