2-Phase Stepper Motor Unipolar Driver ICs (2-Phase Excitation)
SLA7022MU/SLA7029M/SMA7022MU/SMA7029M
Application Notes
sDetermining
the Output Current
Fig. 1 shows the waveform of the output current (motor coil cur-
rent). The method of determining the peak value of the output
current (I
O
) based on this waveform is shown below.
(Parameters for determining the output current I
O
)
V
b
: Reference supply voltage
r
1
,r
2
: Voltage-divider resistors for the reference supply voltage
R
S
: Current sense resistor
(1) Normal rotation mode
I
O
is determined as follows when current flows at the maximum
level during motor rotation. (See Fig.2.)
V
b
................................................................
r
2
(1)
I
O
r
1
+r
2
R
S
(2) Power down mode
The circuit in Fig.3 (rx and Tr) is added in order to decrease the
coil current. I
O
is then determined as follows.
I
OPD
1+
1
r
1
(r
2
+r
X
)
r
2
r
X
Fig. 1 Waveform of coil current (Phase A excitation ON)
I
O
Phase A
0
Phase A
Fig. 2 Normal mode
V
b
(5
V
)
r
1
r
5
r
2
C
3
3,(13)
7,(9)
R
S
r
6
V
b
......................................................... (2)
R
S
Equation (2) can be modified to obtain equation to determine rx.
1
r
X
=
1
1
V
b
−1 −
r
1
R
s
I
OPD
r
2
Fig. 4 and 5 show the graphs of equations (1) and (2) respec-
tively.
Fig. 3 Power down mode
V
b
(5
V
)
r
6
r
1
r
5
r
x
Power down
signal
T
r
R
S
r
2
C
3
7,(9)
3,(13)
Fig. 4 Output current I
O
vs. Current sense resistor R
S
4
Fig. 5 Output current I
OPD
vs. Variable current sense resistor rx
2.0
2
r
2
· V
b
I
O
=
r
1
+r
2
R
S
r
1
=510Ω
r
2
=100Ω
r
x
=∞
V
b
=5V
Output current I
OPD
(A)
Output current I
O
(A)
3
1.5
R
S
=0.5Ω
1
· V
b
r
1
(r
2+
r
X
)
R
S
1+
r
2 ·
r
X
r
1
=510Ω
r
2
=100Ω
V
b
=5V
I
OPD
=
1.0
R
S
=0.8Ω
R
S
=1Ω
1
0.5
0
0
1
2
3
4
00
200
400
600
800
1000 1200
Current sense resistor R
S
(Ω)
Variable current sense resistor r
X
(Ω)
(NOTE)
Ringing noise is produced in the current sense resistor R
S
when
the MOSFET is switched ON and OFF by chopping. This noise
is also generated in feedback signals from R
S
which may there-
fore cause the comparator to malfunction. To prevent chopping
malfunctions, r
5
(r
6
) and C
3
(C
4
) are added to act as a noise filter.
However, when the values of these constants are increased,
the response from R
S
to the comparator becomes slow. Hence
the value of the output current I
O
is somewhat higher than the
calculated value.
8
SLA7022MU/SLA7029M/SMA7022MU/SMA7029M
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