Abstract:
A load driving device capable of preventing thermal destruction even when a load short-circuit or an overcurrent occurs, thereby having improved reliability, is provided. A load driving device, in which a power switch element for driving a load and a circuit for controlling the power switch element according to a signal V IN  supplied from the outside are formed on one chip, is provided with an OFF-time delaying circuit for delaying an OFF-time transition of a level of an input signal at which the power switch element makes the transition from an ON state to an OFF state, according to a result of detection of a current flowing through the load and the level of the input signal to the power switch element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a load driving device for driving a load such as a lamp, an LED, or an inductor, and particularly to a technique for preventing thermal destruction of a load driving device due to heat of a power switch element composing the load driving device when an overcurrent or a short-circuit current is generated. 
     2. Related Background Art 
     Conventionally, according to a technique for driving a load such as a lamp or a coil, as shown in FIG. 4, a power source  9  is connected to a high-potential side of a load  17  while a load driving device  1 ′ is provided on a low-potential side of the load  17 , and the load  17  is driven by turning a power switch element  2  of the load driving device  1 ′ ON/OFF according to a signal supplied to an input terminal  22  from the outside so that current from the power source  9  is passed/stopped. Such a load driving device  1 ′ generally has various protective functions. To realize functions of load short-circuit protection and overcurrent protection among the protective functions, it is necessary to protect the load driving device  1 ′ from thermal destruction by detecting a voltage applied to the low-potential side of the load  17 , that is, a potential of a drain terminal  21  of the load driving device  1 ′, with current detecting resistors  13  and  14 , and turning the power switch element  2  OFF according to the potential detected. 
     The following description will depict specific load short-circuit protection and overcurrent protection, while referring to FIG.  5 . FIG. 5 is a waveform diagram illustrating voltages and currents at respective parts of the load driving device  1 ′ upon the occurrence of a load short-circuit or an overcurrent. 
     First of all, when a voltage V IN  is applied to an input terminal  22  from the outside and a voltage V T  obtained by dividing the voltage V IN  with resistors  10 ,  11  and a resistor  12  exceeds a threshold value of a gate voltage detecting element  16 , the gate voltage detecting element  16  is turned ON, thereby being in a state of monitoring a voltage V D  at a drain terminal  21 . 
     In this state, when something abnormal occurs to the load  17  thereby causing a large current to flow, the voltage V C  obtained by dividing the voltage V D  with the current detecting resistors  13  and  14  rises according to the current I L , and a current detecting element  15  is turned ON when the voltage V C  exceeds a threshold value (V TH ) of the current detecting element  15 . Therefore, a gate voltage V G  of the power switch element  2  drops to a ground level since the gate is grounded via a grounding terminal  23 , and the power switch element  2  is turned OFF. 
     When the lowering of the gate voltage V G  of the power switching element  2  causes the voltage V T  obtained by dividing the gate voltage V G  with the resistors  11  and  12  to drop to a level lower than the threshold value of the gate voltage detecting element  16 , this causes the gate voltage detecting element  16  to be turned OFF, and also the current detecting element  15  to be turned OFF. As a result, again the gate voltage V G  of the power switch element  2  rises, thereby again causing the power switch element to be turned ON. 
     The rise of the gate voltage V G  of the power switch element  2  causes a voltage V T  obtained by dividing the voltage V G  with the resistors  11  and  12  to rise, and when the voltage V T  exceeds the threshold value of the gate voltage detecting element  16 , the gate voltage detecting element  16  is turned ON. 
     In the case where the abnormality in the load  17  causes a large current to continue flowing, the device is in an oscillation state in which the foregoing sequence of actions is repeated. Thus, to prevent thermal destruction of the load driving device  1 ′ from occurring when a large current flows through the load  17 , the functions of load short-circuit protection and  25  overcurrent protection are provided. It should be noted that the load short-circuit protection and the overcurrent protection has a difference therebetween only in a value of a current flowing through the load  17 , which results in a difference in the voltage detected by the current detecting resistors  13  and  14 , and the two functions are identical in the actions per se. 
     As shown in FIG. 5, while the voltage V IN  continues to be applied to the input terminal  22 , the oscillation state of the load current I L  continues due to the load short-circuit protection and the overcurrent protection. In such an oscillation state of the load current I L , the load driving device  1 ′ generates heat, thereby causing a temperature to rise, but by providing an overheat protection circuit  18 , the gate voltage V G  of the power switch element  2  is caused to drop when the temperature of the load driving device  1 ′ reaches a heating set temperature, whereby the power switch element  2  is turned OFF. Therefore, thermal destruction of the device is prevented. 
