ML4428
C
VCO AND PHASE DETECTOR CALCULATIONS
VCO
As given in the section on the VCO and phase detector:
The VCO should be set so that at the maximum frequency
of operation (the running speed of the motor) the VCO
−6
control voltage will be no higher than V , or 6V. The
REF
2931×10
C
=
VCO maximum frequency will be:
VCO
N×RPM
MAX
F
= 0.05×N×RPM
MAX
MAX
Where N is the number of poles in the motor, and RPM is
the motor’s maximum operating speed in revolutions per
minute.
where N is the number of poles on the motor and
RPM is the maximum motor speed in Revolutions Per
MAX
Minute.
C
PWM
The minimum VCO gain derived from the specification
table (using the minimum F
This capacitor sets the PWM ramp oscillator frequency.
This is the PWM “switching frequency”. If this value is too
low, <20kHz, then magnetostriction effects in the motor
may cause audible noise. If this frequency is too high,
>30kHz, then the switching losses in the output drivers
may become a problem. 25kHz should be a good
compromise for this value, which can be obtained by
using a 1nF capacitor.
at V
= 6V) is:
VCO
VCO
−5
2.665 ×10
K
=
VCO(MIN)
C
VCO
Assuming that the V
= 5.5V, then
VCO(MAX)
5.5 × 2.665 ×10−5
CVCO
=
R
R
AND R
REF
VCO
VCO
FMAX
should be 10k and R
operation.
should be 120k for normal
REF
or
−6
2931×10
=
VCO FILTER
C
VCO
N×RPM
See the section on the VCO and Phase detector for
information on these components.
MAX
Ω
Gm = 0.23m
+
–
Z
RC
SAMPLED
PHASE
RC
VCO
R
C1
C2
F
OUT
VCO
(Hz/sec/V)
K
VCO
A/RADIAN
V/A
ROTOR
PHASE
BEMF
SAMPLER
(R × C2 × s + 1)
s × (C2 + R × C1 × s × C2 + C1)
Ke × ω × Atten
2 × π
gm = 0.23mA/V
LOOP FILTER
PHASE DETECTOR
RADIAN/sec/V
–5
2.665 × 10
× s
× 2 × π
C
VCO
VCO
Figure 4. Back-EMF Phase Locked Loop Components.
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
