Abstract:
A power cutter, an oscillator, a counter, a S-R latch, and a Hall bias are further disposed inside a DC motor driving circuit so that current consumption of the DC motor driving circuit is reduced significantly under a standby mode. When the counter detects that a received pulse width modulation signal stays at a low electrical level over a predetermined time, the counter triggers the S-R latch so as to activate a disabling signal of the power cutter for shutting down most elements until the pulse width modulation signal returns to a high electrical level. With the aid of the built-in Hall bias, space for externally coupling the Hall bias is saved, and moreover, a Hall sensor retrieves a dynamically-adjusted power and currents so that remarkable current consumption is saved under a standby mode.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a motor driving circuit, and more particularly, to a motor driving circuit having low current consumption under a standby mode. 
   2. Description of the Prior Art 
   A conventional DC motor is equipped with a specific driving circuit, for manipulating a driving voltage of the DC motor or occasions for operating the DC motor. The conventional DC motor is primarily operated by magnetic forces, which are generated from repeated variation of electromagnetic forces generated by two torques, where both the torques are inverse in orientations and are generated by magnetic fields, which are generated by repeatedly changed orientations of currents, in the DC motor. However, under certain circumstances, the conventional DC motor is not required to operate, and therefore, power of the DC motor has to be reduced at this time for avoiding unnecessary power consumption. 
     FIG. 1  is a diagram of a conventional DC motor driving circuit  100 . As shown in  FIG. 1 , the DC motor driving circuit  100  includes a driving module  101 . The driving module  101  includes a control module  102 , an H-shaped full-bridge circuit  104 , an operational amplifier  106 , a comparator  108 , a transistor  110 , a lock/restart module  112 , and a thermal shutdown module  114 . The DC motor driving circuit  100  further comprises a motor  116 , a frequency generating resistor  118 , a motor driving voltage source  120 , a first diode  122 , a capacitor  124 , a Hall sensor  126 , and a conventional resistor  128 . The H-shaped full-bridge circuit  104  has a first input terminal coupled to a first output terminal of the control module  102 , a second input terminal coupled to a second output terminal of the control module  102 , a third input terminal coupled to a third output terminal of the control module  102 , and a fourth input terminal coupled to a fourth output terminal of the control module  102 . The H-shaped full-bridge circuit  104  has a first output terminal coupled to a pin OUT 1  of the driving module  101 , and a second output terminal coupled to a pin OUT 2  of the driving module  101 . The operational amplifier  106  has a first output terminal coupled to a first input terminal of the control module  102 , a second output terminal coupled to a second input terminal of the control module  102 , a first input terminal coupled to a pin H+ of the driving module  101 , and a second input terminal coupled to the pin H− of the driving module  101 . The comparator  108  has a first input terminal coupled to the first input terminal of the control module  102 , and a second input terminal coupled to the second input terminal of the control module  102 . The transistor  110  has a gate coupled to a first output terminal of the comparator  108 , a source coupled to ground, and a drain coupled to a pin FG of the driving module  101 . The lock/restart module  112  has an input terminal coupled to the second output terminal of the comparator  108 , and an output terminal coupled to a third input terminal of the control module  102 . The thermal shutdown module  114  has an output terminal coupled to a fourth input terminal of the control module  102 . The motor  116  has a first terminal coupled to the pin OUT 1  of the driving module  101 , and a second terminal coupled to the pin OUT 2  of the driving module  101 . The frequency generating resistor  118  has a first terminal coupled to the pin FG of the driving module  101 . The motor driving voltage source  120  has a positive terminal coupled to a second terminal of the frequency generating resistor  118 , and a negative terminal coupled to ground. The first diode  122  has a positive bias terminal coupled to the positive terminal of the motor driving voltage source  120 . The capacitor  124  has a first terminal coupled to both the negative terminal of the motor driving voltage source  120  and a pin VDD of the driving module  101 , and a second terminal coupled to ground. The Hall sensor  126  has a first output terminal coupled to the pin H+ of the driving module  101 , a second output terminal coupled to the pin H− of the driving module  101 , and a negative bias terminal coupled to ground. The resistor  128  has a first terminal coupled to a positive bias terminal of the Hall sensor  126 , and a second terminal coupled to the pin VDD. 
