Abstract:
A load driving device according to the present invention has: an internal circuit (DRV) that operates in response to the supply of a power supply voltage (HV or LV); an output circuit (PD 1  and PD 2 ) for driving a load in response to the supply of the power supply voltage (HV or LV); an abnormality detection circuit ( 12 ) for monitoring the power supply voltage (HV or LV) and generating an abnormality detection signal (S 1 ); and a power supply switch (P 0 ) for, according to the abnormality detection signal (S 1 ), conducting or cutting off a power-supply-voltage supply line to the internal circuit (DRV).

Description:
TECHNICAL FIELD 
     The present invention relates to a load driving device, and to an electronic device incorporating a load driving device. 
     BACKGROUND ART 
     Conventionally, load driving devices such as motor driver ICs and switching regulator ICs are in wide and common use. 
     An example of related conventional technology is seen in Patent Document 1 listed below. 
     LIST OF CITATIONS 
     Patent Literature 
     Patent Document 1: Japanese Patent Application Publication No. 2008-263733 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In general, a component (device) that constitutes an output circuit of a load driving device (in particular, an output device such as a power transistor) is designed to have a withstand voltage that suits the electrical characteristics (such as the absolute maximum rated value of a supply voltage) defined in the specification of the load driving device. Thus, when overvoltage destruction testing exceeding the rating is conducted, the load driving device may suffer smoking or destruction. 
     The simplest imaginable solution to the above problem is to raise the withstand voltage of the component. However, raising the withstand voltage (in particular, the gate-source withstand voltage) of the output device among all the components constituting the circuit requires increasing the size of the output device and hence increasing the chip area, posing another problem. In particular, in a load driving device designed to feed a large current to a load, a large output device is used to reduce its on-state resistance, and thus the output device occupies a very large proportion of the entire chip area. Thus, further increasing the size of the output device simply to cope with destructive testing leads to an even larger chip area and hence a higher cost, making the solution impractical. 
     Another imaginable solution to the above problem is to provide an overvoltage protection capability on the part of the target product in which the load driving device is incorporated. However, providing the target product with an overvoltage protection capability requires externally fitting additional components to the load driving device, and thus leads to a higher cost of the product as a whole, thereby posing another problem. 
     In view of the problem experienced by the present inventors, an object of the present invention is to provide a load driving device that withstands destructive testing without unnecessarily increasing the withstand voltage of a component, and to provide an electronic device incorporating such a load driving device. 
     Means to Solve the Problem 
     To achieve the above object, according to one aspect of the present invention, a load driving device includes: an internal circuit which operates by being fed with a supply voltage; an output circuit which drives a load by being fed with the supply voltage; a fault detection circuit which monitors the supply voltage to generate a fault detection signal; and a power switch which connects/disconnects a supply voltage feed line leading to the internal circuit according to the fault detection signal (a first configuration). 
     In the load driving device according to the first configuration described above, preferably, the internal circuit includes a driving circuit which feeds the output circuit with a driving signal (a second configuration). 
     In the load driving device according to the second configuration described above, preferably, there is further provided a pull-down resistor which is connected between a supply voltage input node of the internal circuit and a grounded node (a third configuration). 
     In the load driving device according to the third configuration described above, preferably, the output circuit includes a p-type upper transistor which is connected between a supply voltage node and an output node (a fourth configuration). 
     In the load driving device according to the fourth configuration described above, preferably, there is further provided a first upper switch which connects/disconnects between the gate of the upper transistor and the supply voltage node according to the fault detection signal (a fifth configuration). 
     In the load driving device according to the fifth configuration described above, preferably, there is further provided a second upper switch which connects/disconnects between the gate of the upper transistor and the driving circuit according to the fault detection signal (a sixth configuration). 
     In the load driving device according to any one of the fourth to sixth configurations described above, preferably, the output circuit includes an n-type lower transistor which is connected between the grounded node and the output node (a seventh configuration). 
     In the load driving device according to the seventh configuration described above, preferably, there is further provided a first lower switch which connects/disconnects between the gate of the lower transistor and the grounded node according to the fault detection signal (an eighth configuration). 
     In the load driving device according to the eighth configuration described above, preferably, there is further provided a second lower switch which connects/disconnects between the gate of the lower transistor and the driving circuit according to the fault detection signal (a ninth configuration). 
     In the load driving device according to any one of the first to ninth configurations described above, preferably, the internal circuit includes a first internal circuit which operates by receiving a first supply voltage and a second internal circuit which operates by receiving a second supply voltage lower than the first supply voltage, and the output circuit includes a first output circuit which drives a first load by receiving the first supply voltage and a second output circuit which drives a second load by receiving the second supply voltage (a tenth configuration). 
