May, 2000

PRELIMINARY DATASHEET

9

PRELIMINARY

ML4435

FUNCTIONAL DESCRIPTION

MOTOR START-UP

When power is first applied to the ML4435, the motor is at

rest and thus, 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 ML4435

uses a minimum VCO frequency to begin commutating the

motor. This low frequency commutation is set by the 0.2V

clamp on RVCO, this provides a commutation frequency

at 1/30th of the maximum frequency.

RUN MODE

After the Back EMF sensing PLL has locked on to the

motor’s position, the motor is running in closed loop con-

trol. At this point, the speed control loop should force the

motor speed to the speed that corresponds to the SPEED

SET voltage.

PWM SPEED CONTROL

Speed control is accomplished by setting a speed com-

mand at SPEED SET (pin 5) with an input voltage from 0.2

to 6V. The accuracy of the speed command is determined

by the external components RVCO and CVCO. There are

a number of methods to control the speed command on

the ML4435. One method is to use a potentiometer from

RT to ground with the wiper going to SPEED SET. If SPEED

SET is controlled from a microcontroller, a DAC that uses

RT as its input reference can be used. The RT voltage must

be buffered connecting it to external circuits. The speed

command is compared with the sensed speed from SPEED

FB minus 0.7V (pin 18) through a transconductance error

amplifier. The output of the speed error amplifier is SPEED

COMP (pin 3). SPEED COMP is clamped between 8.2V

and 2.2V. A signal of 8.2V corresponds to full PWM duty

cycle and 2.2V corresponds to 0% duty cycle. Speed

loop compensation components are placed on this pin as

shown in Figure 11.

The speed loop compensation components are calculated

as follows:

C

NxKexVMOTOR R

C

J RI freq

SC

VCO

VCO

2

144

2

2

=

××

×

×× × ×

.

π

Rsc

freq Csc

=

××

×

10

22

π

Csc1 = 10 x Csc2

Where freq is the speed loop bandwidth in Hz.

Figure 11. PWM Oscillator Circuit

The voltage on SPEED COMP (pin 3) is compared with a

triangle wave oscillator to create a PWM duty cycle. The

PWM oscillator creates a triangle wave function from 3V

to 7V as shown in Figure 11. The frequency of the triangle

wave oscillator is set by a resistor to ground on RT (pin 6).

RT can be selected from the graph in Figure 12.

The PWM duty cycle from the speed control loop is gated

the pulse-by-pulse current limit that controls the LA, LB,

and LC output drivers.

Figure 12. RT vs PWM Frequency

SPEED

COMP

SPEED

SET

V

3V

5

SPEED FB

+

–

PWM

COMPARATOR

TRIAN LE

WAVE

COMPARATOR

SPEED

ERROR AMP

LEVEL SHIFT

0. V

+

–

2.2 + 0. V

CSE

.2 - 0. V

–

+

3

PWM FRE UENCY kHz

000

000

00

0