Part # Application Note Printers L6506 A datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: Using the L6506 for current control of stepping motors

Part Details:

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 howthis 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 othersimilar devices. When used in conjunction with power stages like the L293, L298N, or L7180 the chip set formsa constant current drive for inductive loads and performs all the interface functions from the control logic throughthe power stage. The L6506 may be used with either two phase bipolar or four phase unipolar motor configurations. The circuitin 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 currentis sensed by monitoring the voltage across a sense resistor (Rsense) and using a Pulse Width Modulated con-trol to maintain the current at the desired value. An on-chip oscillator drives the dual chopper and sets the operating frequency. An RC network on pin 1 sets theoperating frequency, which is given by the equation : 1 f = --------------- (1) 0.69R1 C1 for R1 > 10 K Figure 1. Application Circuit for Bipolar 2 Phase Stepper Motor. November 2003 1/6 AN469 APPLICATION NOTE The oscillator provides pulses to set the two flipflops, which in turn cause the outputs to activate the power ac-tuator. Once the outputs have been activated the current in the load starts to increase, limited by the inductivecharacteristic 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(Rsense) is equal to reference voltage input (Vref) and the corresponding comparator resets its flip-flop. This in-terrupts the drive and allows the current to decay through a recirculating path until the next oscillator pulse oc-curs. The peak current in each winding is programmed by selecting the value of the sense resistor and Vref andis given by the equation : V I r ef = ---------- (2) p eak Rsense The minimum output pulse width is determined by the pulse width of the oscillator, or other signal applied to thesync input. The internal oscillator is designed to provide narrow pulses to the sync input but the pulse widthshould be considered carefully. In some applications it is desirable to set the pulse width of this sync pulse to be just longer than the turn ondelay time of the actuator stage. This may be useful in systems where the switching noise or recovery currentof the catch diodes, which passes through the sense resistor, causes the comparator to sense a current abovethe peak current. By making the sync pulse wide enough to hold the flip-flop set at the time the switching tran-sient occurs will cause the device to ignore this false data. When the internal oscillator is used the pulse widthcan 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, asshown in figure 1, for bipolar motors or a quad darlington array, as shown in figure 2, for 4 phase unipolar mo-tors. For eigher configuration, half step may be implemented using the 4 phase inputs with the input waveformsshown in figure 3. The recirculation path for the motor current is through a catch diode for unipolar motors, or a catch diode and 2/6 AN469 APPLICATION NOTE one of the lower transistors of the bridge for bipolar motors. Both of these implementations produce a low ripplecurrent since the voltage across the motor during the recirculation time is much less than the power supply volt-age. Figure 4 shows the ripple current for bipolar motor applications using the L6506 and the L298N. Figure 3. Input Signal for Stepper Motor Drive. When implementing a half step drive, both outputs of the L6506 will be low during the half step of one phase.This means a very long time is required for the current in the "off" winding to decay when driving bipolar motors. Alternately, the power stage (L298N) may be inhibited to put the output in the state and achieve a faster currentdecay. Since separate Vref inputs are provided for each channel, each of the loads may be programmed inde-pendently allowing the device to be used to implement microstepping or applications with different peak andhold currents. In this type of application, changing the reference voltage (Vref) will change the load current, ef-fectively implementing a transconductance amplifier. SYNCHRONIZING MULTIPLE DEVICES Ground noise problems in multiple configurations can be avoided by synchronizing the oscillators. This may bedone by connecting the sync pins of each of the devices with the oscillator output of the master device and con-necting the R/C pin of the unused oscillators to ground as shown in figure 5.The devices may be synchronized to external circuits by applying synchronizing pulses to the sync pins. Itshould be noted, however, that the input pulse sets the minimum on time of the outputs and will therefore set aminimum output average current. Figure 4. Ripple Current in Bipolar Motors. Figure 5. Synchronizing Multiple Devices. 3/6

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