NJM3774 New Japan Radio, NJM3774 Datasheet - Page 7

• Manufacturer Part Number: NJM3774
• Description: DUAL STEPPER MOTOR DRIVER
• Manufacturer: New Japan Radio

values result in a clock frequency (= switching frequency) of 26.5 kHz. A lower frequency will result in higher
current ripple, but may improve low-current level linearity. A higher clock frequency reduces current ripple, but
increases the switching losses in the IC and possibly increased iron losses in the motor. If the clock frequency
needs to be changed, the C
current, reference voltage and the value of R
maximum output current which is 1200mA peak when only one channel is activated. Or recommended output
current, which is 1000mA peak, when both channels is activated.
Motor selection
The NJM3774 is designed for two-phase bipolar stepper motors, i.e. motors that have only one winding per phase.
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.
needs to be considered. Typical motors to be used together with the NJM3774 have a voltage rating of 1 to 6 V,
while the supply voltage usually ranges from 12 to 40 V.
give the same torque capability at low speed, the reduced number of turns in the winding of 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 max. current.
General
Phase inputs
A logic HIGH on a Phase input gives a current flowing from pin M
opposite direction. A time delay prevents cross conduction in the H-bridge when changing the Phase input.
Heat sinking
Soldering the batwing ground leads onto a copper ground plane of 20 cm
thickness 35 m, permits the circuit to operate with 650 mA output current, both channels driving, at ambient
temperatures up to 70 C. Consult figures 7,10 and 11 in order to determine the necessary copper ground plane
area for heat sinking at higher current levels.
Thermal shutdown
The circuit is equipped with a thermal shutdown function that turns the output off at temperatures above 160 C.
Normal operation is resumed when the temperature has decreased.
Figure 7. Typical thermal resistance vs. PC Board copper
area and suggested layout
80
70
60
50
40
30
20
The frequency of the clock oscillator is set by the R
The sensing resistor R
The chopping principle of the NJM3774 is based on a constant frequency and a varying duty cycle. This scheme
It is not possible to use a motor that is rated for the same voltage as the actual supply voltage. Only rated current
Low inductance, especially in combination with a high supply voltage, enables high stepping rates. However, to
Thermal resistance [ C/W]
PLCC package
DIP package
5
PCB copper foil area [cm ]
10
15
20
25
S
, should be selected for maximum motor current. The relationship between peak motor
2
30
T
capacitor value should be adjusted. The recommended R
35
S
28-Pin
PLCC
22-Pin
DIP
is described under Current control above. Be sure not to exceed the
T
-C
T
timing components at the RC pin. The recommended
Figure 8. Stepping modes
Phase
Phase
A
Dis
Dis
–100%
–140%
–100%
–140%
V
140%
100%
V
140%
100%
I
140%
100%
I
140%
100%
MA1
MA2
R1
R2
into M
1
2
1
2
Full step mode
B
. A logic LOW gives a current flow in the
2
(approx. 1.8" x 1.8"), copper foil
Half step mode
T
resistor value is 15 kohm.
NJM3774
Modified half step mode