   The H-shaped full-bridge circuit  104  includes four transistors as shown in  FIG. 1 , where the four transistors include a first P-type MOSFET  130 , a second P-type MOSFET  134 , a first N-type MOSFET  138 , and a second N-type MOSFET  142 . The H-shaped full-bridge circuit  104  further includes four diodes as shown in  FIG. 1 , where the four diodes include a second diode  132 , a third diode  134 , a fourth diode  140 , and a fifth diode  144 . The first P-type MOSFET  130  has a gate coupled to the first input terminal of the H-shaped full-bridge circuit  104 . The second diode  132  has a first terminal coupled to a drain of the first P-type MOSFET  130 , and a second terminal coupled to a source of the first P-type MOSFET  130 . The second P-type MOSFET  134  has a gate coupled to the third input terminal of the H-shaped full-bridge circuit  104 . The third diode  136  has a first terminal coupled to a drain of the second P-type MOSFET  134 , and a second terminal coupled to a source of the second P-type MOSFET  134 . The first N-type MOSFET  138  has a gate coupled to the second input terminal of the H-shaped full-bridge circuit  104 , and a drain coupled to the drain of the first P-type MOSFET  130 . The fourth diode  140  has a first terminal coupled to a source of the first N-type MOSFET  138 , and a second terminal coupled to the drain of the first N-type MOSFET  138 . The second N-type MOSFET  142  has a gate coupled to the fourth input terminal of the H-shaped full-bridge circuit  104 , and a drain coupled to the drain of the second P-type MOSFET  134 , and a source coupled to the source of the first N-type MOSFET  134 . The fifth diode  144  has a first terminal coupled to the source of the second N-type MOSFET  142 , and a second terminal coupled to the drain of the second N-type MOSFET  142 . Note that both the sources of the first P-type MOSFET  130  and the second P-type MOSFET  134  are coupled to the pin VDD for receiving a voltage inputted at the pin VDD. Both the sources of the first N-type MOSFET  138  and the second N-type MOSFET  142  are coupled to ground. All of the first P-type MOSFET  130 , the second P-type MOSFET  134 , the first N-type MOSFET  138 , and the second N-type MOSFET  142  are utilized for providing required currents for driving the motor  116 . 
   The DC motor driving circuit  100  is biased with both a DC voltage, which is inputted through the pin VDD, and ground, which is coupled through the pin GND. The control module  102  is utilized for controlling voltage levels of gates of the first P-type MOSFET  130 , the second P-type MOSFET  134 , the first N-type MOSFET  138 , and the second N-type MOSFET  142 , for switching on or switching off the listed MOSFETs, and for tuning a required current for driving the motor  116 . The control module  102  may be implemented with a digital logic circuit or an analog amplifier control circuit. Bias voltages of the Hall sensor  126  are determined by both the voltage level at the pin VDD and the resistance of the resistor  128 . The operational amplifier  106  is utilized for amplifying voltage levels, which are outputted from the Hall sensor  126  and at the pins H+ and H− , so that the amplified voltage levels are respectively outputted at nodes PO and NO, as shown in  FIG. 1 , for usage of succeeding elements. The lock/restart module  112  transmits a command for ordering the control module  102  to shut down transistors of the H-shaped full-bridge circuit  104  when fans of the motor  116  are jammed. After the transistors of the H-shaped full-bridge circuit  104  are shut down for a while, the lock/restart module  112  transmits another command to the control module  102  for activating the motor  116  by turning on the transistors. The comparator  108  is utilized for switching on or switching off the transistor  110 . When the transistor  110  is switched on by receiving an output signal from the first output terminal of the comparator  108 , the voltage level at the drain of the transistor  110 , i.e., the voltage level at the pin FG, may be detected from an external system, where the signal at the pin FG indicates a rotational velocity of the motor  116 . Moreover, when an output signal is outputted from the first output terminal of the comparator  108 , a reset signal is also outputted from the second output terminal of the comparator  108  to the lock/restart module  112  for resetting the status of the lock/restart module  112 , where the reset signal is a one shot pulse. The thermal shutdown module  114  is utilized for ordering the control module  102  to shut down the transistors of the H-shaped full-bridge circuit  104  when the motor  116  is overheated. Therefore, the H-shaped full-bridge circuit  104  