Patent Publication Number: US-11381184-B2

Title: Driving circuit for stepping motor, method of driving stepping motor, and electronic device using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2019-025185, filed on Feb. 15, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a driving technique for a stepping motor. 
     BACKGROUND 
     A stepping motor has been widely used in electronic devices, industrial machines, and robots. The stepping motor is a synchronous motor which rotates in synchronization with an input clock generated by a host controller, and has excellent controllability for starting, stopping, and positioning. Further, the stepping motor has characteristics that the position control in an open loop is possible and it is suitable for digital signal processing. 
     In a normal state, a rotor of the stepping motor rotates in synchronization with each step angle proportional to the number of input clocks. However, when sudden load variation or speed variation occurs, the synchronization is lost. This is referred to as step-out. Once step-out occurs, since special processing is then required for normally driving the stepping motor, it is desired to prevent the step-out. 
     In order to solve such a problem, in many cases, a margin is provided for the assumed maximum load, and a driving circuit is designed so as to obtain output torque in consideration of the step-out margin. However, when the margin is increased, power loss is also increased. 
     In a related art, a technique for reducing power consumption and improving efficiency has been proposed by optimizing output torque (i.e., current amount) by feedback while preventing step-out.  FIG. 1  is a block diagram of a motor system including a conventional stepping motor and a driving circuit therefor. 
     A host controller  2  supplies an input clock CLK to a driving circuit  4 . The driving circuit  4  changes an excitation position in synchronization with the input clock CLK. 
       FIG. 2  is a diagram illustrating the excitation position. The excitation position is recognized as a combination of coil currents (driving currents) I OUT1  and I OUT2  flowing through two coils L 1  and L 2  of a stepping motor  6 . Eight excitation positions 1 to 8 are illustrated in  FIG. 2 . In one-phase excitation, a current alternately flows through the first coil L 1  and the second coil L 2 , and transitions between the excitation positions 2, 4, 6, and 8. In two-phase excitation, a current flows through both the first coil L 1  and the second coil L 2 , and transitions between the excitation positions 1, 3, 5, and 7. One-two-phase excitation is a combination of the one-phase excitation and the two-phase excitation to transition between the excitation positions 1 to 8. In microstep driving, the excitation position is further finely controlled. 
       FIG. 3  is a diagram illustrating a driving sequence of the stepping motor. At the time of starting, a frequency f IN  of the input clock CLK increases with time, and the stepping motor is thus accelerated. Then, when the frequency f IN  reaches a certain target value, it is kept constant and the stepping motor is rotated at a constant speed. Thereafter, when the stepping motor is stopped, the frequency of the input clock CLK is lowered to decelerate the stepping motor. The control in  FIG. 3  is also referred to as trapezoidal wave driving. 
     In a normal state, the rotor of the stepping motor rotates in synchronization with each step angle proportional to the number of input clocks. However, when sudden load variation or speed variation occurs, the synchronization is lost. This is referred to as step-out. Once step-out occurs, since a special process is then required for normally driving the stepping motor, it is desired to prevent the step-out. 
     Therefore, during the acceleration and deceleration having a high possibility of step-out, a target value I REF  of the driving currents is set to a fixed value I FULL  so as to obtain sufficiently large fixed output torque in consideration of the step-out margin (high torque mode). 
     In situations where a rotational speed is stable and the possibility of step-out is low, the target value I REF  of the driving currents is reduced to improve the efficiency (high efficiency mode). In a related art, a technique for reducing power consumption and improving efficiency has been proposed by optimizing output torque (i.e., current amount) by feedback while preventing step-out. Specifically, a load angle φ is estimated based on a counter electromotive force V BEMF , and the target value I REF  of the driving currents (coil currents) is feedback-controlled so that the load angle φ approaches a target value φ REF . The counter electromotive force V BEMF  is expressed by Eq. (1).
 
 V   BEMF   =K   E ×ω×cos φ  Eq. (1)
 
wherein ω is a rotational speed of the stepping motor, and K E  is a counter electromotive force constant.
 
     In the technique described in the relate art, a feedback loop is formed so that a detected value cow based on the load angle approaches its target value cos(φ REF ), and the coil currents I OUT1  and I OUT2  in the high efficiency mode are optimized. 
