Abstract:
A voltage output circuit includes: an oscillator circuit configured to output an oscillation signal while changing an oscillation frequency thereof; and a voltage generating circuit configured to convert a first voltage into a second voltage higher than the first voltage, and output the second voltage, based on the oscillation signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority to Japanese Patent Application No. 2012-029868, filed Feb. 14, 2012, of which full contents are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a voltage output circuit, a load drive circuit, and a method for outputting a voltage. 
         [0004]    2. Description of the Related Art 
         [0005]    Today, while motors are incorporated in a variety of electric products, such electric products consume a comparatively large amount of power as well as cause noise. Thus, various techniques are developed (see, e.g., Japanese Laid-Open Patent Publication No. 2009-177894). 
         [0006]    A motor drive circuit is usually configured such that two transistors connected in series between a power supply and the ground are provided for a coil of each phase, and the coil is connected to a connection point of these two transistors. Then these transistors are on/off controlled in appropriate timing, thereby on/off controlling energization of each coil to rotate the motor. 
         [0007]    At this time, in order to turn on the transistor on the upper arm side (power supply side) out of the two transistors connected in series, a voltage between a control terminal (e.g., gate terminal) and the connection point is required to be set higher than the predetermined threshold voltage. When the transistor on the upper arm side is turned on, however, the potential at the connection point increases up to close to a power supply voltage, and thus a voltage higher than the power supply voltage is required to be applied to the control terminal. 
         [0008]    Therefore, it is necessary to generate a voltage higher than the power supply voltage using a voltage generating circuit such as a charge pump circuit, however, an oscillation signal such as a clock signal is required for the voltage generating circuit such as the charge pump circuit. Thus, the noise caused by this oscillation signal is generated. This noise is generated centering on a particular frequency according to the frequency of the oscillation signal, which can cause a significant effect on other surrounding electronic devices. This may occur similarly in the case where loads other than the motor are used. 
       SUMMARY OF THE INVENTION 
       [0009]    A voltage output circuit according to an aspect of the present invention includes: an oscillator circuit configured to output an oscillation signal while changing an oscillation frequency thereof; and a voltage generating circuit configured to convert a first voltage into a second voltage higher than the first voltage, and output the second voltage, based on the oscillation signal. 
         [0010]    Other features of the present invention will become apparent from descriptions of this specification and of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1  is a diagram illustrating a configuration example of a load drive circuit and a voltage output circuit according to an embodiment of the present invention; 
           [0013]      FIG. 2  is a diagram illustrating a configuration example of a voltage changing circuit according to an embodiment of the present invention; 
           [0014]      FIG. 3  is a diagram illustrating an operation of the voltage changing circuit according to an embodiment of the present invention; 
           [0015]      FIG. 4  is a diagram illustrating a configuration example of an oscillator circuit according to an embodiment of the present invention; and 
           [0016]      FIG. 5  is a diagram illustrating a configuration example of an inverter circuit according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    At least the following details will become apparent from descriptions of this specification and of the accompanying drawings. 
         [0018]    A description will be given of a configuration example of a load drive circuit  1000  and a voltage output circuit  2000  of an embodiment of the present invention with reference to  FIGS. 1 to 5 . 
       ==Load Drive Circuit and Voltage Output Circuit== 
       [0019]    As depicted in  FIG. 1 , the load drive circuit  1000  according to an embodiment of the present invention includes a charge pump circuit  200 , a transistor drive circuit  300 , an output circuit  400 , an FG circuit  600 , a voltage changing circuit  500 , and an oscillator circuit  100 . The voltage output circuit  2000  according to an embodiment of the present invention includes the charge pump circuit  200 , the voltage changing circuit  500 , and the oscillator circuit  100 . 
         [0020]    In the following description, a description will be given of the case, as an example, where the load drive circuit  1000  is a motor drive circuit configured to drive a three-phase motor  700 . Needless to say, the load drive circuit  1000  is also applicable to the case of driving loads other than the motor  700 . 
         [0021]    In an embodiment of the present invention, the load drive circuit  1000  and the voltage output circuit  2000  are configured such that some constituent elements of the charge pump circuit  200  are connected to predetermined terminals of a motor drive IC  800 , as depicted in  FIG. 1 . 
