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L6238S Datasheet(PDF) 24 Page - STMicroelectronics

Part # L6238S
Description  12V SENSORLESS SPINDLE MOTOR CONTROLLER
PDF  31 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

L6238S Datasheet(HTML) 24 Page - STMicroelectronics

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The relation between these variables is given by:
V
= Lmtr
dimtr
dt
Rmtr imtr
+ Eg
(5.1.2)
where:
V
=
Applied Voltage
imtr
=
Motor Current
Lmtr
=
Total
inductance
of
the
motor
windings
Rmtr
=
Resistance in series with the motor
Eg
=
The internally generated voltage of
the motor, proportional to the motor
velocity
Since:
Eg
=KE
ω
(5.1.3)
The above equations can be combined to form
the basic electrical equation for a motor:
V
= Lmtr
dimtr
dt
Rmtr imtr
+ KEω
(5.1.4)
Figure 5.5 is a simplified electrical equivalent of
the output stage of the L6238S along with the
model of the motor during the time that the Out-
put Drives are conducting.
The additional resistance associated with the out-
put stage and sensing resistor are also in series
with the motor. If we let Rs equal the total series
resistence:
Rs = 2*RdsON +Rmtr +Rsense
(5.1.5)
then (5.1.4) becomes:
V
= Lmtr
dimtr
dt
Rs imtr
+ Eg
(5.1.6)
Figure 5-6 is an equivalent circuit of the output
stage during the Constant-OFF period. During the
OFF time the lower driver for the particular phase
beign driven remains ON.
The internally generated voltage forces the path
of current though the motor, its series resistance,
the RdsON of the Lower Driver and finally through
the opposite lower driver.
PWM Example (Refer to Figure 5-7)
The following is an example on how to select the
timing parameters.
Given:
DCStart Current
=
1.25A
Ripple Current
=
100mA
Duty Cycle
=
50%
Motor Interface (L)
=
880
µH
Total Series Resistance (Rs)
=
4.8
Ω
If the worst case start current is 1.25A and the
duty cycle is 50%, then the Peak Current, It will
be:
it
= 1.25+
0.1
2
it = 1.30A
Eg
+
-
Rmtr
Lmtr
D95IN321
Figure 5-4
+
-
Rmtr
Lmtr
D95IN322
UPPER
Rdson
KEW
LOWER
Rdson
Rsense
Figure 5-5
+
-
Rmtr
Lmtr
D95IN323
KEW
LOWER
Rdson
LOWER
Rdson
Figure 5-6
L6238S
24/31



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