Abstract:
A motor drive circuit comprising: first and second transistors connected in series, a voltage of a connection-point therebetween being a drive-voltage applied to one end of a motor coil; an operational amplifier for controlling the transistors such that the drive-voltage is a voltage according to a difference between first and second control voltages; a switch circuit for driving the transistors such that the motor coil is in an undriven state regardless of control by the operational amplifier when a pulse-signal is at one logic level, and driving the transistors based on the control when the pulse-signal is at the other logic level; and an auxiliary drive circuit for driving the transistors to increase the drive-voltage for a predetermined time period shorter than a time period of the pulse signal being at the other level regardless of the control, when the pulse-signal changes from the one level to the other.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority to Japanese Patent Application No. 2007-302075, filed Nov. 21, 2007, of which full contents are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a motor drive circuit, a fan motor, electronic equipment, and a notebook computer. 
     2. Description of the Related Art 
     In electronic equipment such as a notebook computer, a fan motor is used for cooling a heat-producing component such as a processor. In the fan motor, a drive voltage applied to a motor coil is controlled according to a signal indicating a rotational position of the motor, so that the motor runs at the desired rotation. In a case where the drive voltage applied to the motor coil is thus controlled, change of the drive voltage may be moderated for silencing of noise, reduction of flyback voltage, etc. (see Japanese Publication Laid-Open No. 2004-166379.) 
     When using the fan motor to cool a component, a cooling effect can be enhanced by increasing rotational speed of the motor, i.e., rotational speed of a fan. However, constantly keeping the fan at high rotation speed causes problems such as an increase in power consumption, fan noise. Therefore, the rotational speed of the fan is often controlled to be at a necessary level by intermittently driving the motor according to an amount of heat released by a component to be cooled, an amount of processing, etc. 
       FIG. 6  is a diagram showing an example of a configuration of a motor drive circuit that drives a single phase fan motor. In a motor drive circuit  100 , drive voltages V OUT1  and V OUT2 , which are applied to terminals OUT 1  and OUT 2  connected to a motor coil L, are controlled based on voltages V H1  and V H2  opposite in phase from each other according to the rotational position of the motor that are output by a Hall element  110 . The voltage of a connection point between a P-channel MOSFET  112  and an N-channel MOSFET  113  connected in series is denoted as the drive voltage V OUT1 . An operational amplifier  115  controls a voltage applied to gates of the P-channel MOSFET  112  and the N-channel MOSFET  113  so that the drive voltage V OUT1  becomes a voltage according to a difference between the voltages V H1  and V H2  output by the Hall element  110 . The drive voltage V OUT2  is controlled to have a phase opposite to that of the drive voltage V OUT1  based on the voltages V H1  and V H2  output by the Hall element  110 . Accordingly, in the motor drive circuit  100 , the drive voltages V OUT1  and V OUT2  are controlled to change moderately, so that silencing of noise, reduction of flyback voltage, etc., are realized. 
     The motor coil L is intermittently controlled according to a PWM signal output by a PWM signal output circuit  120 . Specifically, when the PWM signal is at H level, the P-channel MOSFETs  122  and  123  are OFF, and therefore, the drive voltage V OUT1  is controlled by the operational amplifier  115 , however, when the PWM signal is at L level, the P-channel MOSFETs  122  and  123  are ON, and therefore, the P-channel MOSFET  112  is OFF, the N-channel MOSFET  113  is ON, and the drive voltage V OUT1  changes to the L level regardless of control by the operational amplifier  115 . Similarly, the drive voltage V OUT2  is also controlled according to the PWM signal. Accordingly, the motor coil L is in a state of not being driven while the PWM signal is at L level. In other words, in the motor drive circuit  100 , the rotational speed of the fan can be controlled by changing duty of the PWM signal according to circumstances. 
       FIG. 7  is a diagram showing an example of change of the drive voltage V OUT1  according to the PWM signal. When the PWM signal changes from the H level to the L level, the P-channel MOSFET  112  is turned OFF, the N-channel MOSFET  113  is turned ON, and the drive voltage V OUT1  quickly changes to the L level. On the other hand, when the PWM signal changes from the L level to the H level, the drive voltage V OUT1  changes under a feedback control by the operational amplifier  115 , and therefore, time is required according to frequency characteristics of the operational amplifier  115  to reach a target level according to the difference between the voltages V H1  and V H2  output by the Hall element  110 . 
     In a fan motor that cools a processor or the like of a notebook computer, the rotational speed of the fan is changed according to circumstances, and in such circumstances that little heat is released such as a standby state, sleep mode, etc., in order to reduce the power consumption, it is desirable to operate the fan at a low rotation speed as much as possible. Therefore, in a case of the motor drive circuit  100 , in order to operate the fan at the low rotation speed, it is necessary to reduce a duty ratio of the H level of the PWM signal, i.e., a pulse width of the PWM signal. 