       &lt;Charge Pump Circuit&gt; 
       [0022]    The charge pump circuit  200  is a circuit configured to convert a constant power supply voltage VCC (first voltage) into a second voltage and output the second voltage, when an oscillation signal is inputted from the oscillator circuit  100 , which will be described later. 
         [0023]    The charge pump circuit  200  includes transistors SW 1  and SW 2 , the constant power supply voltage VCC, capacitors C 1  and C 2 , and diodes D 1  and D 2 . 
         [0024]    The transistors SW 1  and SW 2  are connected in series between a power supply voltage VS and the ground. Specifically, the source terminal of the transistor SW 1  is connected to the power supply voltage VS, the drain terminal of the transistor SW 1  and the drain terminal of the transistor SW 2  are connected at the connection point, and the source terminal of the transistor SW 2  is connected to the ground. The oscillation signal outputted from the oscillator circuit  100  is inputted to the gate terminal of each of the transistors SW 1  and SW 2 . 
         [0025]    One end of the capacitor C 1  is connected to the connection point between the transistors SW 1  and SW 2  via a “CL” terminal of the motor drive IC  800 . The other end of the capacitor C 1  is connected to the cathode of the diode D 1 . The anode of this diode D 1  is connected to the constant power supply voltage VCC. 
         [0026]    Whereas, the cathode of the diode D 1  is connected to the anode of the diode D 2  as well. 
         [0027]    The capacitor C 2  has one end thereof connected to the source terminal of the transistor SW 1  and the power supply voltage VS via a “C−” terminal of the motor drive IC  800 , and has the other end thereof connected to the cathode of the diode D 2 . The other end of the capacitor C 2  is connected to the transistor drive circuit  300  via a “C+” terminal of the motor drive IC  800 . 
         [0028]    In the configuration in which the charge pump circuit  200  is connected as such, when the oscillation signal from the oscillator circuit  100  is inputted to the gate terminal of each of the transistors SW 1  and SW 2 , the transistors SW 1  and SW 2  repeat on and off alternately in synchronization with the oscillation signal. 
         [0029]    Firstly, when the transistor SW 1  is turned off and the transistor SW 2  is turned on, the voltage at the connection point between the transistors SW 1  and SW 2 , i.e., the “CL” terminal of the motor drive IC  800  reaches 0 volt. On the other hand, the capacitor C 1  is charged by a current flowing from the constant power supply voltage VCC via the diode D 1 . 
         [0030]    In this state, when the transistor SW 1  is changed from off to on and the transistor SW 2  is changed from on to off, the voltage at the one end of the capacitor C 1 , i.e., the voltage at the “CL” terminal of the motor drive IC  800 , increases to a voltage almost equal to the power supply voltage VS. Thus, the voltage at the other end of the capacitor C 1  also increases, and the current flows to the capacitor C 2  via the diode D 2 . The capacitor C 2  is charged by this current. 
         [0031]    With the transistors SW 1  and SW 2  repeating on and off alternately in synchronization with the oscillation signal from the oscillator circuit  100 , the voltage of the capacitor C 2  increases to a voltage (second voltage) higher than the power supply voltage VS. 
         [0032]    The voltage of the capacitor C 2  is applied, as an output voltage of the charge pump circuit  200 , to the transistor drive circuit  300  via the “C+” terminal of the motor drive IC  800 . 
       &lt;Transistor Drive Circuit&gt; 
       [0033]    The transistor drive circuit  300  is a circuit configured to apply the output voltage, generated by the charge pump circuit  200 , to a control terminal (gate terminal, base terminal, etc.) of transistors TRs included in the output circuit  400 , thereby turning on the transistors TRs. 
         [0034]    The transistor drive circuit  300  includes an upper arm drive circuit  310  and a lower arm drive circuit  320 . The upper arm drive circuit  310  is a circuit configured to apply the output voltage, generated by the charge pump circuit  200 , to the control terminal of the transistor TR connected on the power supply voltage VS side out of the transistors TRs included in the output circuit  400 . The lower arm drive circuit  320  is a circuit configured to apply the output voltage generated by the charge pump circuit  200  to the control terminal of the transistor TR connected on the ground side out of the transistors TRs included in the output circuit  400 . 
       &lt;Output Circuit&gt; 
       [0035]    The output circuit  400  includes the transistors TRs (electronic devices) for performing on/off control of the motor  700 . The output circuit  400  according to an embodiment of the present invention is configured such that two transistors TRs are connected in series between the power supply voltage VS and the ground. A coil  710  of the motor  700  is connected to the connection point between these two transistors TRs. 