     In a motor system using the driving circuit of the related art, the current amount I REF  is generated inside the driving circuit  4 . Although the amount of the coil currents includes information useful for system design or control, there is no way to know how much torque allows the stepping motor  6  to be currently driven from the outside of the driving circuit. 
     SUMMARY 
     Some embodiments of the present disclosure provide a driving circuit capable of outputting information useful for system design or control to outside. 
     According to one embodiment of the present disclosure, there is provided a driving circuit for a stepping motor, including: a counter electromotive force detection circuit configured to detect a counter electromotive force generated in a coil; a current value setting circuit configured to generate a current set value based on the counter electromotive force; a constant current chopper circuit configured to generate a pulse-modulated signal which is pulse-modulated so that a detected value of a coil current flowing through the coil approaches a target amount based on the current set value; and a logic circuit configured to control a bridge circuit connected to the coil according to the pulse-modulated signal, wherein the driving circuit is configured to output the current set value to outside or to access the current set value from the outside. 
     The driving circuit may further include an interface circuit configured to output the current set value to the outside as a digital signal. 
     The driving circuit may further include a D/A converter configured to convert the current set value into an analog signal; and a buffer circuit configured to output the current set value, which has been converted into the analog signal, to the outside. 
     According to another embodiment of the present disclosure, there is provided a driving circuit for a stepping motor, including: a counter electromotive force detection circuit configured to detect a counter electromotive force generated in a coil; a current value setting circuit configured to generate a current set value based on the counter electromotive force; a constant current chopper circuit configured to generate a pulse-modulated signal which is pulse-modulated so that a detected value of a coil current flowing through the coil approaches a target amount based on the current set value; and a logic circuit configured to control a bridge circuit connected to the coil according to the pulse-modulated signal, wherein the driving circuit is configured to output the detected value of the coil current to outside or to access the detected value of the coil current from the outside. 
     The detected value of the coil current may be according to a voltage drop of a detection resistor installed at the bridge circuit, and the driving circuit may further include a buffer configured to output the voltage drop of the detection resistor to the outside. 
     The detection value of the coil current may be according to a voltage drop of a detection resistor installed at the bridge circuit, and the driving circuit may further include an A/D converter configured to convert the voltage drop of the detection resistor into a digital value; and an interface circuit configured to output the digital value to the outside. 
     The current value setting circuit may include a load angle estimation part configured to estimate a load angle based on the counter electromotive force; and a feedback controller configured to generate the current set value so that the estimated load angle approaches a predetermined target angle. 
     The constant current chopper circuit may include a comparator configured to compare the detected value of the coil current with a threshold value based on the current set value; an oscillator configured to oscillate at a predetermined frequency; and a flip-flop configured to output the pulse-modulated signal which transitions to an OFF level in response to an output of the comparator and transitions to an ON level in response to an output of the oscillator. 
     The driving circuit may be integrated as one body on a single semiconductor substrate. “Integrated as one body” may include the case where all of the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated as one body, and some of a resistor, a capacitor, and the like for adjusting circuit constants may be installed outside the semiconductor substrate. By integrating circuits on one chip, the circuit area can be reduced and the characteristics of the circuit elements can be kept uniform. 
     Further, any combination of the above-described components, and any replacement of the components and expressions of the present disclosure between methods, apparatuses, systems, and the like are also effective as embodiments of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the present disclosure. 
         FIG. 1  is a block diagram of a motor system including a conventional stepping motor and a driving circuit therefor. 
         FIG. 2  is a diagram illustrating an excitation position. 
         FIG. 3  is a diagram illustrating a driving sequence of the stepping motor. 
         FIG. 4  is a block diagram of a motor system including a driving circuit according to a first embodiment of the present disclosure. 
         FIGS. 5A to 5C  are diagrams illustrating configuration examples of an interface circuit. 
         FIG. 6  is a circuit diagram illustrating a configuration example of the driving circuit. 
         FIG. 7  is a block diagram of a motor system including a driving circuit according to a second embodiment of the present disclosure. 
         FIGS. 8A to 8C  are diagrams illustrating configuration examples of an interface circuit. 
         FIG. 9  is a diagram illustrating another configuration example of a current value setting circuit. 
