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TC649VUATR

PWM Fan Speed Controller with Auto-Shutdown and FanSense™ Technology

Microchip Technology 

VDD

R1 IIN

IDIV

VAS

R2

GND

FIGURE 5-3:

VAS Circuit.

Per Section 1.0, “Electrical Characteristics”, the leak-

age current at the VAS pin is no more than 1 µA. It is

conservative to design for a divider current, IDIV, of

100 µA. If VDD = 5.0V then:

EQUATION

IDIV = 1e– 4A

R1 + R2 =

5.0V

, therefore

R1 + R2

5.0V

1e–4A

= 50,000Ω = 50 kΩ

We can further specify R1 and R2 by the condition that

the divider voltage is equal to our desired VAS. This

yields the following:

EQUATION

VDD x R2

VAS =

R1 + R2

Solving for the relationship between R1 and R2 results

in:

EQUATION

R1 = R2 x

VDD - VAS

VAS = R2 x

5 -1.53

1.53

In the case of this example, R1 = (2.27) R2. Substituting

this relationship back into the VAS equation above

yields the resistor values:

R2 = 15.3 kΩ, and

R1 = 34.7 kΩ

In this case, the standard values of 34.8 kΩ and

15.4 kΩ are very close to the calculated values and

would be more than adequate.

TC649

5.3 Operations at Low Duty Cycle

One boundary condition which may impact the

selection of the minimum fan speed is the irregular

activation of the Diagnostic Timer due to the TC649

“missing” fan commutation pulses at low speeds. This

is a natural consequence of low PWM duty cycles (typ-

ically 25% or less). Recall that the SENSE function

detects commutation of the fan as disturbances in the

current through RSENSE. These can only occur when

the fan is energized (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 TC649 may fail

to capture a pulse within the 32-cycle missing pulse

detector window. If 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 accelerates briefly when the Diagnostic

Timer fires. For this reason, it is recommended that VAS

be set no lower than 1.8V.

5.4 FanSense™ Network

(RSENSE and CSENSE)

The FanSense network, comprised of RSENSE and

CSENSE, allows the TC649 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 nec-

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

version ratio). A 0.1 µF ceramic capacitor is

recommended for CSENSE. Smaller values require

larger sense resistors, and higher value capacitors are

bulkier and more expensive. Using a 0.1 µF capacitor

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 net-

work.

 2002 Microchip Technology Inc.

DS21449C-page 11

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