         [0036]    Since the motor  700  according to an embodiment of the present invention is a three-phase motor, a pair of two transistors TRs connected in series is provided for each phase, resulting in that three pairs thereof are provided in total. Although  FIG. 1  depicts only the pair of two transistors TR 1  and TR 2  provided with respect to U phase, for the sake of simplification, a configuration is made similarly with respect to other phases (V phase and W phase) as well. 
         [0037]    These transistors TRs are on/off-controlled in accordance with control voltages applied to the control terminals of transistors TR, respectively, by the transistor drive circuit  300 , thereby on/off-controlling the energization of each of the coils in the motor  700 . 
         [0038]    The transistor TR included in the output circuit  400  may be realized by IGBT (Insulated Gate Bipolar Transistor) or may be realized by MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), for example. 
         [0039]    When describing the case of MOSFET, a configuration may be such that the transistor TR included in the output circuit  400  is closed between the drain and the source when a voltage greater than a gate-source threshold voltage is applied to the gate terminal thereof or the transistor TR is opened between the drain and the source when the voltage greater than the gate-source threshold voltage is applied to the gate terminal thereof. 
       &lt;Motor&gt; 
       [0040]    The motor  700  includes the coils (load)  710  of the phases and Hall ICs  720  configured to detect the rotation of the motor  700 . 
         [0041]    Each of the Hall ICs  720  detects a change in magnetism when the motor  700  rotates, and outputs a pulse signal. Thus, the Hall IC  720  outputs a pulse signal having a frequency corresponding to the rotational speed of the motor  700 . The motor  700  according to an embodiment of the present invention is of four pole-pairs (eight poles), for example. Thus, the Hall IC  720  outputs four pulses per rotation of motor. 
         [0042]    In an embodiment of the present invention, the motor  700  includes three Hall ICs  720 , and these three Hall ICs  720  output their respective pulse signals during one rotation of the motor  700 . These pulse signals are inputted to an “HU” terminal, an “HV” terminal, and an “HW” terminal of the motor drive IC  800 . 
       &lt;FG Circuit&gt; 
       [0043]    The FG circuit  600  is a circuit configured to receive the pulse signals from the Hall ICs  720 , generates an FG signal indicative of the rotational state of the motor, using these pulse signals, and output this FG signal as a clock signal to the voltage changing circuit  500 , which will be described later. 
         [0044]    The position of a rotor is detected using a position detecting element such as the Hall IC  720  provided in the motor  700 , for example, thereby being able to generate the FG signal. 
         [0045]    In the motor drive circuit of a senseless drive system without the position detecting element, the position of the rotor is detected using the induced voltage (reverse voltage) generated in the drive coil  710  of the motor  700 , thereby being able to generate the FG signal. 
       &lt;Voltage Fluctuating Circuit&gt; 
       [0046]    The voltage changing circuit  500  is a circuit configured to oscillate an applied voltage VCNT applied to the oscillator circuit  100 , which will be described later. Although the details will be described later, the voltage changing circuit  500  is configured to change the applied voltage VCNT within the range in which the level of the output voltage outputted from the charge pump circuit  200  reaches the level capable of turning on the transistor TR of the output circuit  400 . 
         [0047]    The voltage changing circuit  500  includes a counter circuit  510 , switches  530 , current sources  520 , and a capacitor  540  with one end grounded, as depicted in  FIG. 2 . The voltage between the one end and the other end of the capacitor  540  is outputted as the applied voltage VCNT. 
         [0048]    The counter circuit  510  is configured such that four flip-flops  511  are connected in series. The counter circuit  510  is configured to update the logic level (0 or 1) of output signals (A, B, C, and D) of the flip-flops  511  and outputs the output signals, in accordance with the clock signal outputted from the FG circuit  600 . 
         [0049]    The voltage changing circuit  500  includes four switches  530 . These four switches  530  are associated with the output signals (A, B, C, and D) of the four flip-flops  511 , respectively, and are opened/closed in accordance with the logic levels of these output signals, respectively. Specifically, for example, when the logic level of the output signal of the flip-flop  511  is “1”, the switch  530  is closed, and when the logic level thereof is “0”, the switch  530  is opened. 
         [0050]    In the following description, when a description is given individually of the four switches  530 , the switches  530  corresponding to the output signals (A, B, C, and D) of the flip-flops  511  are given as SWA, SWB, SWC, and SWD, respectively. 
