TC647B Microchip Technology Inc., TC647B Datasheet - Page 15

• Manufacturer Part Number: TC647B
• Description: Pwm Fan Speed Controllers With Minimum Fan Speed, Fan Restart And Fansense Technology For Fault Detection
• Manufacturer: Microchip Technology Inc.

.
FIGURE 4-7:
Commutation Pulses To Zero.
The SENSE pin senses positive voltage pulses that
have an amplitude of 70 mV (typical value). When a
pulse is detected, the missing pulse detector timer is
reset. As previously stated, if the missing pulse detec-
tor 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 that are shown in
Figures 4-6 and 4-7 can be sensed with the sensing
scheme shown in Figure 4-8.
FIGURE 4-8:
Current.
The fan current flowing through R
voltage that is proportional to the current. The C
capacitor removes any DC portion of the voltage
2003 Microchip Technology Inc.
TC64XB
GND
SENSE
V
OUT
(0.1 µF typical)
Fan Current With
Sensing Scheme For Fan
C
SENSE
R
ISO
SENSE
R
SENSE
generates a
FAN
SENSE
across R
portion to the SENSE pin of the TC642B/TC647B
devices.
The R
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 R
and C
Key features of the SENSE pin circuitry are an initial
blanking period after every V
pulse blanker.
The TC642B/TC647B sense circuitry has a blanking
period that occurs at the turn-on of each V
During this blanking period, the sense circuitry ignores
any pulse information that is seen at the SENSE pin
input. This stops the TC642B/TC647B device from
falsely sensing a current pulse which 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 TC642B/TC647B device is in PWM mode, it
will see one current pulse each time the V
turned on. The initial pulse blanker allows the TC642B/
TC647B device to ignore this pulse and recognize that
the fan is in a fault condition.
4.9
The behavioral algorithm for the TC642B/TC647B
devices is shown in Figure 4-9.
The behavioral algorithm shows the step-by-step deci-
sion-making process for the fan speed controller oper-
ation. The TC642B and TC647B devices are very
similar with one exception: the TC647B device does
not implement the over-temperature portion of the
algorithm.
SENSE
SENSE
Behavioral Algorithms
SENSE
TC642B/TC647B
.
and C
and presents only the voltage pulse
SENSE
values need to be selected so
OUT
pulse and an initial
DS21756B-page 15
OUT
OUT
output is
SENSE
pulse.