Minute.
ML4411/ML4411A
The minimum VCO gain derived from the specification
table (using the minimum Fvco at V
= 6V) is:
VCO
START-UP SEQUENCING
−6
2.42×10
K
=
When the motor is initially at rest, it is generating no
back-EMF. Because a back-EMF signal is required for
closed loop commutation, the motor must be started
“open-loop” until a velocity sufficient to generate some
back-EMF is attained (around 100 RPM). The following
steps are a typical procedure for starting a motor which is
at rest.
VCO(MIN)
C
VCO
Assuming that the V
= 9.5V, then
VCO(MAX)
−6
9.5× 2.42×10
C
=
VCO
F
MAX
or
Step 1: The IC is held in reset (state R) with full power
applied to the windings (see figure 6). This aligns the rotor
to a position which is 30° (electrical) before the center of
the first commutation state.
460
CVCO
=
µF
POLES ×RPM
3000
Step 2: Reset is released, and a fixed current is input to
pin 21 and appears as a current on pin 20, and will ramp
the VCO input voltage, accelerating the motor at a fixed
rate.
2500
2000
1500
1000
500
0.01µF
Step 3: When the motor speed reaches about 100 RPM,
the back EMF loop can be closed by pulling pin 18 high.
0.02µF
RESET/
ALIGN
OPEN-LOOP
(STEPPING)
CLOSED LOOP
0
P1, P3, N2 ON
0
2
4
6
8
10
12
V
VCO
(VOLTS)
Figure 3. VCO Output Frequency vs. V
(Pin 20)
VCO
Figure 4 shows the transfer function of the Phase Lock
Loop with the phase detector formed from the sampled
phase through the Gm amplifier with the loop filtered
formed by R, C1, and C2.
0
RESET
The impedance of the loop filter is
ENABLE E/A
(s + ω
C s (s + ω
)
1
LEAD
Z
RC
(s) =
)
LAG
1
Figure 6. Typical Start-up Sequence.
Using this technique, some reverse rotation is possible.
The maximum amount of reverse rotation is 360/N, where
N is the number of poles. For an 8 pole motor, 45° reverse
rotation is possible.
–4
Gm = 1.25 x 10
+
Z
RC
SAMPLED
PHASE
RC
–
For quick recovery following a momentary power failure,
the following steps can be taken:
R
C1
C2
PIN
16
PIN
18
PIN
21
ILIMIT
ICMD
F
OUT
VCO
(HZ/V)
STEP
1
2
3
0
1
1
0
0
1
FIXED
FIXED
0
IMAX
IMAX
IMAX
K
VCO
Figure 4. Back EMF Phase Lock Loop Components
Table 2. Start-up Sequence.
Where the lead and lag frequencies are set by:
1
RC
ω
=
LEAD
2
C1 + C2
RC1C2
ωLAG
=
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