     In the load driving device according to the tenth configuration described above, preferably, the fault detection circuit includes a first overvoltage detector which monitors the first supply voltage to generate a first overvoltage detection signal, a second overvoltage detector which monitors the second supply voltage to generate a second overvoltage detection signal, and a fault detection signal generator which generates the fault detection signal based on the first and second overvoltage detection signals (an eleventh configuration). 
     In the load driving device according to the eleventh configuration described above, preferably, the first overvoltage detector includes a first comparator which generates the first overvoltage detection signal by comparing the first supply voltage with a first overvoltage detection voltage, and the second overvoltage detector includes a second comparator which generates the second overvoltage detection signal by comparing the second supply voltage with a second overvoltage detection voltage (a twelfth configuration). 
     In the load driving device according to the eleventh or twelfth configuration described above, preferably, there is further provided a first level shifter which shifts the signal level of, and then feeds to the fault detection signal generator, the first overvoltage detection signal (a thirteenth configuration). 
     In the load driving device according to the thirteenth configuration described above, preferably, the first level shifter includes a transistor of which the drain is connected to a node to which the first supply voltage is applied and the gate is connected to a node to which the first overvoltage detection signal is applied; a current source which is connected between the source of the transistor and the grounded node; and an inverter of which the input node is connected to the source of the transistor, the output node is connected to the fault detection signal generator, the first supply voltage node is connected to a node to which the second supply voltage is applied, and the second supply voltage node is connected to the grounded node (a fourteenth configuration). 
     In the load driving device according to any one of the eleventh to fourteenth configurations described above, preferably, the fault detection circuit further includes an undervoltage detector which monitors the first supply voltage to generate an undervoltage detection signal, and the fault detection signal generator generates the fault detection signal based on the first overvoltage detection signal, the second overvoltage detection signal, and the undervoltage detection signal (a fifteenth configuration). 
     In the load driving device according to the fifteenth configuration described above, preferably, the undervoltage detector generates the undervoltage detection signal by comparing the first supply voltage with an undervoltage detection voltage (a sixteenth configuration). 
     In the load driving device according to any one of the first to sixteenth configuration described above, preferably, there is further provided a second level shifter which shifts the signal level of the fault detection signal (a seventeenth configuration). 
     In the load driving device according to the seventeenth configuration described above, preferably, the second level shifter shifts the signal level of the fault detection signal from a state where the fault detection signal pulsates between the second supply voltage and a ground voltage to a state where the fault detection signal pulsates between the first supply voltage and a first compensated supply voltage which is lower than the first supply voltage by a predetermined value or to a state where the fault detection signal pulsates between the second supply voltage and a second compensated supply voltage which is lower than the second supply voltage by a predetermined value (an eighteenth configuration). 
     In the load driving device according to the eighteenth configuration described above, preferably, there is further provided a compensated supply voltage generator which generates from the first or second supply voltage, and feeds to the second level shifter, the first or second compensated supply voltage (a nineteenth configuration). 
     In the load driving device according to the nineteenth configuration described above, preferably, the compensated supply voltage generator includes: a first transistor of which the source is connected to a node to which the first or second compensated supply voltage is applied and the drain is connected to the grounded node; a second transistor of which the emitter is commented to the gate of the first transistor; a zener diode of which the anode is connected to the base and the collector of the second transistor and the cathode is connected to the node to which the first or second supply voltage is applied; a first resistor which is connected between the source of the first transistor and the node to which the first or second supply voltage is applied; and a second resistor which is connected between the emitter of the second transistor and the grounded node (a twelfth configuration). 
     According to another aspect of the present invention, an electronic device includes the load driving device according to any one of the first to twelfth configurations described above and a load driven by the load driving device (a twenty-first configuration). 
     Preferably, the electronic device according to the twenty-first configuration described above is an optical disc drive which is incorporated in a computer for playback from, or for recording to and playback from, an optical disc, and the load is at least one of a spindle motor, a sled motor, a loading motor, a focus actuator, a tracking actuator, and a tilt actuator (a twenty-second configuration). 
     Advantageous Effects of the Invention 
     According to the present invention, it is possible to provide a load driving device that withstands destructive testing without unnecessarily increasing the withstand voltage of a component, and to provide an electronic device incorporating such a load driving device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing an exemplary configuration of an optical disc drive according to the present invention; 
         FIG. 2  is a circuit diagram of an exemplary configuration of a load driving circuit  11  and a fault detection circuit  12 ; 
         FIG. 3  is a circuit diagram showing a modified example of the load driving circuit  11 ; 
         FIG. 4  is a circuit diagram showing an exemplary configuration of a switching regulator IC; 
         FIG. 5  is a circuit diagram showing an exemplary configuration of a first overvoltage detector  121 ; 
         FIG. 6  is a diagram showing an exemplary configuration of an undervoltage detector  122 ; 
         FIG. 7  is a circuit diagram showing an exemplary configuration of a second overvoltage detector  123 ; 
         FIG. 8  is a circuit diagram showing an exemplary configuration of a level shifter  124   a;    
         FIG. 9  is a circuit diagram showing an exemplary configuration of a compensated supply voltage generator CVG; 
         FIG. 10  is a timing chart showing an example of fault protection operation; and 
         FIG. 11  is an external view of a desktop PC incorporating an optical disc drive. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Optical Disc Drive&gt; 
     Hereinafter, a detailed description will be given of an example where the present invention is applied to a motor driver IC incorporated in an optical disc drive. 
