NJM3776D2 データシートの表示（PDF） - Japan Radio Corporation

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NJM3776D2

DUAL CONTROLLER FOR HIGH-CURRENT STEPPER MOTOR

Japan Radio Corporation  

NJM3776

Phase inputs

A logic HIGH on a Phase input causes the TxBL pin to sink current, low voltage, and the TxAU pin to source current,

high voltage. A logic LOW causes the TxAL to sink current, low voltage, and the TxBU to source current, high

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

See truth table fig. 8.

Dis (Disable) inputs

A logic HIGH on the Dis inputs will turn off all four transistors of the outputs, which results in a rapidly decreasing output

current to zero. See truth table fig 8.

VR (Reference) inputs

The Vref inputs of the NJM3776 have a voltage divider with a ratio of 1 to 10 to reduce the external reference

voltage to an adequate level. The divider consists of closely matched resistors . Nominal input reference voltage is 5

V.

Interference

Due to the switching operation of NJM3776, noise and transients are generated and coupled into adjacent circuitry.

To reduce potential interference there are a few basic rules to follow:

• Use separate ground leads for power ground (the ground connection of RS), the ground leads of NJM3776, and the

ground of external analog and digital circuitry. The grounds should be connected together close to the main filtering

capacitor at the power supply.

• Decouple the supply voltages close to the NJM3776 circuit. Use a ceramic capacitor in parallel with an electrolytic

type for both VCC and VBB. Route the power supply lines close together.

• Do not place sensitive circuits close to the driver. Avoid physical current loops, and place the driver close to both

the motor and the power supply connector. The motor leads could preferably be twisted or shielded.

Motor selection

The NJM3776 is designed for two-phase bipolar stepper motors, i.e. motors that have only one winding per phase.

The chopping principle of the NJM3776 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 select a motor with a low winding

resistance and inductance, i.e. windings with fewer 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 NJM3776 in a high current application, have a

voltage rating of 0.5 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 in the low resistive, low

inductive motor must be compensated by a higher current. A compromise has to be made. Select 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 maximum current.

To achieve the best utilization of the motor driver combination it is important to find the correct operation conditions in

terms of motor voltage, winding current and stepping mode to fit the motor type and the motor winding.

To find the correct operation conditions for a certain application the following procedure can be used.

1. If low noise and low resonance’s or high resolution is required, use half step or even better modified half step,

quarter step, etc. In order to implement modified half step or modes with better resolution an external sequence

generator must be used. See the testboard manual for TB 313i testboard for more information.

If the required stepping rate is high or if low cost is more important than low noise use full step mode.

2. Set the motor supply voltage and the winding currents to their maximum values (limited by the motor or the driver).

Run the motor in the application at the lowest frequency with maximum load.

3. Decrease the current, by decreasing the Vref voltage, until the motor phases out, then raise the current with the

selected torque margin, 25 to 50% as a guideline. This sets a first approximation of the suitable current level.

4. Run the motor at the highest frequency with maximum load. Decrease the motor voltage until the motor phases

out. Increase the motor voltage with 15 to 30% as a guideline to find a first estimation of the required motor voltage.

To get an even better estimation continue to adjust the current in the low frequency range and the voltage in the high

frequency range. This is a very simplified method for finding the correct operating conditions for the motor but it will

be helpful in most cases. If the motor fails to run in the high frequency range at maximum voltage a motor with lower

winding resistance should be selected. If the problems occur in the low frequency range a larger motor or a gearbox

will have to be used.

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