10

ML4425

FB A

22

FB B

23

FB C

24

BACK

EMF

SAMPLER

VDD

500nA

SPEED

FB

20

VOLTAGE

CONTROLLED

OSCILLATOR

PHASE

LOCKED

LOOP

VCO/TACH

13

A

B

C

D

E

F

R

COMMUTATION

STATE MACHINE

BACK EMF SENSING PLL COMMUTATION CONTROL

Three blocks form a phase locked loop that locks the

commutation clock onto the back EMF signal: the

commutation state machine, the voltage controlled

oscillator, and the back EMF sampler. The complete phase

locked loop is illustrated in Figure 7. The phased locked

loop requires a lead lag filter that is set by external

components on SPEED FB. The components are selected

as follows:

C

K

M

N

d

f

SPEEDFB

OS

VCO

1

1

2

2

2

025

100

=™

™





























.

ln

(6a)

RM

d

f

NK

M

SPEEDFB

VCO

SO

2

100

1

1

ln

CC

M

SPEEDFB

SPEEDFB

21

1

=™

-

05

(6c)

START-UP SEQUENCE

When power is first applied to the ML4425 and the motor

is at rest, the back EMF is equal to zero. The motor needs

to be rotating for the back EMF sampler to lock onto the

rotor position and commutate the motor. The ML4425 uses

an open loop start-up technique to bring the rotor from rest

up to a speed fast enough to allow back EMF sensing.

Start-up is comprised of three modes: align mode, ramp

mode, and run mode.

Align Mode (RESET)

Before the motor can be started, the rotor must be in a

known position. When power is first applied to the

ML4425, the controller is reset into the align mode. Align

mode turns on the output drivers LB, HA, and HC which

aligns the motor into a position 30 electrical degrees

before the center of the first commutation state. This is

shown as state R in the commutation states of Table 1.

Align mode must last long enough to allow the motor and

its load to settle into this position. The align mode time is

GND to 1.5 V until the align comparator trips to end the

follows:

C

tamp

V

AT

S

=

™™

™

-

75 10

15

7

.

.

(7)

If the align time is not long enough to allow the rotor to

desired performance is achieved.

Ramp Mode

At the end of align mode the controller goes into ramp

mode. Ramp mode starts commutating through the states

A through F as shown in Table 1. This ramps up the

commutation frequency, and therefore the motor speed,

for a fixed length of time. This allows the motor to reach a

sufficient speed for the back EMF sampler to lock

commutation onto the motor's back EMF. The amount of

time the ML4425 stays in ramp mode is determined by a

1.5 V until the ramp comparator trips to end the ramp

follows:

C

Jamp K

IK

N

RT

V

MAX

t

=

™™ ™

™

™

™™ ™

-

25 10

3

7

p

(8)

The rate at which the ML4425 ramps up the motor speed

is determined by a fixed 500µA current source on the

SPEED FB pin. The current sources charges up the PLL

filter components causing the VCO frequency to ramp up.

During ramp mode, the back EMF sampler is disabled to

allow control of the ramping to be set only by the 500µA

current source. The ramp based on the SPEED FB filter is

generally too fast for the motor to keep up, so a capacitor

ramping rate. The optimal ramp rate is based on the motor

and load parameters and is can be adjusted by varying

Figure 7. Back EMF Commutation Phase Locked Loop