NJM3771

External components

The voltage across the sensing resistor is fed back to the comparator via a low-pass filter section, to prevent

erroneous switching due to switching transients. The recommended filter component values, 1 kohm and 820 pF,

are suitable for a wide range of motors and operational conditions.

Since the low-pass filtering action introduces a small delay of the signal to the comparator, peak voltage across

the sensing resistor, and hence the peak motor current, will reach a slightly higher level than the threshold, V

by the reference voltage

(V

The time constant of the low-pass filter may therefore be reduced to minimize the delay and optimize low-current

performance, especially if a low (12 V) supply voltage is used. Increasing the time constant may result in unstable

switching.

The frequency of the clock oscillator is set by the R-C combination at pin RC. The recommended values give a

nominal frequency of 26.5 kHz. A lower frequency will result in higher current ripple and may cause audible noise

from the motor, while increasing the frequency results in higher switching losses and possibly increased iron losses

in the motor.

The sensing resistor, R

current, reference voltage and the value of R

the maximum output current which is 650 mA per channel (or 500 mA per channel, both channels fully on, see

“Recommended Operating Conditions”).

Motor selection

The NJM3771 is designed for bipolar motors, i.e., motors that have only one winding per phase. A unipolar motor,

having windings with a center tap, can also be used, see figure 8.

The chopping principle in the

NJM3771 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%. To avoid this, it is necessary to choose a motor with a low winding resistance. Low

winding resistance means less inductance and will therefore enable higher stepping rates, however it also means

less torque capability. 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.

Since the NJM3771 produces a regulated, constant output current it is not necessary to use a motor that is rated

at the same voltage as the actual supply voltage. Only rated current needs to be considered. Typical motors to be

used together with the NJM3771 have voltage ratings of 5 to 12 V, while the supply voltage usually ranges from 24

to 40 V.

Figure 8. Connection of unipolar motors

Best for high speed

NJM3771

Best for high torque

NJM3771