         FIGS. 10A to 10C  are perspective views illustrating examples of an electronic device including the driving circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one of ordinary skill in the art that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, systems, and components have not been described in detail so as not to unnecessarily obscure aspects of the various embodiments. 
     The present disclosure will now be described with reference to the drawings based on an exemplary embodiment. Like or equivalent components, members, and processes illustrated in each drawing are given like reference numerals and a repeated description thereof will be properly omitted. Further, the embodiment is presented by way of example only, and is not intended to limit the present disclosure, and any feature or combination thereof described in the embodiment may not necessarily be essential to the present disclosure. 
     In the present disclosure, “a state where a member A is connected to a member B” includes a case where the member A and the member B are physically directly connected or even a case where the member A and the member B are indirectly connected via any other member that does not affect an electrical connection state between the members A and B or does not impair functions and effects achieved by combinations of the members A and B. 
     Similarly, “a state where a member C is installed between a member A and a member B” includes a case where the member A and the member C or the member B and the member C are indirectly connected via any other member that does not affect an electrical connection state between the member A and the member C or the member B and the member C or does not impair function and effects achieved by combinations of the member A and the member C or the member B and the member C, in addition to a case where the member A and the member C or the member B and the member C are directly connected. 
     The vertical axis and the horizontal axis of the waveform diagram or the time chart referred to herein are appropriately enlarged and reduced for ease of understanding, and each waveform shown is also simplified, exaggerated, or emphasized for ease of understanding. 
     Embodiment 1 
       FIG. 4  is a block diagram of a motor system  100 A including a driving circuit  200 A according to a first embodiment of the present disclosure. The driving circuit  200 A constitutes the motor system  100 A together with a stepping motor  102  and a host controller  2 . The stepping motor  102  may be any of a PM (permanent magnet) type, a VR (variable reluctance) type, and an HB (hybrid) type. 
     An input clock CLK is input from the host controller  2  to an input pin IN of the driving circuit  200 A. In addition, a direction indication signal S DIR  indicating clockwise (CW) or counterclockwise (CCW) is input to a direction indication pin DIR of the driving circuit  200 A. 
     The driving circuit  200 A rotates a rotor of the stepping motor  102  at a predetermined angle in a direction corresponding to the direction indication signal S DIR  whenever the input clock CLK is input. 
     The driving circuit  200 A is integrated as one body on a single semiconductor substrate that includes bridge circuits  202 _ 1  and  202 _ 2 , a current value setting circuit  210 , a counter electromotive force detection circuit  230 , constant current chopper circuits  250 _ 1  and  250 _ 2 , a logic circuit  270 , and an interface circuit  280 A. 
     In the present embodiment, the stepping motor  102  is a two-phase motor, and includes a first coil L 1  and a second coil L 2 . A driving method of the driving circuit  200 A is not particularly limited, and may be any of one-phase excitation, two-phase excitation, one-two-phase excitation, or microstep driving (W1-two-phase driving, 2W1-two-phase driving, or the like). 
     The bridge circuit  202 _ 1  of a first channel CH 1  is connected to the first coil L 1 . The bridge circuit  202 _ 2  of a second channel CH 2  is connected to the second coil L 2 . 
     Each of the bridge circuits  202 _ 1  and  202 _ 2  is an H-bridge circuit including four transistors M 1  to M 4 . The transistors M 1  to M 4  of the bridge circuit  202 _ 1  are switched based on a control signal CNT 1  from the logic circuit  270 , whereby a voltage V OUT1  of the first coil L 1  (also referred to as a first coil voltage) is switched. 
     The bridge circuit  202 _ 2  is configured similarly to the bridge circuit  202 _ 1 , and the transistors M 1  to M 4  thereof are switched based on a control signal CNT 2  from the logic circuit  270 , whereby a voltage V OUT2  of the second coil L 2  (also referred to as a second coil voltage) is switched. 
     The current value setting circuit  210  generates a current set value I REF . Immediately after the start of the stepping motor  102 , the current set value I REF  is fixed to any predetermined value (referred to as a full torque set value) I FULL . The predetermined value I FULL  may be a maximum value within a range that the current set value I REF  can take, and in this case, the stepping motor  102  is driven by full torque. This state will be referred to as a high torque mode. 