ceases generating biasing currents, and the temperature of the motor  116  ceases increasing as well. The motor driving voltage source  120  is utilized for providing required bias voltages of the H-shaped full-bridge circuit  104  (or the DC motor driving circuit  100  as well) through the pin VDD. The first diode  122  is utilized for preventing currents from the pin VDD from reversely flowing to the motor driving voltage source  120  with its reverse bias. Besides, when the motor driving voltage source  120  is erroneously connected in poles, the first diode  122  prevents the DC motor driving circuit  100  from being burnt down as well. The capacitor  124  is utilized for draining backflow currents of the motor  116 , and for stabilizing the voltage level at the pin VDD. The Hall sensor  126  is utilized for detecting the magnetic filed generated by operations of the motor  116  to output corresponding signals to both the pins H+ and H− so that the DC motor driving circuit  100  is informed with variations of the magnitude of the magnetic field. Note that the motor  116  indicates an inductive loading so as to store electric power. The pin PWM receives pulse width modulation (PWM) signals from a system terminal, where switching on or switching off the transistors of the H-shaped full-bridge circuit  104  by the control module  102  primarily follows the pulse width modulation signals when the motor  116  is normally operated. For example, when the motor  116  is normally operated, and when the pulse width modulation signal stays high, the transistors of the H-shaped full-bridge circuit  104  switches on or switches off by following voltage levels of both the pins H+ and H−, and the motor  116  is biased by the voltage level at the pin VDD. When the motor  116  is normally operated, and when the pulse width modulation signal stays low, both the first P-type MOSFET  130  and the second P-type MOSFET  134  are switched off, and both the first N-type MOSFET  138  and the second N-type MOSFET  142  are switched on. At this time, the voltage level at the pin VDD is isolated by both the shut-down transistors, and the motor  116  cannot be biased with the pin VDD so that power consumption is saved. 
   Please refer to  FIG. 2 , which is a waveform diagram of voltage levels at pins of the DC motor driving circuit  100  shown in  FIG. 1  when the control module  102  shown in  FIG. 1  is implemented with an analog amplifier controlling circuit. Note that the symbol “Imotor” shown in  FIG. 2  indicates a bias current of the motor  116 . As shown in  FIG. 2 , at the moment when the voltage levels at both the pins H+ and H− intersect, i.e., when the magnetic pole of the motor  116  is changed, envelopes of the voltage levels at both the pins OUT 1  and OUT 2  vary smoothly between a positive voltage level and a negative voltage level, and therefore, the current Imotor varies smoothly so that less noises from the motor  116  are generated. However, when the control module  102  is implemented with a digital logic circuit, the envelopes of the voltage levels at both the pins OUT 1  and OUT 2  vary significantly between the high voltage level and the low voltage level, and therefore, the current Imotor vary sharply so that more noises from the motor  116  are generated. But the power consumption of the motor  116  is smaller when the control module  102  is implemented with the digital logic circuit. Note that the analog amplifier controlling circuit is merely an exemplary embodiment of the control module  102  in voltage level transition, and other conventional embodiments are not further described herein. Note that the frequency of the signal at the pin FG is the same with the signals at both the pins H+ and H−, and therefore, the frequency of the signal at the pin FG is able to indicate a rotational frequency of the motor  116  so that the system terminal may be informed with a corresponding rotational velocity of the motor  116 . At last, the system terminal accordingly outputs a pulse width modulation signal having an adequate duty cycle for ordering the control module  102  to tune the rotational velocity of the motor  116 . When the system terminal is overheated, the duty cycle of the pulse width modulation signal is increased for increasing both the bias current and the rotational velocity of the motor  116  to enhance heat dissipation of the system terminal. When the temperature of the system terminal is decreased so that the motor  116  is not required to enhance heat dissipation, the system terminal accordingly outputs a pulse width modulation signal having a smaller duty cycle (even 0%) for reducing the bias current of the motor  116 , and for saving unnecessary power consumption of the motor  116  as well. 
