TC654 datasheet(22/36 Pages) MICROCHIP

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TC654/TC655

DS21734A-page 22

 2002 Microchip Technology Inc.

7.0

APPLICATIONS INFORMATION

7.1

Connecting to the SMBus

The SMBus is an open collector bus, requiring pull-up

resistors connected to the SDA and SCLK lines. This

configuration is shown in Figure 7-1.

FIGURE 7-1:

Pull-up Resistors On

SMBus.

The number of devices connected to the bus is limited

only by the maximum rise and fall times of the SDA

and SCLK lines. Unlike I2C specifications, SMBus

does not specify a maximum bus capacitance value.

Rather, the SMBus specification calls out that the max-

imum current through the pull-up resistor be 350 µA

(minimum, 100 µA, is also specified). Therefore, the

value of the pull-up resistors will vary depending on the

system’s bias voltage, VDD. Minimizing bus capaci-

tance is still very important as it directly effects the rise

and fall times of the SDA and SCLK lines. The range

for pull-up resistor values for a 5 V system are shown

in Figure 7-1.

Although SMBus specifications only require the SDA

and SCLK lines to pull down 350 µA, with a maximum

voltage drop of 0.4 V, the TC654/TC655 has been

designed to meet a maximum voltage drop of 0.4 V,

with 3 mA of current. This allows lower values of pull-

up resistors to be used, which will allow higher bus

capacitance. If this is to be done, though, all devices on

the bus must be able to meet the same pull down

current requirements as well.

A possible configuration using multiple devices on the

SMBus is shown in Figure 7-2.

FIGURE 7-2:

Multiple Devices on SMBus.

7.2

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 TC654/TC655 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 monitor the fan cur-

rent pulses. The goal is to be able to monitor at least

two fan current pulses during the on time of the VOUT

output.

Example: Your system design requirement is to oper-

ate the fan at 50% duty cycle when ambient tempera-

tures 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 approxi-

mately 1500 RPM.

EQUATION

If one fan revolution occurs in 40 msec, then 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 frequency of 20 Hz is recommended. This would

define a CF capacitor value of 1.5 µF.

SDA

SCLK

VDD

TC654/TC655

Range for R: 13.2 k

Ω to 46 kΩ for VDD = 5.0 V

SDA SCLK

PIC16F876

Microcontroller

TCN75

Temperature

Sensor

24LC01

EEPROM

TC654/TC655

Fan Speed

Controller

Time for one revolution (msec.)

60 1000

1500

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

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