     However, in the motor drive circuit  100  as illustrated in  FIG. 7 , time is required according to the frequency characteristics of the operational amplifier  115  for the drive voltage V OUT1  to reach the target after the PWM signal changes from the L level to the H level. Therefore, as the pulse width of the PWM signal is reduced, the PWM signal may change to the L level and the drive voltage V OUT1  may undesirably change to the L level before reaching the target level. Accordingly, the rotational speed of the fan cannot be linearly controlled according to the duty of the PWM signal especially in a range of the low rotation speed, and therefore, it is difficult to operate the fan at a sufficiently required low rotation speed. 
     SUMMARY OF THE INVENTION 
     A motor drive circuit according to an aspect of the present invention, comprises: a first transistor and a second transistor connected in series, a voltage of a connection point between the first transistor and the second transistor being a drive voltage applied to one end of a motor coil; an operational amplifier configured to control the first transistor and the second transistor such that the drive voltage is a voltage according to a difference between a first control voltage and a second control voltage for controlling driving of the motor coil; a switch circuit configured to drive the first transistor and the second transistor such that the motor coil is in a state of not being driven regardless of control by the operational amplifier when a pulse signal for intermittently driving the motor coil is at one logic level, and drive the first transistor and the second transistor based on the control by the operational amplifier when the pulse signal is at the other logic level; and an auxiliary drive circuit configured to drive the first transistor and the second transistor to increase the drive voltage for a predetermined time period shorter than a time period during which the pulse signal is at the other logic level regardless of the control by the operational amplifier, when the pulse signal changes from the one logic level to the other logic level. 
     Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For more thorough understanding of the present invention and advantages thereof, the following description should be read in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a configuration of a motor drive circuit that is an embodiment of the present invention; 
         FIG. 2  illustrates a configuration example of an auxiliary drive circuit; 
         FIG. 3  illustrates an example of an operation of an auxiliary drive circuit; 
         FIG. 4  illustrates an example of an operation of a motor drive circuit; 
         FIG. 5  illustrates an example of a change of a drive voltage according to a PWM signal and an auxiliary pulse; 
         FIG. 6  illustrates an example of a configuration of a motor drive circuit that drives a single phase fan motor; and 
         FIG. 7  illustrates an example of a change of a drive voltage according to a PWM signal. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At least the following details will become apparent from descriptions of this specification and of the accompanying drawings. 
       FIG. 1  illustrates a configuration of a motor drive circuit that is an embodiment of the present invention. A motor drive circuit  10  is included in a fan motor for cooling a heat-producing component (a device to be cooled) such as a processor in electronic equipment such as a notebook computer, and is used for driving the motor for rotating a fan for cooling, for example. 
     The motor drive circuit  10  according to an embodiment of the present invention is a circuit that drives a single phase fan motor and includes operational amplifiers  11 A and  11 B, P-channel MOSFETs  12 A to  14 A and  12 B to  14 B, N-channel MOSFETs  15 A and  15 B, resistors  17 A,  17 B,  18 A, and  18 B, inverting circuits  20 A,  20 B,  21 A, and  21 B, a PWM signal output circuit  25 , and an auxiliary drive circuit  26 . According to an embodiment of the present invention, the motor drive circuit  10  is integrated, a motor coil L is connected between terminals OUT 1  and OUT 2 , and a Hall element  30  that outputs a voltage V H1  (first control voltage) and a voltage V H2  (second control voltage) according to a rotational position of the motor is connected between terminals H 1  and H 2 . The voltages V H1  and V H2  are sinusoidally-varying voltages and are opposite in phase from each other. 
     The operational amplifier  11 B, the P-channel MOSFETs  12 B to  14 B, the N-channel MOSFET  15 B, the resistors  17 B and  18 B, and the inverting circuits  20 B and  21 B provided on an output terminal OUT 2  side have a configuration similar to that on an output terminal OUT 1  side, except that an input relationship of the voltages V H1  and V H2  output by the Hall element  30  to the operational amplifier  11 B is opposite to that thereof to the operational amplifier  11 A. 
     The voltage V H1  is applied to a + input terminal of the operational amplifier  11 A, and the voltage V H2  is applied to a − input terminal of the operational amplifier  11 A via the resistor  17 A (first resistor.) The operational amplifier  11 A includes a feedback circuit where the terminal OUT 1  and the − input terminal are connected via the resistor  18 A (second resistor.) In other words, the operational amplifier  11 A performs a feedback control so that a drive voltage V OUT1  applied to the terminal OUT 1  becomes a voltage obtained by amplifying the difference between the voltages V H1  and V H2  by a gain according to a resistance ratio of the resistors  17 A and  18 A. A drive voltage V OUT2  applied to the terminal OUT 2  and controlled by the operational amplifier  11 B has a phase opposite to that of the drive voltage V OUT1 . 