   Please refer to  FIG. 3 , which is a waveform diagram of voltage levels at certain pins illustrated in  FIG. 2  when the motor  116  shown in  FIG. 1  is locked by unknown reasons. As shown in  FIG. 3 , in the operating period, voltage levels at the pins H+, H−, OUT 1 , OUT 2 , and FG are normal when the motor  116  are normally operated. However, in the first restart period, since fans of the motor  116  are jammed or interrupted magnetically, voltage levels at the pins H+ and H− are kept constant. The voltages levels at the pins OUT 1  and OUT 2  for indicating a voltage difference of the motor  116  are kept constant as well so that there are no changes in the magnetic field, but the voltage level at the pin OUT 1  stays high for keeping on restarting the motor  116 . At this time, since the motor  116  is not operated, the voltage level at the pin FG for indicating a rotational frequency of the motor  116  stays at low. After the first restart period is over, since there are no responses in the motor  116  for a while, for saving power consumption, the voltage level at the pin OUT 1  is changed to be low for significantly weakening the current flowing through the motor  116  during the standby periods shown in  FIG. 3 . After several successive restart periods along with several standby periods pass, when the factor for hindering the motor  116  from operating is removed in a certain restart period or a certain standby period, the restart/operating period shown in  FIG. 3  begins, and the motor  116  may be normally operated again by the voltage difference between the pins OUT 1  and OUT 2 . Moreover, voltage levels at other pins shown in  FIG. 3  are back to normal as well. 
   SUMMARY OF THE INVENTION 
   The claimed invention provides a motor driving circuit having low current consumption under a standby mode. The motor driving circuit comprises a driving module, a Hall sensor, a pulse width modulation (PWM) signal source, and a motor. The driving module comprises a power cutter, a control module, an oscillator, a counter, a S-R latch, a Hall bias, a lock/restart module, an H-shaped full-bridge circuit, an operational amplifier, a comparator, and a first transistor. The control module has a first input terminal coupled to an output terminal of the power cutter. The oscillator has an input terminal coupled to the output terminal of the power cutter. The counter has a first input terminal coupled to the output terminal of the power cutter, and a second input terminal coupled to an output terminal of the oscillator. The S-R latch has a Set terminal coupled to an output terminal of the counter, and a positive logic output terminal coupled to an input terminal of the power cutter. The Hall bias has an input terminal coupled to the output terminal of the power cutter. The lock/restart module has a first input terminal coupled to the output terminal of the power cutter, and an output terminal coupled to a second input terminal of the control module. The H-shaped full-bridge circuit has a first input terminal coupled to a first output terminal of the control module, a second input terminal coupled to a second output terminal of the control module, a third input terminal coupled to a third output terminal of the control module, and a fourth input terminal coupled to a fourth output terminal of the control module. The operational amplifier has a first output terminal coupled to the third output terminal of the control module, a second output terminal coupled to a fourth input terminal of the control module, and a first input terminal coupled to the output terminal of the power cutter. The comparator has a first input terminal coupled to the first output terminal of the operational amplifier, a second input terminal coupled to the second output terminal of the operational amplifier, a third input terminal coupled to the output terminal of the power cutter, and a first output terminal coupled to a second input terminal of the lock/restart module. The first transistor has a gate coupled to the second output terminal of the comparator. The Hall sensor has an input terminal coupled to an output terminal of the Hall bias, a first output terminal coupled to the first input terminal of the operational amplifier, and a second input terminal coupled to the second input terminal of the operational amplifier. The pulse width modulation (PWM) signal source is coupled to a third input terminal of the counter, a Reset terminal of the S-R latch, and a fifth input terminal of the control module. The motor has a first input terminal coupled to the first output terminal of the H-shaped full-bridge circuit, and a second input terminal coupled to the second output terminal of the H-shaped full-bridge circuit. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a conventional DC motor driving circuit. 