     When the stepping motor  102  starts to rotate stably, in other words, when the risk of step-out is reduced, it is shifted to a high efficiency mode. The current value setting circuit  210  adjusts the current set value I REF  by feedback control in the high efficiency mode, thereby reducing power consumption. 
     The counter electromotive force detection circuit  230  detects a counter electromotive force V BEMF1  (V BEMF2 ) generated in the coil L 1  (L 2 ) of the stepping motor  102 . A method for detecting the counter electromotive force is not particularly limited, and a known technique may be used. In general, the counter electromotive force may be obtained by setting a certain detection window (detection section), setting both ends of the coil to high impedance, and sampling a voltage of the coil at that time. Therefore, the counter electromotive force V BEMF1  (V BEMF2 ) can be measured each time one end of the coil to be monitored (output of the bridge circuit) becomes high impedance, i.e., for each predetermined excitation position. 
     The constant current chopper circuit  250 _ 1  generates a pulse-modulated signal S PWM1  which is pulse-modulated so that a detected value I NF1  of a coil current I L1  flowing through the first coil L 1  approaches a target amount based on the current set value I REF  while the first coil L 1  is supplied with electric power. The constant current chopper circuit  250 _ 2  generates a pulse-modulated signal S PWM2  which is pulse-modulated so that a detected value I NF2  of a coil current I L2  flowing through the second coil L 2  approaches the current set value I REF  while the second coil L 2  is supplied with electric power. 
     The bridge circuits  202 _ 1  and  202 _ 2  each include a current detection resistor R NF , in which a voltage drop of the current detection resistor R NF  becomes the detected value of the coil current I L . The position of the current detection resistor R NF  is not limited, but it may be installed at a power source side or may be installed in series with the coil between two outputs of the bridge circuit. 
     The logic circuit  270  controls the bridge circuit  202 _ 1  connected to the first coil L 1  depending on the pulse-modulated signal S PWM1 . Also, the logic circuit  270  controls the bridge circuit  202 _ 2  connected to the second coil L 2  depending on the pulse-modulated signal S PWM2 . 
     The logic circuit  270  changes the excitation position and switches the coil (or the pair of coils) for supplying a current whenever the input clock CLK is input. The excitation position is recognized as a combination of magnitudes and directions of the coil current of the first coil L 1  and the coil current of the second coil L 2 . The excitation position may be shifted according to only a positive edge or only a negative edge of the input clock CLK, or both. 
     As described above, the current value setting circuit  210  is configured to be switched between (i) a high torque mode in which the current set value I REF  defining the amplitude of the coil current is fixed to a large value corresponding to the full torque and (ii) a high efficiency mode in which the current set value I REF  is adjusted by feedback control. 
     The driving circuit  200 A is configured to output the current set value I REF  to the outside or to access the current set value I REF  from the outside. To this end, the interface circuit  280 A is installed in the driving circuit  200 A. 
     The interface circuit  280 A may be switched to be enabled or disabled, in which the interface circuit  280 A may be enabled only when the current set value I REF  is desired to be known. 
       FIGS. 5A to 5C  are diagrams illustrating configuration examples of the interface circuit  280 A. In  FIG. 5A , the interface circuit  280 A includes a register  282  and an I 2 C (inter IC) circuit  284 . The current set value I REF  at a predetermined timing is written in the register  282 . Alternatively, the value of the register  282  may be updated at all times by the current set value I REF  changing every moment. The I 2 C circuit  284  may output the current set value I REF  to the outside when the register  282  is accessed from the outside. Instead of the I 2 C, an SPI (serial peripheral interface) or other transmitters or transceivers may be used. 
     Alternatively, the interface circuit  280 A may always output the current set value I REF  to the outside regardless of whether there is a request from the outside. In  FIG. 5B , a digital current set value I REF  is always output to the outside by a transmitter  286 . 
     In  FIG. 5C , a D/A converter  288  converts the digital current set value I REF  into an analog signal (voltage signal). Then, a buffer  289  outputs the analog signal to the outside. 
       FIG. 6  is a circuit diagram illustrating a configuration example of the driving circuit  200 A. Only a portion related to the first coil L 1  is illustrated in  FIG. 6 . 
