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L6238S Datasheet(PDF) 25 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) 25 Page - STMicroelectronics

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The Valley current, Ib will thereforebe:
ib = 1.30 - 0.1A
ib = 1.20A
During the Align and Go Phase (where the power
dissipation requirements are highest, Eg is zero.
The initial time required to reach the Peak current
is:
tinit
= ±
L
R
ln
1
±
I,R
V
(5.1.7)
Substituting values:
tinit
= ±
880e
± 6
4.8
ln
1
±
1.3
⋅ 4.8
12
tinit = 134.6
µs
The ON time can be calculated from:
ton
=
L
Rs
ln
V
Rs
± ib
V
Rs
± it
(5.1.8)
Substituting values:
ton
=
880E
± 6
4.8
ln

12
4.8
± 1.2
12
4.8
± 1.3
ton = 14.67
µs
During the OFF time, the motor current continues
to flow through the DMOS transistors and threfore
the voltage drop remains constant across the
windings.
The time required for the inductor current to reach
the valley current is given by:
toff
=
L
R
ln
It
Ib
(5.1.9)
Substituting values:
toff
=
880e±
6
4.8
ln
1.3
1.2
toff = 14.67
µs
Note: that the parameters for this example were selected to arrive at
a 50% duty cycle. This will not always be the case due to factors such
as fixed motor parameters, etc.
The Constant Off timer period can be determined
from:
toff
= Rslew ⋅ Coff
⋅ ln
Vchrg
Vtrip
(5.1.10)
Where:
Toff
=
Constant-OFF Time
Rslew =
Slew Rate Resistor
Coff
=
Off Time Capacitor
Vchrg =
Initial Capacitor Charge Voltage
Vtrip
=
Capacitor Lower Trip Threshold
Substituting nominal values given:
Toff = 0.75
⋅ Rslew ⋅ Coff
Solving for Coff
Coff =
Toff
0.75Rslew
In the example, to set the OFF timer for a 50%
duty cycle:
Given:
Toff
=
14.67
µs
Rslew =
100K
Ω (typical Value)
Coff
=
14.67e±
6
100e
3
Coff
≈ 146pF
5.1.4 Minimum ON Time
The bandwidth of the PWM loop was optimized to
reject unwanted switching noise while providing
20
µs/DIV
Iout A
200mA
D95IN324
4
It=1.3A
Ib=1.2A
Iavg=1.25A
Figure 5-7
L6238S
25/31



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