     The heating set temperature of the overheat protection circuit  18  usually is set in the vicinity of 150° C., which is an operating upper limit temperature of the load driving device  1 ′. It, however, possibly is close to 200° C. due to variation of circuit constants, and in the case where a load short-circuit or an overcurrent occurs repeatedly, the load driving device  1 ′ possibly undergoes thermal destruction. Thus, in some cases this raises a problem in reliability of the device. 
     SUMMARY OF THE INVENTION 
     Therefore, with the foregoing in mind, it is an object of the present invention to provide a load driving device capable of avoiding thermal destruction even when a load short-circuit or an overcurrent occurs, thereby having improved reliability. 
     To achieve the foregoing object, a load driving device according to the present invention includes a power switch element for driving a load and a circuit for controlling the power switch element according to a signal supplied from the outside, the power switch element and the circuit being provided on one chip, and the load driving device comprises a switching time changing means for changing a time to turn the power switch element ON/OFF, according to a result of detection of a current flowing through the load and an input signal level of the power switch element. Here, the power switch element is an N-channel MOSFET or an insulated-gate bipolar transistor, each of which is a voltage-driven type, or a normal bipolar transistor, which is a current-driven type. 
     In the load driving device, the switching time changing means preferably includes an OFF-time delaying circuit for delaying an OFF-time transition of the input signal level at which the power switch element makes the transition from an ON state to an OFF state. 
     Furthermore, the OFF-time delaying circuit preferably draws its power from the signal supplied from the outside. 
     Furthermore, the OFF-time delaying circuit preferably delays the OFF-time transition of the input signal level for a period according to a temperature rise of the load driving device. 
     In this case, the load driving device preferably further includes an overheat protection circuit that detects a temperature and turns the power switch element OFF according to the detected temperature, wherein the OFF-time delaying circuit delays the OFF-time transition of the input signal level according to the temperature detected by the overheat protection circuit. 
     Furthermore, the OFF-time delaying circuit draws its power from a connection that connects a resistor to which the signal is supplied from the outside and a plurality of diodes connected in series in a forward direction. 
     With the foregoing configuration, the OFF time of the power switch element is prolonged when a load short-circuit or an overcurrent occurs, and the OFF time is set shorter at normal temperature and is increased as the temperature rises. By so doing, it is possible to surely prevent the load driving device from undergoing thermal destruction, and to improve the response speed when the device has recovered from a temporary load short-circuit or overcurrent. Thus, it is possible to improve the reliability of the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view illustrating a configuration of a load driving device according to a first embodiment of the present invention. 
     FIG. 2 is a waveform diagram showing voltages and currents at respective parts of the load driving device shown in FIG. 1 when a load short-circuit or an overcurrent occurs. 
     FIG. 3 is a view illustrating a configuration of a load driving device according to a second embodiment of the present invention. 
     FIG. 4 is a view illustrating a configuration of a conventional load driving device. 
     FIG. 5 is a waveform diagram showing voltages and currents at respective parts of the conventional load driving device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description will depict an embodiment of the present invention, while referring to the drawings. 
     First Embodiment 
     FIG. 1 is a view illustrating a configuration of a load driving device  1  according a first embodiment of the present invention. It should be noted that the same component elements as those of the conventional example shown in FIG. 4 are designated with the same referential numerals and descriptions of the same are omitted. The present embodiment differs from the conventional example in the aspect that an OFF time delaying circuit  3  for delaying the falling of the voltage V T  obtained by division by resistors  11  and  12  for a predetermined time is provided on an input side of the gate voltage detecting element  16 . The OFF time delaying circuit  3  is powered with the voltage V IN  applied to an input terminal  22 . 
     FIG. 2 is a waveform diagram of voltages or currents at respective parts of the load driving device  1  upon the occurrence of a load short-circuit or an overcurrent. 
     The following description will depict functions of load short-circuit protection and overcurrent protection according to the present embodiment, while referring to FIGS. 1 and 2. 
     First of all, when a voltage V IN  is applied to the input terminal  22  from the outside, a voltage V T  obtained by dividing the voltage V IN  with the resistors  10 ,  11  and the resistor  12  rises. The voltage V T  is fed to the OFF-time delaying circuit  3 , but the OFF-time delaying circuit  3  does not operate to delay the rise of the voltage V T . The voltage V T  is outputted to a gate voltage detecting element  16  as a voltage V T1 . When the voltage V T1  exceeds a threshold value of the gate voltage detecting element  16 , the gate voltage detecting element  16  is turned ON, so that a voltage V D  at a drain terminal  21  is monitored. 
