L6238 datasheet(18/35 Pages) STMICROELECTRONICS

18 / 35 page

5.O MOTOR DRIVER

5.1 Output Stage

The output stage forms a 3-phase, full wave

bridge consisting of six Power DMOS FETs capa-

ble of 2.5 amps. Higher output currents are al-

lowed for brief periods. Output Power exceeding

the stand-alone power dissipation capabilities of

the L6238 can be increased with the addition of

an external P-FET.

Table 3 is a reference diagram that lists the pa-

rameters associated with 8-pole motors operating

at 3600 and 5400 RPM.

Figure 15 represents the waveforms associated

with the outpu t stage. The upper portion of fig-

ure 15 shows the flow of current in the motor

windings for each of the 24 phase increments. A

rotational degree index is shown as a reference

along with a base line to indicate the occurrence

of a zero crossing. The 3 output waveforms are

actual digitally reproduced voltage signals as

measured on samples.

A typical sequence starts when the outputs switch

states. Referring to figure 15, during phase 1, out-

put A goes high, while output B is low. During this

phase, output C is floating, and the Bemf is moni-

tored. The outputs remain in this state for 60 elec-

trical degrees as indicated by the first set of

dashed lines. After this period the output switches

to phase 2 with output A high and C low with the

Bemf amplifier monitoring output B.

In order to prevent commutation current noise be-

ing detected as a false zero crossing, a masking

circuit automatically blanks out all incoming sig-

nals as soon as a zero crossing is detected.

When the next commutation occurs an internal

counter starts counting down to set the time that

the masking pulse remains The counter is initially

loaded with a number that is equal to period that

is always 25% of the previous phase period or 15

electrical degrees. This time-out of the masking

pulse shown for reference at the bottom of figure

16. Thus the actual masking period is the total of

the time from the detected zero crossing to the

commutation, plus 25% of the previous period.

The mask pulse operation is further discussed in

section 5.6, Slew Rate Control.

After the masking period, the Bemf voltage at out-

put B is monitored for a zero crossing. Upon de-

tection of the crossing the output is sequenced af-

ter 30 electrical degrees insuring maximum

Table 3.

Rotational Speed

3600 rpm

5400 rpm

Rotational Freq.

60Hz

90Hz

Rotational Period

16.667 ms

11.111 ms

Electrical Period

4.167 ms

2.778 ms

Phase Period

694.5

µs

463.0

µs

L6238

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