TC664 MICROCHIP [Microchip Technology], TC664 Datasheet - Page 27

• Manufacturer Part Number: TC664
• Manufacturer: MICROCHIP [Microchip Technology]

Figure 7-10 shows the voltage pulse at the Sense pin,
which is caused by the fan's "extra" current pulse
during PWM output turn-on.
FIGURE 7-10:
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
C
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, C
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 TC664/TC665 as a true fan
current pulse. Typical values for R
are 1 k
SENSE
2002 Microchip Technology Inc.
scheme can continue to be used to sense the
and 1000 pF, respectively.
Extra Pulse at Sense Pin.
V OUT PWM
Sense Pin Voltage
"Extra Pulse"
SLOW
SENSE
and C
, is not
SLOW
FIGURE 7-11:
Capacitor.
FIGURE 7-12:
C
SLOW
SENSE
SENSE
TC664
TC665
TC664
TC665
Sense Scheme.
V
V
OUT
OUT
GND
GND
TC664/TC665
(0.1 µF typical)
(1 k typical)
R
(0.1 µF
C
typical)
Transistor Drive with C
FET Drive with R
C
R
(1000 pF
ISO
SLOW
C
typical)
SENSE
SLOW
SLOW
FAN
FAN
DS21737A-page 27
R
R
SLOW
SENSE
SENSE
/
SLOW