May, 2000

PRELIMINARY DATASHEET

7

PRELIMINARY

ML4435

mutation to achieve rotational motion. Electronic com-

mutation requires the switching on and off of the power

switches of a 3 phase half bridge. For torque production

to be achieved in one direction the commutation is dic-

tated by the rotor’s position. Electronic commutation in

the ML4435 is achieved by turning on and off, in the

proper sequence, one L output from one phase and one

H

output from another phase. There are six combinations of

L and

H outputs (six switching states) that constitute a full

commutation cycle as illustrated in Table 1 labeled state

A through F. This switching sequence is programmed into

the commutation state machine as illustrated in Figure 6.

Clocking of the commutation state machine is provided by

the output of a VCO.

VOLTAGE CONTROLLED OSCILLATOR

The VCO outputs a TTL compatible clock on the TACH

(pin 2) proportional to the input voltage to the voltage

controlled oscillator SPEED FB (pin 18). The proportion of

frequency to voltage or VCO constant Kv is set by a resistor

to ground on RVCO (pin 4) and capacitor to ground on

CVCO (pin 20) as shown in Figure 7. RVCO sets up a

current proportional to the VCO input voltage SPEED FB

minus 0.7V. This current is used to charge and discharge

CVCO between the threshold voltages of 2V and 3.75V as

shown in Figure 7. RVCO has a lower voltage limit of 0.2V.

The resulting triangle wave of CVCO corresponds to the

clock on the TACH pin, this is also illustrated in Figure 7. Kv

should be set so that the VCO output frequency corresponds

to the maximum commutation frequency FMAX and thus the

maximum motor speed when the VCO input is equal to or

slightly less than 6V. CVCO and RVCO can be selected by

first calculating FMAX and then using the selection graph in

Figure 8. FMAX is calculated as follows:

FMAX = 0.05 x RPMMAX x N

Where: RPMMAX = The maximum speed of operation

(RPM). N = The number of magnetic poles (Unitless).

TACH

The TACH (pin 2) outputs the VCO frequency. This is 6

Figure 6. Commutation State Machine

3s

POWER ON

RESET PULSE

CLK VCO

A

B

C

D

E

F

R

COMMUTATION

STATE MACHINE

Figure 7. VCO Control

RVCO

SPEED FB

0.9V

VOLTA E

CONTROLLED

OSCILLATOR

LEVEL

SHIFT

CVCO

3. 5V

2V

4

20

-

+

Table 1. Communication Control States

OUTPUT DRIVERS

A

LA

LB

LC

HA

HB

HC

LOW LOW HI H LOW HI H HI H

LOW LOW HI H HI H LOW HI H

HI H LOW LOW HI H LOW HI H

HI H LOW LOW HI H HI H LOW

LOW HI H LOW HI H HI H LOW

LOW

= DRIVER ON

HI H LOW LOW HI H HI H

B

C

D

E

F

FUNCTIONAL DESCRIPTION