       FIG. 2  is a waveform diagram of voltage levels at pins of the DC motor driving circuit shown in  FIG. 1  when the control module shown in  FIG. 1  is implemented with an analog amplifier controlling circuit. 
       FIG. 3  is a waveform diagram of voltage levels at certain pins illustrated in  FIG. 2  when the motor shown in  FIG. 1  is locked by unknown reasons. 
       FIG. 4  is a diagram of the DC motor driving circuit provided in the present invention. 
       FIG. 5  is a waveform diagram illustrating voltage levels at pins of the DC motor driving circuit shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   As technologies evolve, reducing power consumption becomes an important topic. For example, reducing power consumption of portable electronic products, such as a laptop or a cell phone, is highly researched and concentrated. In designing a motor driving circuit, reducing the power consumption becomes important as well. A DC motor driving circuit is provided in the present invention. Power consumption of the provided DC motor driving circuit reaches a degree of microamperes or less when the motor is not operated and stays at a standby mode. Compared to conventional motor driving circuits having power consumption in a degree of milliamperes in a standby mode, power consumption is significantly saved in the DC motor driving circuit provided in the present invention. Moreover, as microprocessors evolve, the motor is required to be operated less frequently, i.e., the time for activating the standby mode of the motor is getting more frequently. Therefore, the weaker the current of the motor under the standby mode is, the more the power consumption of the motor driving circuit is saved. The DC motor driving circuit provided in the present invention reduces power consumption of the motor by weakening the current of the motor under the standby mode. 
   Compared to the DC motor driving circuit  100  and the driving module  101  as shown in  FIG. 1 , a few elements are added in the present invention, and both the DC motor driving circuit  200  and the driving module  201  are thus formed. In other words, most utilized elements of both the driving modules  101  and  201  are the same so that overlapped couplings described in  FIG. 1  are not described again in  FIG. 4 . 
   Please refer to  FIG. 4 , which is a diagram of the DC motor driving circuit  200  provided in the present invention. The difference between the DC motor driving circuits  100  and  200  lies in the driving module  201 . Compared to the driving module  101  shown in  FIG. 1 , the driving module  201  further includes a power cutter  250 , an oscillator  260 , a counter  270 , a S-R latch  280 , and a Hall bias  290 . The power cutter  250  has an output terminal coupled to the first input terminal of the control module  102 , the third input terminal of the comparator  108 , the first input terminal of the lock/restart module  112 , an input terminal of the oscillator  260 , a first input terminal of the counter  270 , the input terminal of the thermal shutdown module  114 , the first input terminal of the operational amplifier  106 , and an input terminal of the Hall bias  290 . The power cutter  250  is utilized for generating a disabling signal for disabling the control module  102 , the comparator  108 , the lock/restart module  112 , the oscillator  260 , the counter  270 , the thermal shutdown module  114 , the operational amplifier  106 , and the Hall bias  290 . Note that in the present invention, the power cutter  250  merely generates the disabling signal while the input terminal of the power cutter  250  stays low, whereas the power cutter  250  further generates an enabling signal while the input terminal of the power cutter  250  stays high. The oscillator  260  has an output terminal coupled to the first input terminal of the counter  270 . The oscillator  260  is utilized for outputting a switch current to the counter  270 . The counter  270  has a third input terminal coupled to the pin