Functional Description

The commutation decoder receives Hall-sensor inputs HS1

HS2 and HS3 and a 3060 SELECT input This block de-

codes the gray-code sequence to the required motor-drive

sequence

The dead-time generator monitors the DIRECTION input

and inhibits the outputs (pins 11 thru 16) for a time sufficient

to prevent current-spiking in the external power switches

when the direction is reversed

The six chip outputs drive external power switching devices

which drive the motor Three outputs source current the

remaining three sink current The output transistors provide

up to 50 mA outputs for driving devices or up to 40V output

swings for driving MOSFETs The LM621 logic is powered

from 5V

and if the voltage is not sufficient to permit reliable logic

operation the outputs are shutdown

Three-Phase Motor Commutation

There are two popular conventions for establishing the rela-

tive phasing of rotor-position signals for three-phase mo-

tors While usually referred to as 30-degree and 60-degree

sensor placements this terminology refers to mechanical

degrees of sensor placement not electrical degrees The

electrical angular resolution is the required 60 degrees in

both cases The phasing differences can be noted by com-

paring the sequences of HS1 through HS3 entries in Table I

LM621 Commutation Decoder Truth Table which shows

both the 30- and 60-degree phasings (and the 90-degree

phasing for four-phase motors) and their required decoder

logic truth tables respectively Table I shows the phasing

(or codes) of the Hall-effect sensors for each 60-degree

(electrical) position range of the rotor and correlates these

data to the commutator sink and source outputs required to

drive the power switches These phasings are common to

several motor manufacturers The 60-degree phasing is pre-

ferred to 30-degree phasing because the all-zeros and all-

ones codes are not generated The 60-degree phasing is

more failsafe because the all-zeros and all-ones codes

could be inadvertently generated by things like disconnect-

ed or shorted sensors

Because the above terminology is not used consistently

among all motor manufacturers Table II Alternative Sen-

sor-phasing Names will hopefully clarify some of the differ-

ences Table II shows a different 60-degree phasing and

120- 240- and 300-degree phasings Comparison with Ta-

ble I will show that these four phasings are essentially shift-

ed andor reversed-order versions of those used with the

LM621

Figure 1 shows the waveforms associated with the commu-

tation decoder logic for a motor which has 60-degree rotor-

position phasing along with the generated motor-drive

waveforms As can be seen in the drawing Hall-effect sen-

sor signals HS1 through HS3 are separated by 60 electrical

degrees which is the required angular resolution for three-

phase motors

TLH8679 – 6

FIGURE 1 Commutation Waveforms for 60-degree Phasing

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