       FIG. 1  is a block diagram showing an exemplary configuration of an optical disc drive. The optical disc drive  1  is, for example, incorporated in a personal computer (PC) to allow playback from, or recording to and playback from, optical discs (such as BDs, DVDs, and CDs). The optical disc drive  1  includes a motor driver IC  10 , a plurality of loads  20 , and a microprocessor  30 . 
     The motor driver IC  10  is a multiple-channel load driving device which drives and controls, according to instructions from the microprocessor  30 , a plurality of loads (a spindle motor  21 , a sled motor  22 , a loading motor  23 , a focus actuator  24 , a tracking actuator  25 , and a tilt actuator  26 ). The motor driver IC  10  includes, as a multiple-channel load driving circuit  11 , a spindle motor driver circuit  111 , a sled motor driver circuit  112 , a loading motor driver circuit  113 , a focus actuator driver circuit  114 , a tracking actuator driver circuit  115 , and a tilt actuator driver circuit  116 . The motor driver IC  10  further includes a fault detection circuit  12  which monitors a first supply voltage HV (for a 12 V system) and a second supply voltage LV (for a 5 V system), both fed from outside the IC, to generate a fault detection signal S 1 . 
     The spindle motor driver circuit  111  is fed with the first supply voltage HV, and drives and controls the spindle motor  21  so as to rotate a turntable (not illustrated), on which an optical disc is placed, at a constant linear velocity or at a constant rotational velocity. Usable as the spindle motor  21  is, for example, a brushed DC motor or a three-phase brushless motor. 
     The sled motor driver circuit  112  is fed with the first supply voltage HV, and drives and controls the sled motor  22  so as to slide an optical pickup (not illustrated) in the radial direction of the optical disc. Usable as the sled motor  22  is, for example, a brushed DC motor or a two-phase brushless stepping motor. 
     The loading motor driver circuit  113  is fed with the first supply voltage HV, and drives and controls the loading motor  23  so as to slide a loading tray (not illustrated), on which an optical disc is placed. Usable as the loading motor  23  is, for example, a brushed DC motor. 
     The focus actuator driver circuit  114  is fed with the second supply voltage LV, and drives and controls the focus actuator  24 , thereby to drive an objective lens of the optical pickup so as to control the focus of the beam spot formed on the optical disc. 
     The tracking actuator driver circuit  115  is fed with the second supply voltage LV, and drives and controls the tracking actuator  25 , thereby to drive the objective lens of the optical pickup so as to control the tracking of the beam spot formed on the optical disc. 
     The tilt actuator driver circuit  116  is fed with the second supply voltage LV, and drives and controls the tilt actuator  26 , thereby to drive the objective lens of the optical pickup so as to compensate for fluctuations in signal strength ascribable to deformation of the optical disc. 
       FIG. 2  is a circuit diagram of an exemplary configuration of the load driving circuit  11  and the fault detection circuit  12 . As to the load driving circuit  11  shown there, it should be understood that, for simplicity&#39;s sake, only the circuitry of and around the output stage for one phase is illustrated with respect to one of the spindle motor driver circuit  111 , the sled motor driver circuit  112 , the loading motor driver circuit  113 , the focus actuator driver circuit  114 , the tracking actuator driver circuit  115 , and the tilt actuator driver circuit  116 . 
     The fault detection circuit  12  includes a first overvoltage detector  121 , an undervoltage detector  122 , a second overvoltage detector  123 , and a fault detection signal generator  124 . 
     The first overvoltage detector  121  monitors whether or not the first supply voltage HV is higher than an overvoltage detection voltage Vth 1  (for example, Vth 1 =18 V) to generate a first overvoltage detection signal SA. The first overvoltage detection signal SA is, when the first supply voltage HV is lower than the overvoltage detection voltage Vth 1 , at a normal-mode logical level (low level (GND)) and, when the first supply voltage HV is higher than the overvoltage detection voltage Vth 1 , at an abnormal-mode logical level (high level (HV)). 