PWM, and an output terminal coupled to the Set terminal of the S-R latch  280 . The counter  270  is utilized for counting how long the pin PWM is kept on staying low. The S-R latch  280  has the Reset terminal coupled to the pin PWM, and a positive logic output terminal, which is denoted as Q as well, coupled to the input terminal of the power cutter  250  for activating the disabling signal of the power cutter  250 . Note that the pin VDD and the resistor  128  are not utilized for supplying power to the Hall sensor  126  nor adjusting an input current of the Hall sensor  126  in the DC motor driving circuit  200 , but the Hall bias  290  disposed inside the driving module  201  is utilized for supplying power to the Hall sensor  126  or adjusting the input current of the Hall sensor  126  instead. The Hall bias  290  has an output terminal coupled to the pin HB for supplying a constant power to the Hall sensor  126  to detect variations in the magnetic field of the motor  116 . In the DC motor driving circuit  100 , the pin VDD is originally utilized for supplying power to the Hall sensor  126 , and a corresponding Hall bias has to be disposed outside the DC motor driving circuit  100  as well. As mentioned above, in the DC motor driving circuit  200  provided in the present invention, the Hall sensor  126  is supplied power by the Hall bias  290  disposed inside the driving module  201 . Therefore, room for disposing the Hall bias outside the DC motor driving circuit is saved, and the Hall sensor  126  may be cut in power more rapidly with the aid of the neighboring power cutter  250 . Note that the Hall bias  290  may be implemented with a conventional regulator so that the Hall sensor  126  may be supplied with a power having a variable voltage level. 
   Operations of the DC motor driving circuit  200  are roughly described as follows. When the fans of the motor  116  are not required to rotate, i.e., when the DC motor driving circuit  200  stays at the standby mode, a duty cycle of a pulse width modulation signal at the pin PWM is 0. In other words, the pulse width modulation signal is continuously low at this time. The counter  270  counts how long the pulse width modulation signal stays at low. When the pulse width modulation signal stays at low over a predetermined critical time, the power cutter  250  generates the disabling signal for disabling elements coupled to the output terminal of the power cutter  250 , and for switching off transistors of the H-shaped full-bridge circuit  104  as well. Under such a circumstance, the power consumption of the DC motor driving circuit  200  reaches a degree of microamperes or less, and therefore, the aim of the present invention in significantly reducing the power consumption of the DC motor driving circuit under a standby mode is achieved. The power cutter  250  may be implemented with a conventional digital logic circuit so as to lead the DC motor driving circuit  200  to much less power consumption under the standby mode. Moreover, when the power cutter  250  is implemented with complementary metal-oxide semiconductors (CMOS), the corresponding power consumption is much significantly reduced. As mentioned above, when the Hall bias  290  is implemented with a regulator, the Hall sensor  126  is supplied with a power having a variable voltage level, which is directly proportional to a bias voltage of the Dc motor driving circuit  200 , and therefore, the fact saves much power consumption when the motor is normally operated. Moreover, when the DC motor driving circuit  200  is under the standby mode, the disabling signal generated by the power cutter  250  disables the Hall bias  290  so that there is no power consumption in the Hall bias  290  at this time. The power cutter  250  also disables the comparator  108  so that the gate of the transistor  110  stays low, and the transistor  110  is thus continuously switched off. At this time, the drain of the transistor  110  stays high so that the pin FG continuously stays high as well. 
