11

ML4425

CAT

CAT

FB A

FB B

FB C

BACK

EMF

SAMPLER

CRT

CRT

CRR

–

+

1.5V

750nA

–

+

1.5V

750nA

VDD

VDD

CRR

500nA

VDD

SPEED

FB

TO

SPEED FB

FILTER

CVCO

RVCO

VOLTAGE

CONTROLLED

OSCILLATOR

VCO/TACH

TO RESET INPUT

OF COMMUTATION

STATE MACHINE

Run Mode (Back EMF Sensing)

At the end of ramp mode the controller goes into run

mode. In run mode, the back EMF sensing is enabled and

commutation is now under the control of the phase locked

loop. Motor speed is now regulated by the speed control

loop.

PWM SPEED CONTROL

Speed control is accomplished by setting a speed

command at SPEED SET with an input voltage from 0 to

There are a number of methods that can be used to control

the speed command of the ML4425. One is to use a 10k

W

connected to SPEED SET. If SPEED SET is controlled from

as its input reference.

The speed command is compared with the sensed speed

from SPEED FB through a transconductance error

amplifier. The output of the speed error amplifier is SPEED

COMP. SPEED COMP is clamped between one diode drop

above 3.9V (approximately 4.6V) and one diode drop

below 1.7V (approximately 1V) to prevent speed loop

“wind-up”. Speed loop compensation components are

connected to this pin as shown in Figure 9. The speed loop

compensation components are calculated as follows:

C

NV

C

fK

m

f

SC

MOTOR

VCO

SB

e

SB

=

™™

™

™+

™

™

26 9

25 98 696

2

2

.

..

t

(9a)

R

fC

SC

SB

SC

=

™™

10

2

p

(9b)

Figure 8. ML4425 Start-up Circuitry for Controlling the Align and Ramp Times

–

+

+

–

SPEED SET

SPEED COMP

3.9V

1.7V

10k

Ω

20kHz

PWM ON/OFF

FROM ILIMIT

ONE-SHOT

FROM

SPEED FB

TO

GATING

LOGIC &

OUTPUT

DRIVERS

CT

CT

CSC

RSC

1.7V

VREF

The voltage on SPEED COMP is compared with a ramp

oscillator to create a PWM duty cycle. The PWM ramp

oscillator creates a sawtooth function from 1.7V to 3.9V

as shown in Figure 9. A negative clamp at one diode drop

below 1.7V (approximately 1V) starts the oscillator on

power up. The frequency of the ramp oscillator is set by a

following equation:

C

f

A

V

T

PWM

=

™

1

50

24

m

.

(10)

cycle from the speed control loop is gated the current

limit one shot that controls the LA, LB, and LC output

drivers.

Figure 9. Speed Control Loop Component Connections