       FIG. 5  is a circuit diagram showing an exemplary configuration of the first overvoltage detector  121 . The first overvoltage detector  121  includes a comparator  121   a  which compares the first supply voltage HV fed to its non-inverting input node (+) with the overvoltage detection voltage Vth 1  fed to its inverting input node (−) to generate the first overvoltage detection signal SA. The first power node (high-potential node) of the comparator  121   a  is connected to a node to which the first supply voltage HV is applied. The second power node (low-potential node) of the comparator  121   a  is connected to a node to which a ground voltage GND is applied. 
     The undervoltage detector  122  monitors whether or not the first supply voltage HV is lower than an undervoltage detection voltage Vth 2  (for example, Vth 2 =6 V) to generate an undervoltage detection signal SB. The undervoltage detection signal SB is, when the first supply voltage HV is higher than the undervoltage detection voltage Vth 2 , at a normal-mode logical level (low level (GND)) and, when the first supply voltage HV is lower than the undervoltage detection voltage Vth 2 , at an abnormal-mode logical level (high level (HV)). 
       FIG. 6  is a diagram showing an exemplary configuration of the undervoltage detector  122 . The undervoltage detector  122  includes a comparator  122   a  which compares the first supply voltage HV fed to its inverting input node (−) with the undervoltage detection voltage Vth 2  fed to its non-inverting input node (+) to generate the undervoltage detection signal SB. The first power node (high-potential node) of the comparator  122   a  is connected to the node to which the first supply voltage HV is applied. The second power node (low-potential node) of the comparator  122   a  is connected to the node to which the ground voltage GND is applied. 
     The second overvoltage detector  123  monitors whether or not the second supply voltage LV is higher than an overvoltage detection voltage Vth 3  (for example, Vth 3 =8.5 V) to generate a second overvoltage detection signal SC. The second overvoltage detection signal SC is, when the second supply voltage LV is lower than the overvoltage detection voltage Vth 3 , at a normal-mode logical level (low level (GND)) and, when the second supply voltage LV is higher than the overvoltage detection voltage Vth 3 , at an abnormal-mode logical level (high level (LV)). 
       FIG. 7  is a circuit diagram showing an exemplary configuration of the second overvoltage detector  123 . The second overvoltage detector  123  includes a comparator  123   a  which compares the second supply voltage LV fed to its non-inverting input node (+) with the overvoltage detection voltage Vth 3  fed to its inverting input node (−) to generate the second overvoltage detection signal SC. The first power node (high-potential node) of the comparator  123   a  is connected to a node to which the second supply voltage LV is applied. The second power node (low-potential node) of the comparator  123   a  is connected to the node to which the ground voltage (GND)) is applied. 
     The fault detection signal generator  124  monitors the first overvoltage detection signal SA, the undervoltage detection signal SB, and the second overvoltage detection signal SC to generate the fault detection signal S 1 . The fault detection signal generator  124  includes a level shifter  124   a  and an OR (logical addition) operator  124   b.    
     The level shifter  124   a  shifts the level of the first overvoltage detection signal SA, which is driven to pulsate between the first supply voltage HV and the ground voltage GND, to generate a (shifted) first overvoltage detection signal SA′, which is driven to pulsate between the second supply voltage LV and the ground voltage GND. Using the level shifter  124   a  eliminates the need to give the OR operator  124   b  an unnecessarily high withstand voltage. 
       FIG. 8  is a circuit diagram showing an exemplary configuration of the level shifter  124   a . The level shifter  124   a  of this exemplary configuration includes an N-channel MOS field-effect transistor a 1 , a current source a 2 , and an inverter a 3 . The drain of the transistor a 1  is connected to the node to which the first supply voltage HV is applied. The source of the transistor a 1  is connected, via the current source a 2 , to the node to which the ground voltage GND is applied. The gate of the transistor a 1  is connected to a node to which the first overvoltage detection signal SA is connected. The input node of the inverter a 3  is connected to the source of the transistor a 1 . The output node of the inverter a 3  is connected to the node to which the first overvoltage detection signal SA′ is applied. The first power node (high-potential node) of the inverter a 3  is connected to the node to which the second supply voltage LV is applied. The second power node (low-potential node) of the inverter a 3  is connected to the node to which the ground voltage GND is applied. 
     The OR operator  124   b  calculates the OR (logical sum) of the first overvoltage detection signal SA′, the undervoltage detection signal SB, and the second overvoltage detection signal SC to generate the fault detection signal S 1 . The fault detection signal S 1  is, when any of the first overvoltage detection signal SA′, the undervoltage detection signal SB, and the second overvoltage detection signal SC is at high level, at high level (LV) and, when those signals are all at low level, at low level (GND). 
     The load driving circuit  11  includes a P-channel DMOS field-effect transistor PD 1 , an N-channel DMOS field-effect transistor ND 1 , P-channel MOS field-effect transistors P 0  and P 1 , an N-channel MOS field-effect transistor N 1 , a resistor R 1 , a pre-driver DRV, a buffer BUF, an inverter INV, and a compensated supply voltage generator CVG. 