   Please refer to  FIG. 5 , and please refer to  FIG. 4  as well.  FIG. 5  is a waveform diagram illustrating voltage levels at pins of the DC motor driving circuit  200  shown in  FIG. 4 . As mentioned before, when the DC motor driving circuit is normally operated, i.e., during the operating period, voltage levels at the pins of the DC motor driving circuit  200  stay normal. When the voltage level at the pin PWM is kept on going low for a predetermined time, i.e., when the DC motor driving circuit  200  enters at the standby mode, the counter  270  outputs a signal having a high voltage level to the Set terminal of the S-R latch  280  so that the S-R latch  280  satisfies a condition that the Set terminal stays high whereas the Reset terminal stays low. At this time, the positive logic output terminal of the S-R latch  280  stays low, which is common with conventional S-R latches so as not to be discussed further. After sensing the low voltage level at the positive logic output terminal of the S-R latch  280 , the power cutter  250  generates the disabling signal for disabling most elements of the driving module directly or indirectly. For example, because the power cutter  250  disables the Hall bias  290 , the pin HB stays low, the Hall sensor  126  is not supplied with power, and both the pins H+ and H− stay low. After receiving the disabling signal, the operational amplifier  106  pulls voltage levels at both the nodes PO and NO down to be low. Both the lock/restart module  112  and the thermal shutdown module  114  also shut down its analog elements after receiving the disabling signal, and disable its digital elements so as not to affect the control module  102 . After the comparator  108  receives the disabling signal, the first output of the comparator  108  stays low so that the gate of the transistor  110  stays low as well, the transistor  110  is thus switched off, and the voltage level at the drain of the transistor  110  is raised to be high. In other words, at this time, the pin FG continuously stays at high so that the pin FG cannot indicate the rotational frequency of the motor  116 . After the control module  102  receives the disabling signal, the control module  102  switches off both the first P-type MOSFET  130  and the second P-type MOSFET  134  so that the motor  116  cannot be supplied with power through the pin VDD. The control module  102  also switches on both the first N-type MOSFET  138  and the second N-type MOSFET  142  so that the voltage levels at both the pins OUT 1  and OUT 2  continuously stay at ground. The oscillator  260  and the counter  270  are shut down after receiving the disabling signal. Therefore, the oscillator  260  ceases outputting the switch current, and the counter  270  ceases counting for saving unnecessary power consumption. Note that both the Set terminal and the Reset terminal of the S-R latch  280  stay low because the counter  270  is shut down. Herein, the current of the driving module  201  is weakened to a degree of microamperes or less, and the aim of reaching a least current consumption in the DC motor driving circuit  200  under the standby mode is achieved. At last, when the system terminal tends to reactivate the motor  116 , the pulse width modulation signal is restored to a status of interleaving high and low voltage levels. When the pulse width modulation signal is restored to be high, the S-R latch is reset, and satisfies a condition that the Set terminal stays high whereas the Reset terminal stays low. Therefore, the positive logic output terminal of the S-R latch  280  stays high so as to have the power cutter  250  to output the enabling signal. The elements disabled by the power cutter  250  directly or indirectly are enabled after receiving the enabling signal, and the motor  116  is returned to be normally operated again. Note that whether the power cutter  250  outputs the enabling signal or the disabling signal, there is a certain operating order between the elements enabled or disabled. For example, the power cutter  250  merely outputs the enabling signal instead of the disabling signal after the pin HB is completely changed from low to high, therefore, the precision of the voltage levels at the pins H+ and H− is ensured, and the precision of other functions replying on both the voltage levels at the pins H+ and H− is ensured also. 
   The present invention provides a DC motor driving circuit having a low current consumption under a standby mode, for improving the defect of having a higher current consumption under the standby mode in the prior art, where the higher current consumption leads to a higher power consumption as well. In the DC motor driving circuit provided by the present invention, a counter counts how long a pulse width modulation signal, which is inputted to the DC motor driving circuit, is kept on staying low, and has the DC motor driving circuit of the present invention enter the standby mode when the pulse width modulation signal stays at low over a predetermined period of time. Then the counter triggers a S-R latch to have a power cutter to output a disabling signal. Any element receiving the disabling signal is disabled or shut down, and thus has least current consumption, where the motor has least current consumption as well. At this time, the current consumption of the DC motor driving circuit of the present invention reaches a degree of microamperes or less. When the system terminal generates a high pulse width modulation signal for reactivating the motor, the S-R latch and the power cutter are utilized again for restoring the DC motor driving circuit back to normal operations. Besides, since the Hall sensor utilized in the present invention is supplied with power through a built-in Hall bias, room for coupling an external Hall bias is saved. The Hall bias may also be implemented with a regulator. Therefore, under normal operations of the DC motor driving circuit of the present invention, the Hall bias may dynamically adjust the supplied power according to a bias voltage of the DC motor driving circuit, and significant current consumption may thus be saved. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.