     The P-channel MOSFET  12 A (first transistor) and the N-channel MOSFET  15 A (second transistor) are connected in series between a source voltage Vdd and a ground voltage, and a connection point is connected to the terminal OUT 1 . An output voltage of the operational amplifier  11 A is applied to gates of the P-channel MOSFET  12 A and the N-channel MOSFET  15 A via the inverting circuits  20 A and  21 A. Each of the inverting circuits  20 A and  21 A is a circuit for inverting the output voltage of the operational amplifier  11 A with respect to a midpoint voltage (Vdd/2, for example), to be output. Accordingly, in a case of the voltage V H1 &gt;the voltage V H2 , the output voltage of the operational amplifier  11 A is increased, a current in the P-channel MOSFET  12 A is increased while a current in the N-channel MOSFET  15 A is decreased, and the drive voltage V OUT1  is increased. On the other hand, in a case of the voltage V H1 &lt;the voltage V H2 , the output voltage of the operational amplifier  11 A is decreased, the current in the P-channel MOSFET  12 A is decreased while the current in the N-channel MOSFET  15 A is increased, and the drive voltage V OUT1  is decreased. Under such a control, the drive voltage V OUT1  is a voltage according to the difference between the voltages V H1  and V H2 . 
     The PWM signal output circuit  25  outputs a PWM signal (pulse signal) for intermittently driving the motor coil L. According to an embodiment of the present invention, when the PWM signal is at L level, the P-channel MOSFETs  13 A,  13 B,  14 A, and  14 B (switch circuit) are ON. When the P-channel MOSFETs  13 A,  13 B,  14 A, and  14 B are ON, regardless of the output voltage of the operational amplifiers  11 A and  11 B, the P-channel MOSFETs  12 A and  12 B are OFF, the N-channel MOSFETs  15 A and  15 B are ON, and both of the drive voltages V OUT1  and V OUT2  are at L level, so that, the motor coil L is in a state of not being driven. On the other hand, when the PWM signal is at H level, the P-channel MOSFETs  13 A,  13 B,  14 A and  14 B are OFF, and the drive voltages V OUT1  and V OUT2  are controlled by the operational amplifiers  11 A and  11 B, so that the motor coil L is driven by the difference between the voltages V OUT1  and V OUT2 . In other words, the rotational speed of the motor can be increased by increasing the duty ratio of the H level of the PWM signal, and the rotational speed of the motor can be decrease by decreasing the duty ratio. 
     The auxiliary drive circuit  26  outputs an auxiliary pulse for increasing responsiveness of the drive voltages V OUT1  and V OUT2  when the PWM signal changes from the L level to the H level and control of the voltages V OUT1  and V OUT2  by the operational amplifiers  11 A and  11 B is resumed. In other words, when the PWM signal changes to the H level, such control by the operational amplifiers  11 A and  11 B that the drive voltages V OUT1  and V OUT2  change from voltages of the L level to those of a level according to the difference between the voltages V H1  and V H2 , is resumed. At this time, an auxiliary pulse is used for reducing an amount of time which it takes the voltages V OUT1  and V OUT2  to reach the target level. The auxiliary pulse according to an embodiment of the present invention is a signal that is at H level for a predetermined time period from a time when the PWM signal changes from the L level to the H level, and whose pulse width is shorter than that of the PWM signal. Accordingly, while the auxiliary pulse is at H level, the P-channel MOSFET  12 A and the N-channel MOSFET  15 A are controlled such that the current in the P-channel MOSFET  12 A is increased and the current in the N-channel MOSFET  15 A is decreased, and thus, the drive voltage V OUT1  is increased more quickly than the drive voltage V OUT1  is increased when being controlled only by the operational amplifier  11 A. A similar description may be made for the drive voltage V OUT2 . 
       FIG. 2  illustrates a configuration example of the auxiliary drive circuit  26 . The auxiliary drive circuit  26  includes P-channel MOSFETs  40  and  41 , N-channel MOSFETs  42  and  43 , a resistor  46 , a capacitor  48 , a NOT circuit  50 , and an AND circuit  51 . The P-channel MOSFET  40  and the N-channel MOSFET  42  make up an inverter. The P-channel MOSFET  41  and the N-channel MOSFET  43  also make up an inverter. 