     The source of the transistor PD 1  is connected to the node to which the first supply voltage HV (or the second supply voltage LV) is applied. The drain of the transistor PD 1  is connected to an output node of an output signal OUT. The gate of the transistor PD 1  is connected to the pre-driver DRV. The source of the transistor ND 1  is connected to the node to which the ground voltage GND is applied. The drain of the transistor ND 1  is connected to the output node of the output signal OUT. The gate of the transistor ND 1  is connected to the pre-driver DRV. 
     The source of the transistor P 0  is connected to the node to which the first supply voltage HV (or the second supply voltage LV) is applied. The drain of the transistor P 0  is connected to the supply voltage input node of the pre-driver DRV. The gate of the transistor P 0  is connected to the output node of the buffer BUF. The input node of the buffer BUF is connected to a node to which the fault detection signal S 1  is applied. The first end of the resistor R 1  is connected to the supply voltage input node of the pre-driver DRV. The second node of the resistor R 1  is connected to the node to which the ground voltage GND is applied. 
     The source of the transistor P 1  is connected to the node to which the first supply voltage HV (or the second supply voltage LV) is applied. The drain of the transistor P 1  is connected to the gate of the transistor PD 1 . The gate of the transistor P 1  is connected to the output node of the inverter INV. The input node of the inverter INV is connected to the node to which the fault detection signal S 1  is applied. The source of the transistor N 1  is connected to the node to which the ground voltage GND is applied. The drain of the transistor N 1  is connected to the gate of the transistor ND 1 . The gate of the transistor N 1  is connected to a node to which the fault detection signal S 1  is applied. 
     In the load driving circuit  11  configured as described above, the transistors PD 1  and ND 1  correspond to a push-pull output circuit which, fed with the first supply voltage HV (or the second supply voltage LV), drives a load. More specifically, the transistor PD 1  corresponds to an upper transistor which is connected between the node to which the first supply voltage HV (or the second supply voltage LV) is applied and the output node of the output signal OUT; the transistor ND 1  corresponds to a lower transistor which is connected between the node to which the ground voltage GND is applied and the output node of the output signal OUT. 
     The pre-driver DRV is one of internal circuits which operate by being fed with the first supply voltage HV (or the second supply voltage LV), and corresponds to a driving circuit that generates driving signals for the push-pull output circuit (the gate signals of the transistors PD 1  and ND 1 ) according to instructions from the microprocessor  30 . 
     The transistor P 0  corresponds to a power switch which connects/disconnects (that is, makes conduct/cuts off) a supply voltage feed line leading to the internal circuits (including the pre-driver DRV) according to the fault detection signal S 1 . When the fault detection signal S 1  is at low level (the normal-mode logical level), the transistor P 0  is turned on to conduct the supply voltage feed line to the internal circuits. On the other hand, when the fault detection signal S 1  is at high level (the abnormal-mode logical level), the transistor P 0  is turned off to disconnect the supply voltage feed line leading to the internal circuits. 
     The transistor P 1  corresponds to a first upper switch which, according to the fault detection signal S 1 , connects/disconnects between the gate of the transistor PD 1  and the node to which the first supply voltage HV (or the second supply voltage LV) is applied. When the fault detection signal S 1  is at low level (the normal-mode logical level), the transistor P 1  is turned off to disconnect between the gate of the transistor PD 1  and the node to which the first supply voltage HV (or the second supply voltage LV) is applied. On the other hand, when the fault detection signal S 1  is at high level (the abnormal-mode logical level), the transistor P 1  is turned on to connect between the gate of the transistor PD 1  and the node to which the first supply voltage HV (or the second supply voltage LV) is applied. 
     The transistor N 1  corresponds to a first lower switch which, according to the fault detection signal S 1 , connects/disconnects between the gate of the transistor ND 1  and the node to which the ground voltage GND is applied. When the fault detection signal S 1  is at low level (the normal-mode logical level), the transistor N 1  is turned off to disconnect between the gate of the transistor ND 1  and the node to which the ground voltage GND is applied. On the other hand, when the fault detection signal S 1  is at high level (the abnormal-mode logical level), the transistor N 1  is turned on to connect between the gate of the transistor ND 1  and the node to which the ground voltage GND is applied. 
     The resistor R 1  corresponds to a pull-down resistor which is connected between the supply voltage input node for the internal circuits and the node to which the ground voltage GND is applied. 
     The buffer BUF shifts the level of the fault detection signal S 1 , which is driven to pulsate between the second supply voltage LV and the ground voltage GND, to generate a gate signal G 0  which is driven to pulsate between the first supply voltage HV (or the second supply voltage LV) and a first compensated supply voltage HV′ (or a second compensated supply voltage LV′), and feeds the gate signal G 0  to the gate of the transistor P 0 . The first compensated supply voltage HV′ (or the second compensated supply voltage LV′) is given, for example, a voltage value lower than the first supply voltage HV (or the second supply voltage LV) by a predetermined value α (for example, α=5 V). Using the buffer BUF having a level shifting capability eliminates the need to give the transistor P 0  an unnecessarily high withstand voltage. 