         [0051]    The current sources  520  are connected in series to these switches  530 , respectively. Some of the current sources  520  are connected to the capacitor  540  on the other end side thereof in the direction in which the capacitor  540  is charged, and others of the current sources  520  are connected to the capacitor  540  on the other end side thereof in the direction in which the capacitor  540  is discharged. 
         [0052]    In an embodiment of the present invention, the current source  520  connected in series to the SWA  530  and the current source  520  connected in series to the SWD  530  are connected to the capacitor  540  in the direction in which the capacitor  540  is charged, and the current source  520  connected in series to the SWB  530  and the current source  520  connected in series to the SWC  530  are connected to the capacitor  540  in the direction in which the capacitor  540  is discharged. 
         [0053]    Thus, when at least one of the SWA  530  and the SWD  530  is closed, the current flows so that the capacitor  540  is charged. Whereas, when at least one of the SWB  530  and the SWC  530  is closed, the current flows so that the capacitor  540  is discharged. By the combinations of opening and closing of the SWA  530 , the SWB  530 , the SWC  530 , and the SWD  530 , the capacitor  540  is charged or discharged integrally, thereby increasing or decreasing the voltage VCNT of the capacitor  540  on the other end side. 
         [0054]      FIG. 3  depicts how the output signal of each of the flip-flops  511  of the counter circuit  510  is updated as well as the voltage VCNT of the capacitor  540  is changed according to the logic level of the output signal of each of the flip-flops  511 , in synchronization with the clock signal inputted from the FG circuit  600 . 
         [0055]    In an embodiment of the present invention, there are 16 combinations of the logic levels of the output signals of the four flip-flops  511 , and the number of combinations of charging to the capacitor  540  are equal to the number of combinations of discharging from the capacitor  540 , and thus the voltage VCNT of the capacitor  540  repeats a periodic change with the voltage of the predetermined level, set in advance, used as the reference (center), every time 12 pulses of the clock signal are inputted. That is, the voltage VCNT of the capacitor  540  returns to the original voltage every time 12 pulses of the clock signals are inputted. 
         [0056]    Therefore, if this reference voltage and the fluctuation range of the voltage are set in advance so that the output voltage outputted from the charge pump circuit  200  has a level within the range capable of turning on the transistor TR of the output circuit  400 , which enable the output voltage from the charge pump circuit  200  not to deviate from the level capable of turning on the transistor TR of the output circuit  400 . 
         [0057]    The fluctuation range of the voltage VCNT of the capacitor  540  can be set at a desired value by setting the capacitance of the capacitor  540 , the current values of the current sources  520 , the time period during which the switch  530  is on, the number (the number of stages) of the flip-flops  511  of the counter circuit  510 , etc. 
         [0058]    Since the FG signal is outputted as the clock signal from the FG circuit  600 , the motor  700  makes one rotation every time 12 pulses (4×3) of the clock signal are inputted. 
         [0059]    Thus, the fluctuation cycle of the voltage VCNT with respect to one rotation of the motor  700  can be changed by changing the frequency of the clock signal. 
       &lt;Oscillator Circuit&gt; 
       [0060]    The oscillator circuit  100  is a circuit configured to output the oscillation signal of the switching speed corresponding to the level of the applied voltage VCNT that is outputted from the voltage changing circuit  500 . 
         [0061]    One example of the oscillator circuit  100  is depicted in  FIG. 4 . The oscillator circuit  100  is configured as a ring oscillator includes an odd number of inverter circuits  111  that are ring-connected. 
         [0062]    Each of the inverter circuits  111  is configured as depicted in  FIG. 5 , for example. In  FIG. 5 , the inverter circuit  111  is configured to output an output voltage Vout obtained by inverting the logic level of an input voltage Vin. When the input voltage Vin is inverted, the output voltage Vout is inverted and outputted after a predetermined delay. 
