TC649B datasheet(15/36 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS21755B-page 15

TC646B/TC648B/TC649B

The SENSE pin senses positive voltage pulses that

have an amplitude of 70 mV (typical value). Each time

a pulse is detected, the missing pulse detector timer

(tMP) is reset. As previously stated, if the missing pulse

detector timer reaches the time for 32 cycles, the loop

for diagnosing a fan fault is engaged (diagnostic timer,

then the start-up timer).

Both of the fan current waveshapes shown in Figures

4-6 and 4-7 can be sensed with the sensing scheme

shown in Figure 4-8.

FIGURE 4-8:

Sensing Scheme For Fan

Current.

The fan current flowing through RSENSE generates a

voltage that is proportional to the current. The CSENSE

capacitor removes any DC portion of the voltage

across RSENSE and presents only the voltage pulse

portion to the SENSE pin of the TC646B/TC649B

devices.

The RSENSE and CSENSE values need to be selected so

that the voltage pulse provided to the SENSE pin is

70 mV (typical) in amplitude. Be sure to check the

sense pulse amplitude over all operating conditions

(duty cycles) as the current pulse amplitude will vary

with duty cycle. See Section 5.0, “Applications Informa-

tion”, for more details on selecting values for RSENSE

and CSENSE.

Key features of the SENSE pin circuitry are an initial

blanking period after every VOUT pulse and an initial

pulse blanker.

The TC646B/TC649B sense circuitry has a blanking

period that occurs at the turn-on of each VOUT pulse.

During this blanking period, the sense circuitry ignores

any pulse information that is seen at the SENSE pin

input. This stops the TC646B/TC649B device from

falsely sensing a current pulse that is due to the fan

drive device turn-on.

The initial pulse blanker is also implemented to stop

false sensing of fan current pulses. When a fan is in a

locked rotor condition, the fan current no longer com-

mutates, it simply flows through one fan winding and is

a DC current. When a fan is in a locked rotor condition

and the TC646B/TC649B device is in PWM mode, it

will see one current pulse each time the VOUT output is

turned on. The initial pulse blanker allows the

TC646B/TC649B device to ignore this pulse and

recognize that the fan is in a fault condition.

4.9

Behavioral Algorithms

The behavioral algorithms for the TC646B/TC649B

and TC648B devices are shown in Figure 4-9 and

Figure 4-10, respectively.

The behavioral algorithms show the step-by-step deci-

sion-making process for the fan speed controller oper-

ation. The TC646B and TC649B devices are very

similar with one exception: the TC649B device does

not implement the over-temperature portion of the

algorithm.

RISO

RSENSE

CSENSE

(0.1 µF typical)

SENSE

VOUT

TC64XB

GND

FAN

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