NJM37770
-
-
Ver.2006-04-11
■ APPLICATIONS INFORMATION
Motor selection
Some stepper motors are not designed for continuous operation at maximum current. As the circuit drives a constant
current through the motor, its temperature can increase, both at low- and high-speed operation.
Some stepper motors have such high core losses that they are not suited for switched-mode operation.
Interference
As the circuit operates with switched-mode current regulation, interference-generation problems can arise in some
applications. A good measure is then to decouple the circuit with a 0.1 µF ceramic capacitor, located near the package
across the power line V
MM
and ground.
Also make sure that the V
Ref
input is sufficiently decoupled. An electrolytic capacitor should be used in the +5V rail,
close to the circuit.
The ground leads between R
S
, C
C
and circuit GND should be kept as short as possible. This applies also to the leads
connecting R
S
and R
C
to pin 16 and pin 10 respectively.
In order to minimize electromagnetic interference, it is recommended to route M
A
and M
B
leads in parallel on the printed
circuit board directly to the terminal connector. The motor wires should be twisted in pairs, each phase separately, when
installing the motor system.
Unused inputs
Unused inputs should be connected to proper voltage levels in order to obtain the highest possible noise immunity.
Ramping
A stepper motor is a synchronous motor and does not change its speed due to load variations. This means that the
torque of the motor must be large enough to match the combined inertia of the motor and load for all operation modes. At
speed changes, the requires torque increases by the square, and the required power by the cube of the speed change.
Ramping, i.e., controlled acceleration or deceleration must then be considered to avoid motor pullout.
V
CC
, V
MM
The supply voltages, V
CC
and V
MM
, can be turned on or off in any order. Normal dv/dt values are assumed.
Before a driver circuit board is removed from its system, all supply voltages must be turned off to avoid destructive
transients being generated by the motor.
Switching frequency
The motor inductance, together with the pulse time, t
off
, determines the switching frequency of the current regulator. The
choice of motor may then require other values on the R
T
, C
T
components than those recommended in figure 3, to obtain
a switching frequency above the audible range. Switching frequencies above 40 kHz are not recommended because the
current regulation can be affected.
Figure 7. Typical stepper motor driver application with NJM37770
