PRELIMINARY DATASHEET

May, 2000

8

PRELIMINARY

ML4435

times the commutation frequency. The TACH out also

indicates a low motor speed by staying TTL high when the

motor is at its minimum speed (SPEED FB below 0.97V).

BACK EMF SAMPLER

The input to the VCO (pin 18) SPEED FB is controlled by

the Back EMF Sampler. The back EMF sense pins FB A,

FB B, and FB C inputs to the back EMF sampler require

a signal from the motor phase leads that is below the

VCC of the ML4435. The phase sense input impedance is

8.7k

Ω. If the motor voltage is greater than the VCC of the

ML4435 then this requires a series resistor RFB from the

motor phase lead shown in Figure 8. RFB can be selected

by using the graph in Figure 9 or using the following

equation:

VNEUTRAL = (FB A + FB B + FB C)/3

This allows the ML4435 to compare the back EMF signal

to the motors neutral point without the need for bringing

out an extra wire on a WYE wound motor. For DELTA

wound motors there isn’t a physical neutral to bring out so

this reference point must be calculated anyway.

The back EMF sampler takes the motor phase that is not

driven (i.e. if LA and

HB are on then phase A is driven

low and phase B is driven high then phase C is sampled).

The sampled phase provides a back EMF signal that is

compared against the neutral of the motor. The sampler

is controlled by the commutation state machine. The

sampled back EMF is compared to the neutral through an

error amplifier. The output of the error amplifier outputs

a charging or discharging current to SPEED FB (pin 18)

which provides the voltage to the VCO.

BACK EMF SENSING PHASE LOCKED LOOP

COMMUTATION CONTROL

The three blocks: The commutation state machine, the

VCO, and the back EMF sampler form a phase locked

loop that locks the commutation clock onto the back EMF

signal. The complete phase locked loop is illustrated in

Figure 10. The phased locked loop requires a lead lag

filter that is set by external components on SPEED FB (pin

18). The filter components in Figure 10 work for most

applications. If performance is unstable C1 and C2 can

go up or down a decade in value as low as the C2 stays

equal to C1 x 10.

Figure 10. Phase Locked Loop

FBA

VCO

BACK EMF

SAMPLER

COMMUTATION

STATE MACHINE

4

SPEED FB

RSFB

CSFB2

CSFB

6

5

FBB

FBC

RSFB2

Figure 9. RFB vs. VMOTOR [MAX]

FUNCTIONAL DESCRIPTION

RFB = 8.7k

(

VMOTOR

12

-1)

W

The back EMF sampler takes the motor phase voltages

divided down to signals that are less than VCC (12V

nominal) and calculates the neutral point of the motor by

the following equation:

Motor Volta e [Max] V

05

20

25

30

2,000

0

4,000

6,000

,000

0,000

2,000

Figure 8. Back EMF Sampler Configuration

BLDC

MOTOR

RFBA

RFBB

RFBC

FBA

ABC

4

FBB

5

FBC

6