NJM3775
Dis (Disable) inputs
A logic HIGH on the Dis inputs will turn off all four transistors of the output H-bridge, which results in a rapidly
decreasing output current to zero.
V
R
(Reference) inputs
The Vref inputs of the NJM3775 have a voltage divider with a ratio of 1 to 10 to reduce the external reference
voltage to an adequate level. The divider consists of closely matched resistors. Nominal input reference voltage is 5
V.
Interference
Due to the switching operation of NJM3775, noise and transients are generated and might be coupled into adjacent
circuitry. To reduce potential interference there are a few basic rules to follow:
• Use separate ground leads for power ground (the ground connection of R
S
), the ground leads of NJM3775, and
the ground of external analog and digital circuitry. The grounds should be connected together close to the GND
pins of NJM3775.
• Decouple the supply voltages close to the NJM3775 circuit. Use a ceramic capacitor in parallel with an electrolytic
type for both V
CC
and V
MM
. Route the power supply lines close together.
• Do not place sensitive circuits close to the driver. Avoid physical current loops, and place the driver close to both
the motor and the power supply connector. The motor leads could preferably be twisted or shielded.
Motor selection
The NJM3775 is designed for two-phase bipolar stepper motors, i.e. motors that have only one winding per phase.
The chopping principle of the NJM3775 is based on a constant frequency and a varying duty cycle. This scheme
imposes certain restrictions on motor selection. Unstable chopping can occur if the chopping duty cycle exceeds
approximately 50 %. See figure 5 for definitions. To avoid this, it is necessary to choose a motor with a low winding
resistance and inductance, i.e. windings with a few turns.
It is not possible to use a motor that is rated for the same voltage as the actual supply voltage. Only rated current
needs to be considered. Typical motors to be used together with the NJM3775 have a voltage rating of 1 to 6 V,
while the supply voltage usually ranges from 12 to 40 V.
Low inductance, especially in combination with a high supply voltage, enables high stepping rates. However, to
give the same torque capability at low speed, the reduced number of turns in the winding in the low resistive, low
inductive motor must be compensated by a higher current. A compromise has to be made. Choose a motor with the
lowest possible winding resistance and inductance, that still gives the required torque, and use as high supply
voltage as possible, without exceeding the maximum recommended 40 V. Check that the chopping duty cycle does
not exceed 50 % at maximum current.
Heat sinking
NJM3775 is a power IC, packaged in a power DIP,SOP or PLCC package. The ground leads of the package (the
batwing) are thermally connected to the chip. External heatsinking is achieved by soldering the ground leads onto a
copper ground plane on the PCB.
Maximum continuous output current is heavily dependent on the heatsinking and ambient temperature. Consult
figures 8,10 and 11 to determine the necessary heatsink, or to find the maximum output current under varying
conditions.
A copper area of 20 cm
2
(approx. 1.8” x 1.8”), copper foil thickness 35 µm on a 1.6 mm epoxy PCB, permits the
circuit to operate at 2 x 450 mA output current, at ambient temperatures up to 85° C.
Thermal shutdown
The circuit is equipped with a thermal shutdown function that turns the outputs off at a chip (junction) temperature
above 160° C. Normal operation is resumed when the temperature has decreased.
Programming
Figure 9 shows the different input and output sequences for full-step, half-step and modified halfstep operations.
