ST AN469 Application note
AN469
APPLICATION NOTE
USING THE L6506 FOR CURRENT CONTROL
OF STEPPING MOTORS
by Thomas Hopkins
Chopper-type current control circuits improve the performance of motor drives. This note shows how
this can be done simply using the L6506 current controller IC.
The L6506 is a linear integrated circuit designed to sense and control the current in stepping motors and other
similar devices. When used in conjunction with power stages like the L293, L298N, or L7180 the chip set forms
a constant current drive for inductive loads and performs all the interface functions from the control logic through
the power stage.
The L6506 may be used with either two phase bipolar or four phase unipolar motor configurati ons. The circuit
in figure 1 shows the L6506 used in conjunction with the L298N in a 2 phase bipolar stepper motor application.
The circuit in figure 2 implements a similar 4 phase unipolar application.
CURRENT CONTROL LOGIC
In these two circuits, the L6506 is used to sense and control the current in each of the load windings. The current
is sensed by monitoring the voltage across a sense resistor (Rsense) and using a Pulse W idth M odulated control to maintain the current at the desired value.
An on -chip osc illator d rives the dual chopper and sets the operating frequency. An RC network on pin 1 sets the
operating frequency, which is given by the equation :
1
f
----------------------------=
0.69R1C1
for R1 > 1 0 K
Figure 1. Application Circuit for Bipolar 2 Phase Stepper Motor.
Ω
(1)
November 2003
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AN469 APPLICATION NO TE
The oscillator provides pulses to set the two flipflops, which in turn cause the outputs to activate the power actuator. Once the outputs have been activated the current in the load starts to increase, limited by the inductive
characteristic of the load.
Figure 2. Application Circuit for Unipolar 4 Phase Stepper Motor.
When the current in the load winding reaches the programmed peak value, the voltage across the sense resistor
(R
) is equal to reference voltage input (V
sense
) and the corresponding comparator resets its flip-flop. This in-
ref
terrupts the drive and allows the current to decay through a recirculating path until the next oscillator pulse occurs. The peak current in each winding is programmed by selecting the value of the sense resistor and Vref and
is given by the equation :
V
I
peak
ref
------------------=
(2)
R
sense
The minimum output pulse width is determined by the pulse width of the oscillator, or other signal applied to the
sync input. The internal oscillator is designed to provide narrow pulses to the sync input but the pulse width
should be considered carefully.
In some applications it is desirable to set the pul se width of this sync pulse to be just longer t han the turn on
delay time of the actuator stage. This may be useful in systems where the switching noise or recovery current
of the catch diodes, which passes through the sense resistor, causes the comparator to sense a current above
the peak current. By making the sync pulse wide enough to hold the flip-flop set at the time the switching transient occurs will cause the device to ignore this false data. When the internal oscillator is used the pulse width
can be modified by changing the value of the capacitor on pin 1.
Increasing the capacitance will widen the pulse width. The L6506 may be used with either a bridge driver, as
shown in figure 1, for bipolar motors or a quad darlington array, as shown in figure 2, for 4 phase unipolar motors. For eigher configuration, half step may be implemented using the 4 phase inputs with the input waveforms
shown in figure 3.
The recirculation path for the motor cur rent is through a c atc h diode for unipolar motors , or a c atch diode and
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