     The current value setting circuit  210  will be described. The current value setting circuit  210  includes a feedback controller  220 , a feedforward controller  240 , and a multiplexer  212 . The feedforward controller  240  outputs a fixed current set value Ix (=I FULL ) used in the high torque mode immediately after the starting. This current set value Ix is set to a large value in order to prevent step-out. 
     The feedback controller  220  becomes active in the high efficiency mode, and outputs a current set value Iy which is feedback-controlled based on the counter electromotive force V BEMF . 
     The multiplexer  212  selects one of the two signals Ix and Iy depending on a mode selection signal MODE to output the selected signal as the current set value I REF . 
     The feedback controller  220  includes a load angle estimation part  222 , a subtractor  224 , and a proportional-integral (PI) controller  226 . 
     The counter electromotive force detection circuit  230  detects a counter electromotive force V BEMF1  (V BEMF2 ) generated in the coil L 1  (L 2 ) of the stepping motor  102 . A method for detecting the counter electromotive force is not particularly limited, and a known technique may be used. In general, the counter electromotive force can be obtained by setting a certain detection window (detection section), setting both ends of the coil to high impedance, and sampling the voltage of the coil at that time. Therefore, the counter electromotive force V BEMF1  (V BEMF2 ) can be measured each time one end of the coil to be monitored (output of the bridge circuit) becomes high impedance, i.e., for each predetermined excitation position. 
     The load angle estimation part  222  estimates a load angle φ based on the counter electromotive force V BEMF1 . The load angle φ corresponds to a difference between a current vector (i.e., a position command) determined by a driving current flowing through the first coil L 1  and a position of a rotor (movable element). As described above, the counter electromotive force V BEMF1  is given by the following equation.
 
 V   BEMF1   =K   E ·ω·cos φ
 
wherein K E  is a counter electromotive force constant and ω is a rotation speed. Therefore, a detected value having a correlation with the load angle φ can be generated by measuring the counter electromotive force V BEMF . For example, cow may be used as the detected value. In this case, the detected value is expressed by Eq. (2).
 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     The feedback controller  220  generates the current set value Iy so that the estimated load angle φ approaches a predetermined target angle φ REF . Specifically, the subtractor  224  generates an error ERR between the detected value cow and the target value cos(φ REF ) based on the load angle φ. The PI controller  226  performs a PI control operation so that the error ERR becomes zero to generate the current set value Iy. The processing of the feedback controller  220  may also be realized by an analog circuit using an error amplifier. 
     The constant current chopper circuit  250 _ 1  includes a D/A converter  252 , a PWM comparator  254 , an oscillator  256 , and a flip-flop  258 . The D/A converter  252  converts the current set value I REF  into an analog voltage V REF . The PWM comparator  254  compares the feedback signal I NF1  with the reference voltage V REF , and asserts (high) an OFF signal S OFF  when I NF1 &gt;V REF . The oscillator  256  generates a periodic ON signal S ON  defining a chopping frequency. The flip-flop  258  outputs a PWM signal S PWM1  which transitions to an ON level (for example, high) in response to the ON signal S ON  and transitions to an OFF level (for example, low) in response to the OFF signal S OFF . 
     Although the interface circuit  280 A is omitted in  FIG. 6 , it may be installed at a position corresponding to the configuration of the interface circuit  280 A. When the interface circuit  280 A in  FIG. 5A  is employed, the output Iy of the feedback controller  220  may be stored in the register  282 . When the interface circuit  280 A in  FIG. 5B  is employed, the output Iy of the feedback controller  220  may be output to the outside by the transmitter  286 . These paths are illustrated as (i) in  FIG. 6 . 
     When the interface circuit  280 A in  FIG. 5C  is employed, the D/A converter  288  may be configured to correspond to the D/A converter  252 , and the output of the D/A converter  252  may be output to the outside by the buffer  289 . This path is illustrated as (ii) in  FIG. 6 . 
     The configuration of the driving circuit  200 A has been described above. Next, an operation thereof will be described. 
