NJM3772

External components

The NJM3772 exhibits substantially less power dissipation than most other comparable stepper motor driver ICs on

the market. This has been achieved by creating an external voltage drop in series with the upper transistor in the

output H-bridge, see figure 3. The voltage drop reduces the collector-emitter saturation voltage of the internal

transistor, which can greatly reduce power dissipation of the IC itself. The series resistor, designated R

selected for about 0.5 V voltage drop at the maximum output current. In an application with an output current of

1000 mA (peak), a 0.47 ohm,

In low current applications where power dissipation is not a critical factor, the R

omitted, and the V

Contributing to the low power dissipation is the fact that the upper recirculation diodes in the output H- bridge are

connected externally to the circuit. These diodes shall be of fast type, with a t

are UF4001 or BYV27.

A low pass filter in series with the comparator input prevents 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 than what is defined

by the comparator threshold, V

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

performance. Increasing the time constant may result in unstable switching. The time constant should be adjusted

by changing the C

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 the 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

1200 mA peak when only one channel is activated. Or recommended output

current, which is 1000 mA peak, when both channels is activated.