
FAN8460MTC/FAN8460MP
12
output PWM duty is proportional to input PWM duty with some dead band. The output PWM frequency is defined by external
capacitor, C
T.
So there is no relationship between VPWM input frequency and output PWM frequency. Table 1 summarizes
the motor speed according to digital VPWM input and V
VCON
Table 1.
.
Operation Tables
Mode
4.3 PWM Speed Control using External PWM Input(Scheme 3)
This scheme indicates that digital PWM input signal becomes directly output PWM signal. In other word, frequency and duty
of output PWM driving signal is the same as this digital PWM input. In case input PWM frequency is very low, active filter
needs large value of capacitor to make speed control voltage in scheme 2. This scheme dosen’t need filter capacitor and has
good input/output characteristics. This means that there is no deadband and output signals are synchronized with input VPWM
signal as shown in figure6.
Figure 6. Interface and it’s Related Waveforms Scheme3
4.4 Offset comparator(Thermistor open protection)
If under 100mV difference between VREF and VCON, fan motor runs at full speed.
5 Locked Rotor Protection with Open Collector Output and Automatic Restart
When the rotor is locked, there is no change in input signal of hall amplifier and thus a internal T
ZERO
pulse is not observed.
A capacitor (C
LD
) connected LD pin is continually charged by internal current source (i
LDC
) to the internal threshold
(V
LDCL
) resulting from no Tzero pulse. When the voltage, V
CLD on
LD pin, reaches V
LDCL
, high side output power TR is
turned-off to protect motor during T
OFF
and the alarm output (AL) becomes floating high. When the V
CLD
reaches upper
threshold, V
LDCL
, V
CLD
starts to decrease with internal current sink (i
LDD
) to the low threshold, V
LDCP
. At that time, the
V
CLD
ramps up again and one of two outputs is turned on depending on locked rotor position during T
ON
. The charging and
discharging repeat until locked condition is removed, or FAN8460MTC/FAN8460MP is powered down. The overall time chart
is shown in figure.6. The auto- retry time (T
ON
), the motor protection time (T
OFF
) and the locked rotor detection time
(T
LOCK
) are proportional to external capacitor, C
LD.
Each value can be calculated as follows;
VPWM
H
L
H/L
GND
VCON
L
H
Speed Condition
Full Speed
Stop
proportional to PWM input duty
The higher TEMP, the faster fan speed
PWM Input
Depend on mother board PWM signal
Depend on thermistor resistance
Thermistor Input
R
PWM
2V
Reference
5
4
VCON
11
VREF
VPWM
PWM
Decoder
6.2K
VCC
H -
H+
VPWM
OUTA
OUTB
20K
12
CT
180K