     (i) The interface circuit  280 A can be effectively utilized in the design stage of the motor system  100 . In the design stage, the load and rotational speed of the stepping motor  102  are determined for each platform and application. When they are determined, the amount of current in the high efficiency mode becomes a substantially constant value. Therefore, in the design stage, when the current set value I REF  stabilized to a certain amount in the high efficiency mode is obtained, it is possible to know how much torque allows the stepping motor to be driven, which can be useful for system design. 
     (ii) The interface circuit  280 A can be effectively utilized even during the actual operation of the motor system  100 . For example, the current set value I REF  stabilized in the high efficiency mode is monitored, and when it is out of an appropriate range, it can be determined that an error has occurred. 
     Embodiment 2 
       FIG. 7  is a block diagram of a motor system  100 B including a driving circuit  200 B according to a second embodiment of the present disclosure. A basic configuration of the motor system  100 B is identical to that of the motor system  100 A in  FIG. 4 , and therefore, only differences will be described. 
     The driving circuit  200 B is configured to output the detected value of the coil current I L  to the outside, or to access the detected value of the coil current from the outside. The driving circuit  200 B includes an interface circuit  280 B instead of the interface circuit  280 A in  FIG. 4 . The interface circuit  280 B is installed to output the detected value of the coil current to the outside as an analog signal or a digital signal, or the interface circuit  280 B is installed to access the detected value of the coil current from the outside. 
     The detected value I NF1  (or I NF2 ) of the coil current corresponds to a voltage drop of the detection resistor R NF  installed in the bridge circuit  202 _ 1  (or  202 _ 2 ). 
       FIGS. 8A to 8C  are diagrams illustrating configuration examples of the interface circuit  280 B. In  FIG. 8A , the interface circuit  280 B includes an A/D converter  290 , a register  292 , and an I 2 C circuit  294 . The A/D converter  290  samples a current-detected value I FB  at a predetermined timing and converts it into a digital signal. The digital signal of the current-detected value is written in the register  292 . The value of the register  292  may be updated at all times at each sampling timing. The I 2 C circuit  294  may output a digital current-detected value I NF  to the outside when the register  292  is accessed from the outside. Instead of the I 2 C, an SPI (serial peripheral interface) or other transmitters or transceivers may be used. 
     Alternatively, the interface circuit  280 B may always output the current-detected value I NF  to the outside regardless of whether there is a request from the outside. In  FIG. 8B , the digital current-detected value I NF  is always output to the outside by the transmitter  296 . 
     In  FIG. 8C , a buffer  299  outputs an analog current-detected value I NF  to the outside. 
     More specifically, the driving circuit  200 B in  FIG. 7  may be configured similarly to the driving circuit  200 A in  FIG. 6 , in which the interface circuit  280 B may be added at an appropriate position. 
     The configuration of the driving circuit  200 B has been described above. Next, an operation thereof will be described. 
     (i) The interface circuit  280 B can be effectively utilized in the design stage of the motor system  100 . Since the constant current chopping control is performed so that the current-detected value I NF  coincides with the current set value I REF , it may be considered that the current-detected value I NF  and the current set value I REF  are substantially equal in a steady state. 
     In the design stage, in a situation where the stepping motor is stably driven in the high-efficiency mode, when the current-detected value I NF  stabilized to a certain amount is obtained, it is possible to know how much torque allows the stepping motor to be driven, which can be useful for system design. 
     (ii) The interface circuit  280 B can be effectively utilized even during the actual operation of the motor system  100 . For example, the current-detected value I NF  stabilized in the high efficiency mode is monitored, and when it is out of a proper range, it can be determined that an error has occurred. 
       FIG. 9  is a diagram illustrating another configuration example of the current value setting circuit  210 . The feedback controller  220  is active in the high efficiency mode, and generates a current-corrected value ΔI whose value is adjusted so that the load angle φ approaches the target value φ REF . The current-corrected value ΔI is zero in the high torque mode. 
     The feedforward controller  240  outputs a predetermined high efficiency set value I LOW  in the high efficiency mode. A relationship of I FULL &gt;I LOW  may be established. The current value setting circuit  210  includes an adder  214  instead of the multiplexer  212  in  FIG. 6 , in which the adder  214  adds the current-corrected value ΔI to the high efficiency set value I LOW  generated by the feedforward controller  240 . Thus, the current set value I REF =I LOW +ΔI is adjusted so that the load angle φ approaches the target value φ REF . 