     In this state, when something abnormal occurs to a load  17  thereby causing a large current to flow, a voltage V C  obtained by dividing the voltage V D  with the current detecting resistors  13  and  14  rises according to the current I L , and a current detecting element  15  is turned ON when the voltage V C  exceeds a threshold value (V TH ) of the current detecting element  15 . Therefore, a gate voltage V G  of the power switch element  2  drops to a ground level since the gate is grounded via a grounding terminal  23 , and the power switch element  2  is turned OFF. 
     Subsequently, the drop of the gate voltage V G  of the power switch  30  element  2  causes the voltage V T  obtained by dividing the gate voltage V G  with the resistors  11  and  12  to drop. Here, the OFF-time delaying circuit  3  causes the falling of the voltage V T1  applied to the gate voltage detecting element  16  to delay for a predetermined delay time. When the voltage V T1  drops to a level lower than the threshold value of the gate voltage detecting element  16  after the predetermined delay time, this causes the gate voltage detecting element  16  to be turned OFF, and also the current detecting element  15  to be turned OFF. As a result, again the gate voltage V G  of the power switch element  2  rises, thereby again causing the power switch element  2  to be turned ON. 
     The rise of the gate voltage V G  of the power switch element  2  causes a voltage V T  obtained by dividing the voltage V G  with the resistors  11  and  12  to rise, and when the voltage V T1  via the OFF-time delaying circuit  3  exceeds the threshold value of the gate voltage detecting element  16 , the gate voltage detecting element  16  is turned ON. 
     In the case where the abnormality in the load  17  causes a large current to continue flowing, the device is in an oscillation state in which the foregoing sequence of actions is repeated. However, in the OFF-time delaying circuit  3 , the delay time is set for the falling of the voltage V T , so that the oscillation state is made to be such that an OFF time T 2  is about 100 times an ON time T 1 . By so doing, the heat generation of the load driving device  1  can be suppressed to approximately {fraction (1/100)} of that in the conventional case. It should be noted that an excessively long OFF time T 2  is not preferable with a view to stabilization of operations since this causes the response speed to lower in the case where the device is recovered after a temporary load short-circuit or overcurrent occurred. 
     For instance, when a load short-circuit or an overcurrent occurs, in the case where a power consumption due to the oscillation state of the load current is 10 W and the load driving device has a package with an allowable dissipation of 1 W, conventionally the load driving device  1 ′ immediately generates heat and the overheat protection circuit  18  operates. On the other hand, in the present embodiment, the power consumption is {fraction (1/100)}, that is, 0.1W, and hence the overheat protection circuit  18  whose heating set temperature tends to be varied need not operate. Thus, the reliability of the load driving device  1  can be improved. 
     Second Embodiment 
     FIG. 3 is a view illustrating a configuration of a load driving device  100  according to a second embodiment of the present invention. It should be noted that, in FIG. 3, the same component elements as those of the first embodiment shown in FIG. 1 are designated with the same reference numerals and descriptions of the same are omitted. 
     While in the first embodiment the OFF-time delay circuit  3  draws its power from the voltage V IN  applied to the input terminal  22 , the OFF-time delay circuit  3  in the second embodiment draws its power from a voltage V F  at a connection between a resistor  20  connected with the input terminal  22  and a plurality of diodes  19  connected in series with each other as shown in FIG.  3 . 
     This configuration allows respective forward voltages of the plurality of diodes  19  to decrease due to temperature characteristics when the temperature of the load driving device  100  rises, thereby allowing the voltage V F  as a sum of the respective forward voltages of the diodes  19  to decrease. Such a decrease in the voltage V F  causes an increase in the OFF delay time of the OFF-time delaying circuit  3  drawing its power from the voltage V F , thereby causing an increase in the OFF time T 2  of the load current I L . Therefore, when the load driving device  100  is at a high temperature, an operation to prevent further heat generation is carried out. 
     This makes it possible to set the OFF time T 2  shorter at a normal temperature, thereby preventing retardation of the response when the device is recovered from a temporary load short-circuit or an overcurrent, which is a defect in the case where the OFF time T 2  is set to be long. Therefore, this allows the load driving device  100  to operate stably. 
     It should be noted that generally the overheat protection circuit  18  is provided with identical circuit elements to the resistor  20  and the plurality of diodes  19  to detect a temperature, and it is possible to decrease the number of component elements by causing the OFF-time delaying circuit  3  to draw its power from the foregoing circuit elements. 
     As described above, according to the present invention, it is possible to achieve a specific effect of preventing the load driving device surely from undergoing thermal destruction even when a load short-circuit or an overcurrent occurs, and hence improving the reliability of the device. 
     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.