L6238 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6238' PDF : 35 Pages View PDF
L6238
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 output 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
Table 3.
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
18/35
Rotational Speed
Rotational Freq.
Rotational Period
Electrical Period
Phase Period
3600 rpm
60Hz
16.667 ms
4.167 ms
694.5 µs
5400 rpm
90Hz
11.111 ms
2.778 ms
463.0 µs
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