TC654 datasheet(27/38 Pages) MICROCHIP

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DS20001734C-page 27

TC654/TC655

which is caused by the fan's "extra" current pulse

during PWM output turn-on.

FIGURE 7-10:

Extra Pulse at Sense Pin.

This problem occurs mainly with fans that have a cur-

rent waveshape like the one shown in Figure 7-9. For

configurations where an NPN transistor is being used

as the external drive device, the typical RSENSE and

CSENSE scheme can continue to be used to sense the

fan current pulses. In order to eliminate the extra cur-

rent pulse, a slow-down capacitor can be placed from

the base of the transistor to ground. A 0.1 µF capacitor

is appropriate in most cases. This arrangement is

shown in Figure 7-11. This capacitor will help to slow

down the turn-on edge of the transistor and reduce the

amplitude of the extra current pulse.

For configurations using an N-channel MOSFET as the

drive device, the slow-down capacitor does not fix all

conditions and the current sensing scheme must be

changed. Since the current for this type of fan always

returns to zero, the coupling capacitor (CSENSE) is not

needed. Instead, it will be replaced by an R-C configu-

ration to eliminate the voltage pulse generated by the

extra current pulse. This new sensing configuration is

shown in Figure 7-12. The values of the resistor/capac-

itor combination should be adjusted so that the voltage

pulse generated by the extra current pulse is smoothed

and is not registered by the TC654/TC655 as a true fan

current pulse. Typical values for RSLOW and CSLOW are

 and 1000 pF, respectively.

FIGURE 7-11:

Transistor Drive with CSLOW

Capacitor.

FIGURE 7-12:

FET Drive with RSLOW/

CSLOW Sense Scheme.

Sense Pin Voltage

"Extra Pulse"

VOUT PWM

FAN

RISO1

RISO2

CSENSE

(0.1 µF typical)

SENSE2

SENSE1

VOUT

TC654/

TC655

(0.1 µF typical)

CSLOW1

(0.1uF

GND

typical)

(0.1 µF typical)

RSENSE2

CSENSE2

RSENSE1

CSLOW2

FAN

RSLOW2

RSLOW1

RSENSE2

RSENSE1

CSLOW1

(1000pF typical)

SENSE2

SENSE1

VOUT

TC654/

TC655

CSLOW2

(1000pF typical)

GND

(1 k

 typical)

(1 k

 typical)

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