TC654 datasheet(26/38 Pages) MICROCHIP

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TC654/TC655

DS20001734C-page 26

 2002-2014 Microchip Technology Inc.

FIGURE 7-8:

Clamp Diode For Fan Turn-

Off.

7.6

Bias Supply Bypassing and Noise

Filtering

The bias supply (VDD) for the TC654/TC655 devices

should be bypassed with a 1 µF ceramic capacitor. This

capacitor will help supply the peak currents that are

required to drive the base/gate of the external fan drive

devices.

As the VIN pin controls the duty cycle in a linear fashion,

any noise on this pin can cause duty cycle jittering. For

this reason, the VIN pin should be bypassed with a

0.01 µF capacitor.

In order to keep fan noise off of the TC654/TC655

device ground, individual ground returns for the TC654/

TC655 and the low side of the fan current sense resis-

tor should be used.

7.7

Determining Current Pulses Per

Revolution of Fans

There are many different fan designs available in the

marketplace today. The motor designs can vary and,

along with it, the number of current pulses in one fan

revolution. In order to correctly measure and communi-

cate the fan speed, the TC654/TC655 must be pro-

grammed for the proper number of fan current pulses

per revolution. This is done by setting the F2PPR and

F1PPR bits in the Configuration Register to the proper

values (see Section 6.3 “Configuration Register

(Config)” for settings). A fan's current pulses per revo-

lution can be determined in the following manner.

The first piece of information required is the fan's full-

speed RPM rating. The fan RPM rating can then be

converted to give the time for one revolution using the

following equation:

EQUATION

The fan current can now be monitored over this time

period. The number of pulses occurring in this time

period is the fan's "Current Pulses per Rotation" rating

which is needed in order to accurately read fan RPM.

Example: The full-speed fan RPM rating is 8200 RPM.

From this, the time for one fan revolution is calculated

to be 7.3 msec, using the previously discussed equa-

tion. Using a current probe, the fan current can be mon-

itored as the fan is operating at full speed. Figure 7-9

shows the fan current pulses for this example. The

7.44 msec window, marked by the cursors, is very near

the 7.3 msec calculated above, and is within the toler-

ance of the fan ratings. Four current pulses occur within

this 7.44 msec time frame. Given this information,

F2PPR (bits 4-3<01>) and F1PPR (bits 2-1<01>) in the

Configuration Register, should be set to '10' to indicate

4 current pulses per revolution.

FIGURE 7-9:

Four Current Pulses Per

Revolution Fan.

7.8

How to Eliminate False Current

Pulse Sensing

During the PWM mode of operation, some fans will

generate an extra current pulse. This pulse occurs

when the external drive device is turned on and is, in

most cases, caused by the fan's electronics that control

the fan motor. This pulse does not represent true fan

current and needs to be blanked out. This is particularly

important for detecting a fan in a locked rotor condition.

Figure 7-10 shows the voltage pulse at the Sense pin,

GND

RSENSE

VOUT

Q1- N-Channel MOSFET

FAN

Time for one revolution (msec.)

1000



Fan RPM

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

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