     As illustrated in  FIG. 3 , when the PWM signal changes from the L level to the H level, an output of the inverter made up of the P-channel MOSFET  40  and the N-channel MOSFET  42  changes to the L level, and therefore a voltage of an A point is decreased with a time constant according to a resistance value of the resistor  46  and capacitance of the capacitor  48 . When the voltage of the A point reaches a threshold voltage of the inverter made up of the P-channel MOSFET  41  and the N-channel MOSFET  43 , a voltage of a B point changes to the H level. When the PWM signal changes from the H level to the L level, the voltage of the A point is increased according to the time constant, and when the voltage of the A point reaches the threshold voltage of the inverter made up of the P-channel MOSFET  41  and the N-channel MOSFET  43 , the voltage of the B point changes to the L level. In other words, the P-channel MOSFETs  40  and  41 , the N-channel MOSFETs  42  and  43 , the resistor  46 , and the capacitor  48  make up a delay circuit, and a signal (delayed pulse signal) which is obtained by delaying the PWM signal by a predetermined time period is output from the B point. The NOT circuit  50  and the AND circuit  51  make up an auxiliary pulse output circuit. A signal obtained by inverting the signal output from the B point in the NOT circuit  50  and the PWM signal are input to the AND circuit  51 , and thus, an auxiliary pulse is generated, which is at H level for the predetermined time period from the time when the PWM signal changes from the L level to the H level. 
       FIG. 4  illustrates an example of an operation of the motor drive circuit  10 . During a period of time in which the PWM signal is maintained at H level, the drive voltages V OUT1  and V OUT2  to be applied to both ends of the motor coil L are controlled by the operational amplifiers  11 A and  11 B to become a voltage according to the difference between the voltages V H1  and V H2  output by the Hall element  30 , so that the motor coil L is driven. During a period of time during which the PWM signal changes in a pulse-like form, when the PWM signal changes from the H level to the L level, the drive voltages V OUT1  and V OUT2  are changed to the L level regardless of the control by the operational amplifiers  11 A and  11 B, and thus, the motor coil L is in the state of not being driven. During the period of time during which the PWM signal changes in the pulse-like form, when the PWM signal changes from the L level to the H level, the drive voltages V OUT1  and V OUT2  is returned to the target level according to the difference between the voltages V H1  and V H2  under the control of the operational amplifiers  11 A and  11 B. In other words, the motor coil L is intermittently driven according to the duty ratio of the H level of the PWM signal. 
       FIG. 5  illustrates an example of a change of the drive voltage according to the PWM signal and the auxiliary pulse. As described above, when the PWM signal changes from the L level to the H level, the auxiliary pulse is output from the auxiliary drive circuit  26 . During a short period of time during which the auxiliary pulse is at H level, the P-channel MOSFET  12 A and the N-channel MOSFET  15 A are controlled such that the current in the P-channel MOSFET  12 A is increased while the current in the N-channel MOSFET  15 A is decreased, and thus, the drive voltage V OUT1  is increased more quickly than the drive voltage V OUT1  is increased when being controlled only by the operational amplifier  11 A. When the auxiliary pulse changes to the L level, the drive voltage V OUT1  is controlled by the operational amplifier  11 A to reach the target level from a level at which the drive voltage V OUT1  is increased by the auxiliary pulse. A similar description may be made for the drive voltage V OUT2 . Thus, in a case where the motor coil L is intermittently driven, when changing the motor coil L from the state of not being driven to a state of being driven, the amount of time which it takes the voltages V OUT1  and V OUT2  to reach the target level can be reduced due to the auxiliary pulse. 
     As described above, the motor drive circuit  10  according to an embodiment of the present invention is described. In the motor drive circuit  10 , when the PWM signal changes from the L level to the H level, the auxiliary pulse having a short pulse width is generated, to help the drive voltages V OUT1  and V OUT2  reach the target level. Accordingly, the drive voltages V OUT1  and V OUT2  can reach the target level more quickly than the drive voltages V OUT1  and V OUT2  can reach when being controlled only by the operational amplifiers  11 A and  11 B. Therefore, a pulse width of the PWM signal, i.e., a switching interval when intermittently driving the motor coil L, can be shortened; and the rotational speed of the motor can be linearly controlled even in a range of a low speed. Also, the power consumption is high during the period of time until when the drive voltages V OUT1  and V OUT2  reach the target level, and therefore, shortening this period enables reduction of the power consumption. 
     As illustrated in  FIG. 2 , the auxiliary pulse can be generated based on the PWM signal and a signal obtained by delaying the PWM signal by a predetermined time period. 
     By using such a motor drive circuit  10 , in the case where a small amount of heat is released by a heat-producing component such as a processor in electronic equipment such as a notebook computer, the rotational speed of the fan can sufficiently be reduced, and thus, the power consumption can be reduced. 
     The above embodiments of the present invention are simply for facilitating the understanding of the present invention and are not in any way to be construed as limiting the present invention. The present invention may variously be changed or altered without departing from its spirit and encompass equivalents thereof.