NJM3717
Figure 1. Block diagram
GND
V
CC
M A
M
B
Phase
I1
I0
V
R
&
&
&
&
+
+
+
1
Monostable
t = 0.69 R C
Current Sensor
Output Stage
off
T T
Schmitt
Trigger
Time
Delay
T
E
NJM3717
1
1
1
1
V
MM
V
MM
NJM3717D2
NJM3717E2
NJM3717FM2
STEPPER MOTOR DRIVER
s
GENERAL DESCRIPTION
s
PACKAGE OUTLINE
NJM3717 is a stepper motor diver, which consists of a LS-TTL
compatible logic input stage, a current sensor, a monostable
multivibrator and a high power H-bridge output stage with built-in
protection diodes.
The output current is up to 1200mA. Two NJM3717 and a small
number of external components form a complete control and drive
unit for stepper motor systems.
s
FEATURES
Half-step and full-step modes
Switched mode bipolar constant current drive
Wide range of current control 5 - 1200 mA
Wide voltage range 10 - 50 V
Thermal overload protection
Packages DIP16 / PLCC28 / EMP20
s
BLOCK DIAGRAM
NJM3717
s
PIN DESCRIPTION
DIP
EMP
PLCC
Symbol
Description
1
1
10
M
B
Motor output B, Motor current flows from M
A
to M
B
when Phase is high.
2
2
11
T
Clock oscillator. Timing pin connect a 56 k
resistor and a 820 pF in
parallel between T and Ground.
3,14
3,18
12,4
V
MM
Motor supply voltage, 10 to 45 V. V
MM
pins should be wired together on
4,5,
4,5,6,7,14
1,2,3,9,13,
PCB.
12,13
15,16,17
14,15,16,17
GND
Ground and negative supply. Note these pins are used for heatsinking.
28
Make sure that all ground pins are soldered onto a suitable large copper
ground plane for efficient heat sinking.
6
8
18
V
CC
Logic voltage supply normally +5 V.
7
9
19
I
1
Logic input, it controls, together with the I
0
input, the current level in the
output stage. The controllable levels are fixed to 100, 60, 20, 0%.
8
10
20
Phase
Controls the direction of the motor current of M
A
and M
B
outputs. Motor
current flows from M
A
to M
B
when the phase input is high.
9
11
21
I
0
Logic input, it controls, together with the I
1
input, the current level in the
output
stage. The controlable levels are fixed to 100, 60, 20, 0%.
10
12
23
C
Comparator input. This input senses the instantaneous voltage across the
sensing resistor, filtered through a RC Network.
11
13
24
V
R
Reference voltage. Controls the threshold voltage of the comparator and
hence the output current. Input resistance: typically 6.8k
20%.
15
19
6
M
A
Motor output A, Motor current flows from M
A
to M
B
when Phase is high.
16
20
8
E
Common emitter. Connect the sense resistor between this pin and ground.
Figure 2. Pin configurations
B
T
MM
GND
GND
CC
1
Phase
E
M
GND
GND
V
C
I
A
V
MM
R
0
I
V
V
M
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
NJM
3717D2
N/C
A
N/C
E
GND
B
T
N/C
V
C
N/C
I
Phase
I
V
GND
GND
GND
GND
N/C
N/C
MM
GND
GND
GND
GND
GND
CC
5
6
7
8
9
10
11
25
24
23
22
21
20
19
4
3
2
1
28
27
26
12
13
14
15
16
17
18
MM
R
0
1
V
V
M
M
NJM3717FM2
B
T
MM
GND
GND
CC
1
Phase
E
M
GND
GND
V
C
I
1
2
3
4
5
6
7
8
20
19
18
17
16
15
14
9
A
V
MM
R
0
I
V
V
M
NJM
3717E2
13
12
11
10
GND
GND
GND
GND
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PIN CONFIGURATIONS
NJM3717
Figure 3. Definition of terms
50 %
V
CH
t
on
t
off
V
E
| V V |
MA
MB
f =
s
ton toff
+
D =
t
t
on
off
+
1
t
on
t
t
t
d
s
FUNCTIONAL DESCRIPTION
The NJM3717 is intended to drive a bipolar constant current through one motor winding of a 2-phase stepper
motor.
Current control is achieved through switched-mode regulation, see figure 4 and 5.
Three different current levels and zero current can be selected by the input logic.
