TC646 datasheet(12/28 Pages) MICROCHIP

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TC646

DS21446C-page 12

 2002 Microchip Technology Inc.

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

We can further specify R1 and R2 by the condition that

the divider voltage is equal to our desired VAS. This

yields:

EQUATION

Solving for the relationship between R1 and R2 results

in:

EQUATION

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

Substituting this relationship back into the original

equation yields the resistor values:

R2 = 15.3 kΩ, and

R1 = 34.7 kΩ

In this case, the standard values of 34.8 k

Ω and

15.4 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 TC646 “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 TC646 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 TC646 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

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.

GND

VDD

VAS

IDIV

IIN

R1 + R2

IDIV = 1e

–4A =

, therefore

5.0V

R1 + R2 =

= 50,000

Ω = 50kΩ

1e–4A

5.0V

VAS =

VDD x R2

R1 + R2

R1 = R2 x

= R2 x

VDD - VAS

VAS

5 - 1.53

1.53

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