     The inverter INV shifts the level of the fault detection signal S 1 , which is driven to pulsate between the second supply voltage LV and the ground voltage GND, and then logically inverts the result to generate a gate voltage G 1  that is driven to pulsate between the first supply voltage HV (or the second supply voltage LV) and the first compensated supply voltage HV′ (or the second compensated supply voltage LV′), and feeds the gate voltage G 1  to the gate of the transistor P 1 . Using the inverter INV having a level shifting capability eliminates the need to give the transistor P 1  an unnecessarily high withstand voltage. 
       FIG. 9  is a circuit diagram showing an exemplary configuration of the compensated supply voltage generator CVG. The compensated supply voltage generator CVG of this exemplary configuration includes a P-channel MOS field-effect transistor b 1 , an npn-type bipolar transistor b 2 , a zener diode b 3 , and resistors b 4  and b 5 . The source of the transistor b 1  is connected to a node to which the first compensated supply voltage HV′ (or the second compensated supply voltage LV′) is applied; it is also connected, via the resistor b 4 , to the node to which the first supply voltage HV (or the second supply voltage LV) is applied. The drain of the transistor b 1  is connected to the node to which the ground voltage GND is applied. The gate of the transistor b 1  is connected to the emitter of the transistor b 2 , and is also connected, via the resistor b 5 , to the node to which the ground voltage GND is applied. The collector and the base of the transistor b 2  are both connected to the anode of the zener diode b 3 . The cathode of the zener diode b 3  is connected to the node to which the first supply voltage HV (or the second supply voltage LV) is applied. The compensated supply voltage generator CVG of this exemplary configuration can generate the first compensated supply voltage HV′ (or the second compensated supply voltage LV′) which is lower than the first supply voltage HV (or the second supply voltage LV) by a predetermined value α. 
     In the load driving circuit  11  configured as described above, when the fault detection signal S 1  turns to high level (the abnormal-mode logical level), the transistor P 0  is turned off, and the supply voltage feed line leading to the internal circuits including the pre-driver DRV is disconnected. Thus, even when there is a fault (an overvoltage or an undervoltage) in the first supply voltage HV or the second supply voltage LV, the internal circuits are prevented from destruction. 
     Incidentally, when the transistor P 0  is turned off and the supply voltage feed line leading to the internal circuits is disconnected, the supply voltage input node for the internal circuits is pulled down, via the resistor R 1 , to the node to which the ground voltage GND is applied. Thus, no indefinite voltage appears at the supply voltage input node for the internal circuits, which are thereby prevented from abnormal operation. 
     On the other hand, in the load driving circuit  11  configured as described above, to avoid a drop in power efficiency ascribable to the on-state resistance component across a switch, no switch for connecting/disconnecting is provided in the supply voltage feed line leading to the push-pull output circuit. 
     Instead, in the load driving circuit  11  configured as described above, as a means for protecting the transistors PD 1  and ND 1 , the transistors P 1  and N 1  are provided. When the fault detection signal S 1  turns to high level (the abnormal-mode logical level), the transistors P 1  and N 1  are turned on so that the transistors PD 1  and ND 1  both have their gate and source short-circuited together. As a result, the transistors PD 1  and ND 1  no longer receives any voltage between their gate and source. In this way, it is possible to protect the transistors PD 1  and ND 1  without unnecessarily increasing their gate-source withstand voltage. Needless to say, the transistors PD 1  and ND 1  need to be given a source-drain withstand voltage high enough to withstand a fault in the first supply voltage HV (or the second supply voltage LV). Incidentally, when the fault detection signal S 1  turns to high level (the abnormal-mode logical level), the transistors PD 1  and ND 1  are both completely turned off, with the result that the output node of the output signal OUT is left in a floating state (a high-impedance state).  FIG. 10  is a timing chart showing an example of the fault protection operation described above, showing, from top, the first supply voltage HV, the fault detection signal S 1 , the gate voltages of the transistors P 1  and N 1 , the gate voltages of the transistors PD 1  and ND 1 , and the output signal OUT. 
     Although not illustrated in  FIG. 2 , an electrostatic protection diode is commonly connected between the node to which the first supply voltage HV (or the second supply voltage LV) is applied and the node to which the ground voltage GND is applied. The electrostatic protection diode lies outside the scope of the protection operation based on the fault detection signal S 1 , and therefore needs to be implemented with a device that has a sufficiently high withstand voltage. 
