TC649B datasheet(18/36 Pages) MICROCHIP

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TC646B/TC648B/TC649B

DS21755B-page 18

 2003 Microchip Technology Inc.

5.0

APPLICATIONS INFORMATION

5.1

Setting the PWM Frequency

The PWM frequency of the VOUT output is set by the

capacitor value attached to the CF pin. The PWM fre-

quency will be 30 Hz (typical) for a 1 µF capacitor. The

relationship between frequency and capacitor value is

linear, making alternate frequency selections easy.

As stated in previous sections, the PWM frequency

should be kept in the range of 15 Hz to 35 Hz. This will

eliminate the possibility of having audible frequencies

when varying the duty cycle of the fan drive.

A very important factor to consider when selecting the

PWM frequency for the TC646B/TC648B/TC649B

devices is the RPM rating of the selected fan and the

minimum duty cycle that you will be operating at. For

fans that have a full-speed rating of 3000 RPM or less,

it is desirable to use a lower PWM frequency. A lower

PWM frequency allows for a longer time-period to mon-

itor the fan current pulses. The goal is to be able to

monitor at least two fan current pulses during the on-

time of the VOUT output.

Example: The system design requirement is to operate

the fan at 50% duty cycle when ambient temperatures

are below 20°C. The fan full-speed RPM rating is

3000 RPM and has four current pulses per rotation. At

50% duty cycle, the fan will be operating at

approximately 1500 RPM.

EQUATION

If one fan revolution occurs in 40 msec, each fan pulse

occurs 10 msec apart. In order to detect two fan current

pulses, the on-time of the VOUT pulse must be at least

20 msec. With the duty cycle at 50%, the total period of

one cycle must be at least 40 msec, which makes the

PWM frequency 25 Hz. For this example, a PWM fre-

quency of 20 Hz is recommended. This would define a

CF capacitor value of 1.5 µF.

5.2

Temperature Sensor Design

As discussed in previous sections, the VIN analog input

has a range of 1.20V to 2.60V (typical), which repre-

sents a duty cycle range on the VOUT output of 0% to

100%, respectively. The VIN voltages can be thought of

as representing temperatures. The 1.20V level is the

low temperature at which the system requires very little

cooling. The 2.60V level is the high temperature, for

which the system needs maximum cooling capability

(100% fan speed).

One of the simplest ways of sensing temperature over

a given range is to use a thermistor. By using a NTC

thermistor, as shown in Figure 5-1, a temperature-

variant voltage can be created.

FIGURE 5-1:

Temperature Sensing

Circuit.

Figure 5-1 represents a temperature-dependent, volt-

age divider circuit. RT is a conventional NTC thermistor,

R1 and R2 are standard resistors. R1 and RT form a

parallel resistor combination that will be referred to as

RTEMP (RTEMP = R1 * RT / R1 + RT). As the temperature

increases, the value of RT decreases and the value of

RTEMP will decrease with it. Accordingly, the voltage at

VIN increases as temperature increases, giving the

desired relationship for the VIN input. R1 helps to linear-

ize the response of the SENSE network and aids in

obtaining the proper VIN voltages over the desired tem-

perature range. An example of this is shown in

Figure 5-2.

If less current draw from VDD is desired, a larger value

thermistor should be chosen. The voltage at the VIN pin

can also be generated by a voltage output temperature

sensor device. The key is to get the desired VIN volt-

age-to-system (or component) temperature relation-

ship.

The following equations apply to the circuit in

Figure 5-1.

EQUATION

In order to solve for the values of R1, R2, VIN and the

temperatures at which they are to occur, need to be

selected. The variables T1 and T2 represent the

selected temperatures. The value of the thermistor at

these two temperatures can be found in the thermistor

Time for one revolution (msec.)

60 1000

1500

------------------------40

IDIV

VIN

VDD

VT1

()

VDD R2

RTEMP T1

() R2

----------------------------------------------

VT2

()

VDD R2

RTEMP T2

() R2

----------------------------------------------

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