NJM3774

The frequency of the clock oscillator is set by the R

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 C

The sensing resistor R

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.

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 M

opposite direction. A time delay prevents cross conduction in the H-bridge when changing the Phase input.

Heat sinking

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.

Figure 8. Stepping modes

V

R1

Dis

1

Phase

1

Phase

2

Dis

2

I

MA1

I

MA2

100%

–100%

–100%

100%

Full step mode

Half step mode

Modified half step mode

V

R2

140%

100%

140%

100%

140%

–140%

140%

–140%

Thermal resistance [

°C/W]

PCB copper foil area [cm ]

2

80

70

60

50

40

30

20

510

15

20

30

35

25

PLCC package

DIP package

28-Pin

PLCC

22-Pin

DIP

Figure 7. Typical thermal resistance vs. PC Board copper

area and suggested layout