       FIG. 3  is a circuit diagram showing a modified example of the load driving circuit  11 . As shown in  FIG. 3 , the load driving circuit  11  may be so configured as to have analog switches SW 1  and SW 2  connected to the gates of the transistors PD 1  and ND 1  respectively. 
     The analog switch SW 1  corresponds to a second upper switch which, according to the fault detection signal S 1 , connects/disconnects between the gate of the transistor PD 1  and the pre-driver DRV. When the fault detection signal S 1  is at low level (the normal-mode logical level), the analog switch SW 1  is turned on to connect between the gate of the transistor PD 1  and the pre-driver DRV. On the other hand, when the fault detection signal S 1  is at high level (the abnormal-mode logical level), the analog switch SW 1  is turned off to disconnect between the gate of the transistor PD 1  and the pre-driver DRV. Providing the analog switch SW 1  makes it possible to more reliably keep the gate of the transistor PD 1  at the first supply voltage HV (or the second supply voltage LV) when the fault detection signal S 1  turns to high level (the abnormal-mode logical level). 
     The analog switch SW 2  corresponds to a second lower switch which, according to the fault detection signal S 1 , connects/disconnects between the gate of the transistor ND 1  and the pre-driver DRV. When the fault detection signal S 1  is at low level (the normal-mode logical level), the analog switch SW 2  is turned on to connect between the gate of the transistor ND 1  and the pre-driver DRV. On the other hand, when the fault detection signal S 1  is at high level (the abnormal-mode logical level), the analog switch SW 2  is turned off to disconnect between the gate of the transistor ND 1  and the pre-driver DRV. Providing the analog switch SW 2  makes it possible to more reliably keep the gate of the transistor ND 1  at the ground voltage GND when the fault detection signal S 1  turns to high level (the abnormal-mode logical level). 
     As described above, a motor driver IC  10  adopting the configuration shown in  FIG. 2 or 3  eliminates the need, in order to cope with overvoltage destruction testing and destructive testing in which supply voltages for two systems are connected the wrong way around (so-called cross connection testing), to give the transistors PD 1  and ND 1  an unnecessarily increased withstand voltage, or to connect an additional component externally to the motor driver IC  10 . This contributes to reducing the chip area and reducing the cost of target products. 
     For example, in a case where overvoltage destruction testing is conducted in which the first supply voltage HV or the second supply voltage LV is intentionally brought into an overvoltage state, while the first supply voltage HV or the second supply voltage LV is held in an overvoltage state, the first overvoltage detection signal SA or the second overvoltage detection signal SC remains at high level, and this causes the fault detection signal S 1  to turn to high level (the abnormal-mode logical level) and thereby activates the fault protection operation described above (see  FIG. 10 ). The IC is thus prevented from smoking or destruction. 
     As discussed above, according to the protection technology described above, with respect to a device that handles a small signal, it can be protected from destruction by turning off a switch in a power supply line and, with respect to an output device that cannot be protected in that way, it can be protected from destruction through the introduction of a switch for turning its gate off That is, as circuit elements for realizing the protection technology described above, both a gate-off switch against destruction of an output device and a switch for disconnecting a power supply line are both essential. The reason that a power supply line connected to an output device is not disconnected is to avoid an apparent increase in the on-state resistance of the output device which would result from the insertion of the switch. 
     On the other hand, in a case where destructive testing is conducted in which the first and second supply voltages LH and LV are connected the wrong way around (so-called cross connection testing), the undervoltage detection signal SB or the second overvoltage detection signal SC turns to high level and thereby activates the protection operation described above. The IC is thus prevented from smoking or destruction. 
     (Desktop PC) 
       FIG. 11  is an external view of a desktop personal computer (PC) incorporating the optical disc drive  1 . The desktop PC X of this exemplary configuration includes a cabinet X 10 , a liquid crystal display monitor X 20 , a keyboard X 30 , and a mouse X 40 . 
     The cabinet X 10  accommodates a central processing unit (CPU) X 11 , a memory X 12 , an optical drive X 13 , a hard disk drive X 14 , etc. 
     The CPU X 11  executes an operating system and various application programs stored on the hard disk drive X 14 , and thereby controls the operation of the desktop PC X in an integrated fashion. 
     The memory X 12  is used as a working area (for example, an area where task data is stored during execution of a program) by the CPU X 11 . 
     The optical drive X 13  reads and writes optical discs. Examples of optical discs include CDs (compact discs), DVDs (digital versatile discs), and BDs (Blu-ray discs). As the optical drive X 13 , the optical disc drive  1  described previously can suitably be used. 
     The hard disk drive X 14  is a type of large-capacity auxiliary storage device that stores programs and data on a non-volatile basis by use of a magnetic disk hermetically sealed inside a housing. 
     The liquid crystal display monitor X 20  outputs video based on instructions from the CPU X 11 . 
