
FAN8460MTC/FAN8460MP
10
motor while PWM switching occurs. PWM occurs on the high side, and the freewheeling current flow on the low side during
T
OFF
.
3 Triangle Waveform Generator (PWM Oscillator)
The PWM oscillator output (V
CT
) sets output PWM frequency using external capacitor (C
T
). When V
C
T
reaches the upper
threshold(1.8V typical) by internal current source(6uA typical), C
T
begins to be discharged by internal current sink(-6uA typ-
ical) until the low threshold(0.8V typical). It repeats the charging and discharging cycle. To have a desired PWM frequency,
f
CT
,
can be calculated as follows;
For example, C
T
= 100pF, then f
CT
is about 25KHz.
4. Speed Control Voltage (V
VCON
) and Active Filter (PWM Decoder)
In general, many PC super IO and hardware monitoring ICs provide one of two fan speed control output to provide variable
fan speed control without an external drive power stage. FAN8460MTC/FAN8460MP have two type of input stage scheme.
This means an end user can control the fan speed with a PWM signal or a DC control voltage (typically thermistor input). Fig-
ure.3 shows two kind of input stages; ambient temperature based input stage using thermistor(figure.3a) and digital PWM
input stage(figure.3b).
Figure 3. Input Stages and Speed Control Voltage Output
4.1 NTC Thermistor based Speed Control(Scheme 1)
When the ambient temperature based speed control is used, the VPWM pin must be connected with ground as shown fig-
ure.3a. The V
VCON
will be adjusted automatically by ambient temperature with NTC thermistor. When the ambient tempera-
ture increases, decreased thermistor resistance results in low V
VCON
and high fan motor speed. An optional resistor, R
MIN,
is
set to a minimum speed when thermistor is accidentally disconnected.
The V
VCON
is calculated as follows;
For example, RPWM = 3K, RMIN =open, RNTC=10K, the V
VCON
is shown in fig4.a. When the temperature is higher than
65, motor will run at full speed. In case, the temperature is under 5
°
C
, no drive will be present. But the practical motor stop
temperature is slightly higher than 5
°
C
because motor needs minimum starting torque depending on mechanics, motor size. In
case, the RMIN is not used, fan motor runs at full speed when the thermistor is accidentally disconnected.
Another example is useage of optinal resistor R
MIN
to limit minimum motor speed. For example, RPWM = 1.5K, RMIN =
3.3K, RNTC=10K, fig4.b shows the resultant VCON voltage is under 1.3V and thus the minimum PWM duty will be over 0.5.
It means motor will runs at medium speed even if NTC is disconnected accidentally.
C
T
I
CTC
2f
CT
V
CTMAX
V
CTMIN
–
(
)
×
--------------------------------------------------------------------
=
R
NTC
R
PWM
2V
Reference
5
4
VCON
11
VREF
VPWM
PWM
Decoder
VS
R
MIN
PWM
Decoder
C
PWM
R
PWM
2V
Reference
5
4
VCON
11
VREF
VPWM
6.2K
VS
120K
R
OPT
12
CT
100pF
12
CT
100pF
V
VCON
+
R
MIN
R
NTC
R
PWM
-------------------------------------------------------------
V
VREF
×
=