L6238S View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6238S' PDF : 31 Pages View PDF
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,VR
(5.1.7)
Substituting values:
tinit
=
±
880e
4.8
±
6
ln
1
±
1.3 ⋅ 4.8
12
tinit = 134.6µs
The ON time can be calculated from:
ton
=
L
Rs
ln
V
Rs
V
Rs
±±ibit
Substituting values:
(5.1.8)
ton
=
880E ±
4.8
6
ln
12
4.8
12
4.8
±
±
11..23
ton = 14.67µs
Figure 5-7
It=1.3A
D95IN324
Ib=1.2A
Iavg=1.25A
Iout A
200mA
4
20µs/DIV
During the OFF time, the motor current continues
to flow through the DMOS transistors and threfore
L6238S
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
Substituting values:
(5.1.9)
toff
=
880e±6
4.8
ln
1.3
1.2
toff = 14.67µs
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 = 14.67µs
Rslew = 100KΩ (typical Value)
Coff
=
14.67e±6
100e3
Coff ≈ 146pF
5.1.4 Minimum ON Time
The bandwidth of the PWM loop was optimized to
reject unwanted switching noise while providing
25/31
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