     Lastly, applications of the driving circuits  200 A and  200 B (generally and simply referred to as a driving circuit  200 ) will be described. The driving circuit  200  is used for various electronic devices.  FIGS. 10A to 10C  are perspective views illustrating examples of an electronic device including the driving circuit  200 . 
     The electronic device in  FIG. 10A  is an optical disk device  500 . The optical disk device  500  includes an optical disk  502  and a pickup  504 . The pickup  504  is installed for writing and reading data into and from the optical disc  502 . The pickup  504  is movable over the recording surface of the optical disk  502  in the radial direction of the optical disk (tracking). The distance between the pickup  504  and the optical disk is also variable (focusing). The pickup  504  is positioned by a stepping motor (not shown). The driving circuit  200  controls the stepping motor. With this configuration, the pickup  504  can be positioned with high efficiency and high accuracy while preventing step-out. 
     The electronic device in  FIG. 10B  is a device  600  having an imaging function, such as a digital still camera, a digital video camera, a portable telephone terminal, or the like. The device  600  includes an imaging element  602  and an auto-focus lens  604 . The stepping motor  102  positions the auto-focus lens  604 . According to this configuration in which the driving circuit  200  drives the stepping motor  102 , the auto-focus lens  604  can be positioned with high efficiency and high accuracy while preventing step-out. The driving circuit  200  may be used for driving a lens for camera shake correction, in addition to the auto-focus lens. Alternatively, the driving circuit  200  may be used for aperture control. 
     The electronic device in  FIG. 10C  is a printer  700 . The printer  700  includes a head  702  and a guide rail  704 . The head  702  is supported so as to be positioned along the guide rail  704 . The stepping motor  102  controls the position of the head  702 . The driving circuit  200  controls the stepping motor  102 . With this configuration, the head  702  can be positioned with high efficiency and high accuracy while preventing step-out. The driving circuit  200  may be used for driving a motor for a paper feeding mechanism, in addition to driving the head. 
     The present disclosure has been described above with reference to the embodiments. It is to be understood by those of ordinary skill in the art that the embodiments are merely illustrative and may be differently modified by any combination of the components or processes, and the modifications are also within the scope of the present disclosure. Hereinafter, the modifications will be described. 
     (Modification 1) 
     The logic circuit  270  may adjust a power source voltage V DD  supplied to the bridge circuit  202 , instead of adjusting the duty ratio of the pulse-modulated signal S 2  or in combination with it, so that the load angle φ approaches the target angle φ REF . By changing the power source voltage V DD , the electric power supplied to the coils L 1  and L 2  of the stepping motor  102  can be changed. 
     (Modification 2) 
     In the embodiments, there has been described a case where the bridge circuit  202  is configured by a full bridge circuit (H bridge). However, the present disclosure is not limited thereto, and the bridge circuit  202  may be configured by a half bridge circuit. Also, the bridge circuit  202  may be a separate chip from the driving circuit  200 A ( 200 B) or may be a discrete component. 
     (Modification 3) 
     The method for generating the current set value Iy in the high efficiency mode is not limited to that described in the embodiments. For example, a target value V BEMF(REF)  of the counter electromotive force V BEMF1  may be determined, and a feedback loop may be configured so that the counter electromotive force V BEMF1  approaches the target value V BEMF(REF) . 
     (Modification 4) 
     In the embodiments, the currents I OUT1  and I OUT2  flowing through the two coils are turned on and off according to an excitation position, but the amount of currents thereof is constant regardless of the excitation position. In this case, the torque is varied in the case of one-two-phase excitation. Instead of this control, the currents I OUT1  and I OUT2  may be corrected so that the torque is constant regardless of the excitation position. For example, in the one-two-phase excitation, the amounts of the currents I OUT1  and I OUT2  at the excitation positions 2, 4, 6, and 8 may be set to √2 times the amounts of the currents at the excitation positions 1, 3, 5, and 7. 
     (Modification 5) 
     In the embodiments, the feedback controller  220  is configured by the PI controller. However, the present disclosure is not limited thereto, and a PID controller or the like may be used. 
     According to the present disclosure in some embodiments, it is possible to output information useful for system design or control to outside. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.