The circuit contains the following functional blocks:
Input logic
Current sense
Single-pulse generator
Output stage
Input logic
Phase input. The phase input determines the direction of the current in the motor winding. High input forces the
current from terminal M
A
to M
B
and low input from terminal M
B
to M
A
. A Schmitt trigger provides noise immunity and
a delay circuit eliminates the risk of cross conduction in the output stage during a phase shift.
Half- and full-step operation is possible.
Current level selection. The status of I
0
and I
1
inputs determines the current level in the motor winding. Three fixed
current levels can be selected according to the table below.
Motor current
I
0
I
1
High level
100% L
L
Medium level
60% H
L
Low level
20% L
H
Zero current
0%
H
H
The specific values of the different current levels are determined by the reference voltage V
R
together with the value
of the sensing resistor R
S
.
The peak motor current can be calculated as follows:
i
m
= (V
R
0.083) / R
S
[A], at 100% level
i
m
= (V
R
0.050) / R
S
[A], at 60% level
i
m
= (V
R
0.016) / R
S
[A], at 20% level
The motor current can also be continuously varied by modulating the voltage reference input.
NJM3717
Current sensor
The current sensor contains a reference voltage divider and three comparators for measuring each of the select-
able current levels. The motor current is sensed as a voltage drop across the current sensing resistor, R
S
, and
compared with one of the voltage references from the divider. When the two voltages are equal, the comparator
triggers the single-pulse generator. Only one comparator at a time is activated by the input logic.
Single-pulse generator
The pulse generator is a monostable multivibrator triggered on the positive edge of the comparator output. The
multivibrator output is high during the pulse time, t
off
, which is determined by the timing components R
T
and C
T
.
t
off
= 0.69 R
T
C
T
The single pulse switches off the power feed to the motor winding, causing the winding to decrease during t
off
.If a
new trigger signal should occur during t
off
, it is ignored.
Output stage
The output stage contains four transistors and four diodes, connected in an H-bridge. The two sinking transistors
are used to switch the power supplied to the motor winding, thus driving a constant current through the winding.
See figures 4 and 5.
Overload protection
The circuit is equipped with a thermal shut-down function, which will limit the junction temperature. The output
current will be reduced if the maximum permissible junction temperature is exceeded. It should be noted, however,
that it is not short circuit protected.
Operation
When a voltage V
MM
is applied across the motor winding, the current rise follows the equation:
i
m
= (V
MM
/ R) (1 - e
-(R t ) / L
)
R =
Winding resistance
L =
Winding inductance
t =
time
(see figure 5, arrow 1)
The motor current appears across the external sensing resistor, R
S
, as an analog voltage. This voltage is fed
through a low-pass filter, R
C
C
C
, to the voltage comparator input (pin 10). At the moment the sensed voltage rises
above the comparator threshold voltage, the monostable is triggered and its output turns off the conducting sink
transistor.
The polarity across the motor winding reverses and the current is forced to circulate through the appropriate
upper protection diode back through the source transistor (see figure 5, arrow 2).
After the monostable has timed out, the current has decayed and the analog voltage across the sensing resistor is
below the comparator threshold level.
The sinking transistor then closes and the motor current starts to increase again, The cycle is repeated until the
current is turned off via the logic inputs.
By reversing the logic level of the phase input (pin 8), both active transistors are turned off and the opposite pair
turned on after a slight delay. When this happens, the current must first decay to zero before it can reverse. This
current decay is steeper because the motor current is now forced to circulate back through the power supply and
the appropriate sinking transistor protection diode. This causes higher reverse voltage build-up across the winding
which results in a faster current decay (see figure 5, arrow 3).
For best speed performance of the stepper motor at half-step mode operation, the phase logic level should be
changed at the same time the current-inhibiting signal is applied (see figure 6).
NJM3717
0
200 mA/div
1 ms/div
100
s/div
Figure 6. Principal operating sequence
Figure 5. Output stage with current
paths for fast and slow current decay
Figure 4. Motor current (I
M
),
Vertical : 200 mA/div, Horizontal: 1
ms/div, expanded part 100
s/div
I
0A
I
1A
Ph
A
Ph
B
I
0B
I
1B
I
MA
I
MB
100%
100%
60%
60%
20%
20%
100%
100%
60%
60%
Half step mode at 100 %
Full step mode at 60 %
Stand by mode
at 20 %
Full step position
Half step position
Phase shift here
gives fast
current decay
Phase shift here
gives slow
current decay
3
2 1
R
S
Fast Current Decay
Slow Current Decay
Motor Current
Time
1 2
3
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