6

ML4411/ML4411A

Minute.

The minimum VCO gain derived from the specification

K

C

VCO MIN

VCO

()

.

=

C

F

VCO

MAX

=

..

or

C

POLES RPM

F

×

460

µ

02

468

10

12

3000

2500

2000

1500

1000

500

0

0.01

µF

0.02

µF

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.

The impedance of the loop filter is

Zs

Cs

s

s

RC

LEAD

LAG

()

()

()

=

+

+

1

1

ω

ω

R

C1

C2

VCO

–

+

RC

SAMPLED

PHASE

Figure 4. Back EMF Phase Lock Loop Components

Where the lead and lag frequencies are set by:

RC

= 1

2

CC

RC C

=

+

12

12

START-UP SEQUENCING

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.

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.

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.

Step 3: When the motor speed reaches about 100 RPM,

the back EMF loop can be closed by pulling pin 18 high.

RESET/

ALIGN

P1, P3, N2 ON

OPEN-LOOP

(STEPPING)

CLOSED LOOP

0

RESET

ENABLE E/A

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.

For quick recovery following a momentary power failure,

the following steps can be taken:

PIN

PIN

PIN

STEP

16

18

21

1

0

0

FIXED

2

1

0

FIXED

3110

Table 2. Start-up Sequence.