NJM3774D2 데이터 시트보기 (PDF) - Japan Radio Corporation
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NJM3774D2 Datasheet PDF : 9 Pages
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NJM3774
The frequency of the clock oscillator is set by the RT-CT timing components at the RC pin. The recommended
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 CT capacitor value should be adjusted. The recommended RT resistor value is 15 kohm.
The sensing resistor RS, should be selected for maximum motor current. The relationship between peak motor
current, reference voltage and the value of RS is described under Current control above. Be sure not to exceed the
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.
The chopping principle of the NJM3774 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 NJM3774 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 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 MA into MB. A logic LOW gives a current flow in the
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 cm2 (approx. 1.8" x 1.8"), copper foil
thickness 35 µm, 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.
Thermal resistance [°C/W]
80
70
28-Pin
PLCC
60
50
40
30
22-Pin
DIP
20
5
10
15 20
25 30
35
PCB copper foil area [cm2]
PLCC package
DIP package
Figure 7. Typical thermal resistance vs. PC Board copper
area and suggested layout
Phase 1
Dis 1
Phase 2
Dis 2
V R1
140%
100%
V R2
140%
100%
I MA1
140%
100%
–100%
–140%
I
MA2
140%
100%
–100%
–140%
Full step mode
Half step mode
Figure 8. Stepping modes
Modified half step mode
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