         [0063]    Each of the inverter circuits  111  is configured with a first NMOSFET  111 A, a second NMOSFET  111 B, a first PMOSFET  111 C, and a second PMOSFET  111 D. The gates of the first NMOSFET  111 A and the first PMOSFET  111 C are connected to each other and the drains of the first NMOSFET  111 A and the first PMOSFET  111 C are connected to each other. That is, the first NMOSFET  111 A and the first PMOSFET  111 C are connected so as to act as an inverter that inverts the input voltage Vin and outputs the output voltage Vout. The second NMOSFET  111 B is connected in series between a power supply Vdd and the first NMOSFET  111 A, and the second PMOSFET  111 D is connected in series between the first PMOSFET  111 C and the ground. A voltage VICNTp that changes with increase/decrease in the applied voltage VCNT is applied to the gate of the second NMOSFET  111 B. A voltage VICNTn that changes with increase/decrease in the applied voltage VCNT in the direction opposite to the change in the voltage VICNTp is applied to the gate of the second PMOSFET  111 D. 
         [0064]    In  FIG. 5 , when the voltage VICNTp is decreased and the voltage VICNTn is increased, the charging/discharging speed with respect to the parasitic capacitance, etc., in the inverter circuit  111  is increased, resulting in decrease in the delay time of the inverter circuit  111 . That is, the response time, which is a time period from inverting of the input voltage Vin until inverting of the output voltage Vout, is decreased in level. 
         [0065]    Whereas, when the voltage VICNTp is increased and the voltage VICNTn is decreased, the charging/discharging speed with respect to the parasitic capacitance, etc., in the inverter circuit  111  is decreased, resulting in increase in the delay time of the inverter circuit  111 . That is, the response time, which is a time period from inverting of the input voltage Vin until inverting of the output voltage Vout is increased in level. 
         [0066]    Firstly, when the applied voltage VCNT applied to the oscillator circuit  100  increases above Vr, a current IR 1  flows through a resistor R 1  in the direction indicated by an arrow in  FIG. 4  in accordance with increase in the applied voltage VCNT, which results in increase in current IINV. Then, the current IINV is increased, thereby decreasing the voltage VICNTp and increasing the voltage VICNTn. Thus, in this case, since the response time of the inverter circuit  111  is decreased, the frequency of the oscillation signal is increased. 
         [0067]    Whereas, when the applied voltage VCNT applied to the oscillator circuit  100  decreases below Vr, the current IR 1  flows through the resistor R 1  in the direction opposite to that indicated by the arrow in  FIG. 4  in accordance with decrease in the applied voltage VCNT, which results in decrease in current IINV. Then, the current IINV is decreased, thereby increasing the voltage VICNTp and decreasing the voltage VICNTn. Thus, in this case, since the response time of the inverter circuit  111  is increased, the frequency of the oscillation signal is decreased. 
         [0068]    As such, the switching speed of the oscillation signal outputted from the oscillator circuit  100  changes with the level of the applied voltage VCNT outputted from the voltage changing circuit  500 . 
         [0069]    This oscillation signal is inputted to the charge pump circuit  200  described above, and the transistors SW 1  and SW 2  configuring the charge pump circuit  200  repeat on and off in an alternate manner in synchronization with this oscillation signal. 
         [0070]    As such, according to the load drive circuit  1000  according to an embodiment of the present invention, it is possible to change the switching speed of the oscillation signal inputted to the charge pump circuit  200 . 
         [0071]    Thus, the frequency of the noise caused by this oscillation signal can be diffused, thereby being able to reduce the peak of the noise generated from the charge pump circuit  200 . 
         [0072]    This makes it possible, for example, to reduce the effect caused by the noise on other electronic devices. For example, when using the load drive circuit  1000  and the voltage output circuit  2000  according to an embodiment of the present invention in vehicle-mounted electronic devices such as a car air conditioner, it becomes possible to reduce the effect on other vehicle-mounted electronic devices such as a car radio and a transceiver. Similarly, when using the load drive circuit  1000  and the voltage output circuit  2000  according to an embodiment of the present invention in an air conditioner, an air cleaner, a water heater, etc., it becomes possible to reduce the effect on other surrounding electronic devices, irrespective of whether they are in vehicle-mounted use or not. 
         [0073]    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. 
         [0074]    For example, in an embodiment described above, a description has been given of an example of the case where the charge pump circuit  200  is used, but an embodiment of the present invention can be applied similarly in the case where a booster circuit using the coil and a bootstrap circuit, etc., are used. 
         [0075]    Further, a description has been given citing the example of the case where a ring oscillator is used as the oscillator circuit  100 , but the oscillator circuit can also be configured with a pseudo random number oscillator circuit capable of performing a pseudo random-number oscillation with respect to the switching frequency within a certain range, for example.