     The keyboard X 30  and the mouse X 40  are each a type of human interface device that accepts user operation. 
     &lt;Modifications and Variations&gt; 
     Although the embodiment described above deals with, as an example, a configuration where the present invention is applied to a motor driver IC  10 , this is not meant to limit the scope of application of the present invention; the present invention finds wide application in load driving devices in general, such as switching regulator ICs like the one shown in  FIG. 4 . 
       FIG. 4  is a circuit diagram showing an exemplary configuration of a switching regulator IC to which the present invention is applicable. The switching regulator IC  40  of this exemplary configuration is a semiconductor integrated circuit device including a P-channel MOS field-effect transistor  41 , a rectification diode  42 , a switch controller  43 , an overvoltage protector  44 , and a power switch  45 ; it further has, externally connected to it as discrete devices constituting an output stage, a coil L 11 , a capacitor C 11 , and resistors R 11  and R 12 . Although in this exemplary configuration, the output stage is configured as a step-down type, this is not meant to limit the configuration of the output stage; it may instead be of a step-up type or a step-up and -down type. 
     In the switching regulator IC  40  of this exemplary configuration, the transistor  41  corresponds to the transistor PD 1  in  FIG. 2 , and the rectification diode  42  corresponds to the transistor ND 1  in  FIG. 2 . The rectification diode  42  may be replaced with a synchronous-rectification transistor. The switch controller  43  corresponds to the pre-driver DRV (an internal circuit) in  FIG. 2 , and the overvoltage protector  44  corresponds to the fault detection circuit  12  in  FIG. 2 . The power switch  45  corresponds to the transistor P 0  in  FIG. 2 . Although not explicitly shown in  FIG. 4 , a device corresponding to the transistor P 1  in  FIG. 2  may be provided, for example, between the gate and the source of the transistor  41 . 
     Although the embodiment described above deals with, as an example, a configuration where the motor driver IC  10  is fed with supply voltages (HV and LV) for two systems, this is not meant to limit the present invention; even in cases where it is fed with supply voltages for three or more systems, it is possible to flexibly cope with them by providing an overvoltage detector and an undervoltage detector for each system and performing level shifting so as to adapt the signal level of a fault detection signal for each load driving circuit. 
     As discussed above, the present invention may be implemented in any other manners than specifically described by way of an embodiment above, with many modifications made without departing from the spirit of the present invention. That is, it is to be understood that the embodiment described above is in every way illustrative and not restrictive. The technical scope of the present invention is defined not by the description of the embodiment above but by the scope of the appended claims, and is to be understood to encompass any modifications made in the sense and scope equivalent to those of the claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention contributes to enhancing the reliability of load driving devices. 
     LIST OF REFERENCE SIGNS 
     
         
         
           
               1  optical disc drive 
               10  load driving device (motor driver IC) 
               11  load driving circuit 
               111  spindle motor driver circuit 
               112  sled motor driver circuit 
               113  loading motor driver circuit 
               114  focus actuator driver circuit 
               115  tracking actuator driver circuit 
               116  tilt actuator driver circuit 
               12  fault detection circuit 
               121  first overvoltage detector 
               121   a  comparator 
               122  undervoltage detector 
               122   a  comparator 
               123  second overvoltage detector 
               123   a  comparator 
               124  fault detection signal generator 
               124   a  level shifter 
             a 1  N-channel MOS field-effect transistor 
             a 2  current source 
             a 3  inverter 
               124   b  OR operator 
               20  load (motor/actuator) 
               21  spindle motor 
               22  sled motor 
               23  loading motor 
               24  focus actuator 
               25  tracking actuator 
               26  tilt actuator 
               30  microprocessor 
               40  switching regulator IC 
               41  P-channel MOS field-effect transistor 
               42  rectification diode 
               43  switch controller (internal circuit) 
               44  overvoltage protector (fault detection circuit) 
               45  power switch 
             PD 1  P-channel DMOS field-effect transistor 
             ND 1  N-channel DMOS field-effect transistor 
             P 0 , P 1  P-channel MOS field-effect transistor 
             N 1  N-channel MOS field-effect transistor 
             R 1  resistor 
             DRV pre-driver 
             BUF buffer (with a level shifting capability) 
             INV inverter (with a level shifting capability) 
             CVG compensated supply voltage generator 
             b 1  P-channel MOS field-effect transistor 
             b 2  npn-type bipolar transistor 
             b 3  zener diode 
             b 4 , b 5  resistor 
             SW 1 , SW 2  analog switch 
             L 11  coil 
             C 11  capacitor 
             R 11 , R 12  resistor 
             X desktop PC 
             X 10  cabinet 
             X 11  CPU 
             X 12  memory 
             X 13  optical drive 
             X 14  hard disk drive 
             X 20  liquid crystal display monitor 
             X 30  keyboard 
             X 40  mouse