NJM3772

Figure 8. Typical thermal resistance vs. PC Board copper area and suggested layout

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

Motor selection

The NJM3772 is designed for two-phase bipolar stepper motors, i.e., motors that have only one winding per

phase.

The chopping principle of the NJM3772 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 NJM3772 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, a reduced number of turns in the winding must be compensated by a

higher current. A compromise has to be made.

Choose a motor with the lowest possible winding resistance 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.

Phase inputs.

A logic HIGH on a Phase input gives a current flowing from pin M

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

Heat sinking.

ness 35

µm, permits the circuit to operate with 750 mA output current, both channels driving, at ambient tempera-

tures up to 70

°C. Consult figures 8, 9, 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 chip temperatures above

160

°C. Normal operation is resumed when the temperature has decreased.

Operating temperature.

The max recommended operating temperature is 125

°C. This gives an estimated lifelength of about 5 years at

continuous drive, A change of

±10° would increase/decrease the lifelength of the circuit about 5 years.