NJM3770A 데이터 시트보기 (PDF) - Japan Radio Corporation

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NJM3770A

STEPPER MOTOR DRIVER

Japan Radio Corporation  

NJM3770A

Current level selection.

The status of I0 and I1 inputs determines the current level in the motor winding. Three fixed current levels can be

selected according to the table below.

Motor current

High level 100%

I0 I1

LL

Medium level 60% H L

Low level 20% L H

Zero current 0% H H

The specific values of the different current levels are determined by the reference voltage VR together with the

value of the sensing resistor RS.

The peak motor current can be calculated as follows:

im = (VR • 0.080) / RS [A], at 100% level

The motor current can also be continuously varied by modulating the voltage reference input.

Current sensor

The current sensor contains a reference voltage divider and three comparators for measuring each of the selectable

current levels. The motor current is sensed as a voltage drop across the current sensing resistor, RS, and compared

with one of the voltage references from the divider. When the two voltages are equal, the comparator triggers the

single-pulse generator. Only one comparator at a time is activated by the input logic.

Single-pulse generator

The pulse generator is a monostable multivibrator triggered on the positive edge of the comparator output. The

multivibrator output is high during the pulse time, toff , which is determined by the timing components RT and CT.

toff = 0.69 • RT • CT

The single pulse switches off the power feed to the motor winding, causing the winding to decrease during toff .

If a new trigger signal should occur during toff , it is ignored.

Output stage

The output stage contains four transistors and two diodes, connected in an H-bridge. Note that the upper recircula-

tion diodes are connected to the circuit externally. The two sinking transistors are used to switch the power supplied

to the motor winding, thus driving a constant current through the winding. See figures 3 and 4.

Overload protection

The circuit is equipped with a thermal shut-down function, which will limit the junction temperature. The output current

will be reduced if the maximum permissible junction temperature is exceeded. It should be noted, however, that it is

not short circuit protected.

Operation

When a voltage V is applied across the motor winding, the current rise follows the equation:

MM

im = (VMM / R) • (1 - e-(R • t ) / L )

R = Winding resistance

L = Winding inductance

t = time

(see figure 3, arrow 1)

The motor current appears across the external sensing resistor, RS, as an analog voltage. This voltage is fed

through a low-pass filter, RCCC, to the voltage comparator input (pin 10). At the moment the sensed voltage rises

above the comparator threshold voltage, the monostable is triggered and its output turns off the conducting sink

transistor.The polarity across the motor winding reverses and the current is forced to circulate through the appropri-

ate upper protection diode back through the source transistor (see figure 3, arrow 2).

After the monostable has timed out, the current has decayed and the analog voltage across the sensing resistor is

below the comparator threshold level.The sinking transistor then turns on and the motor current starts to increase

again, The cycle is repeated until the current is turned off via the logic inputs.When both I and I are high, all four

1

0

transistors in the output H-bridge are turned off, which means that inductive current recirculates through two opposite

free-wheeling diodes (see figure 3, arrow 3). this method of turning off the current results in a faster current decay

than if only one transistor was turned off and will therefore improve speed performance in half-stepping mode.

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