Patent Publication Number: US-2011062890-A1

Title: Drive circuit

Description:
CROSS-REFERENCE OF THE INVENTION 
     This application claims priority from Japanese Patent Application No. JP2009-212901, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a drive circuit, in particular, to a drive circuit having an open drain type output transistor. 
     A drive circuit having an open drain type output transistor is conventionally known. A drive circuit of this type is used to drive a self emission element such as a vacuum fluorescent display, for example. 
       FIG. 4  is a diagram showing a structure of a drive circuit  10  for vacuum fluorescent displays. As shown in the figure, this drive circuit  10  includes output transistors T 1  to Tm made of high breakdown voltage P-channel type MOS transistors and output terminals P 1  to Pm made of metal pads on a semiconductor die  11 . 
     A positive supply voltage VDD (+5 V) is applied to the sources of the output transistors T 1  to Tm, and switching control signals C 1  to Cm are applied to the gates thereof, respectively. The drains of the output transistors T 1  to Tm are connected to the output terminals P 1  to Pm, respectively. 
     The output terminals P 1  to Pm are connected to the anodes of vacuum fluorescent displays VFD 1  to VFDm provided outside the semiconductor die  11 . A negative high voltage VPP (e.g. −80 V) is applied to the cathodes called filaments of the vacuum fluorescent displays VFD 1  to VFDm. 
     When the output transistor Tx turns on, the voltage of the corresponding output terminal Px becomes the supply voltage VDD and a potential difference of the VDD+VPP (e.g. 85 V) occurs between the anode and cathode. Therefore, thermoelectrons emitted from the cathode of the corresponding vacuum fluorescent display VFDx flow in the anode through a phosphor disposed between the anode and cathode. By this, the vacuum fluorescent display VFDx lights. 
     When the output transistor Tx turns off, a potential difference does not occur between the cathode and anode of the corresponding vacuum fluorescent display VFDx and thermoelectrons do not flow in the phosphor. Therefore, the vacuum fluorescent display VFDx does not light. A high voltage of VDD+VPP is applied between the source and drain of the output transistor Tx at this time, the output transistor Tx need be a high breakdown voltage transistor against such a high voltage. 
     A relevant drive circuit is described in Japanese Patent Application publication No. 2001-209343. 
     The output transistors T 1  to Tm need have a high breakdown voltage characteristic as described above, and in order to secure the characteristic a breakdown voltage test is performed when the wafer is completed. In this test, the output transistors T 1  to Tm are turned off. Then a probe needle is connected to each of the output terminals P 1  to Pm, a negative high voltage VPP (e.g. −80 V) is applied thereto, and a leakage current of each of the output transistors T 1  to Tm is measured. 
     Then the wafer is cut into multiple semiconductor dies  11  by a scribing process, only the semiconductor dies  11  that pass the breakdown voltage test are selected, and an assembling process of the semiconductor dies  11  is performed. 
     However, when there are a large number of output terminals P 1  to Pm, like several hundreds to thousands of output terminals P 1  to Pm, there occurs a problem such as a shortage of probe needles of a prober, a long measurement time and so on. 
     SUMMARY OF THE INVENTION 
     The provides a drive circuit that includes a plurality of output transistors. Each of the output transistors includes a P-channel type MOS transistor having a source configured to receive a positive voltage. The drive circuit also includes a plurality of switching control circuits controlling switching of corresponding output transistors, and a plurality of output terminals. Each of the output terminals is connected to a drain of a corresponding output transistor. The drive circuit further includes a plurality of rectifiers and a control terminal connected to cathodes of the rectifiers. Each of the rectifiers includes an anode connected to a drain of a corresponding output transistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a structure of a drive circuit of a first embodiment of the invention. 
         FIG. 2  is a diagram showing a structure of a drive circuit of a second embodiment of the invention. 
         FIG. 3  is a partial cross-sectional view of the drive circuit of the second embodiment of the invention. 
         FIG. 4  is a diagram showing a structure of a conventional drive circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of the invention will be described referring to  FIG. 1 . As shown in the figure, a drive circuit  1  includes output transistors T 1  to Tm made of high breakdown voltage P-channel type MOS transistors, switching control circuits SCL 1  to SCLm, output terminals P 1  to Pm made of metal pads, diodes DO 1  to DOm (as an example of a “rectifier”) and a control terminal PX on a semiconductor die  2 . 
     A positive supply voltage VDD (+5 V) is applied to the sources s 1  to sm of the output transistors T 1  to Tm. Switching control signals from the switching control circuits SCL 1  to SCLm to which the supply voltage VDD is supplied are applied to the gates g 1  to gm of the output transistors T 1  to Tm respectively so as to control the on and off of the output transistors T 1  to Tm. In detail, when the switching control signal of the switching control circuit SCLx is H level (=VDD), the corresponding output transistor Tx turns off, and when the switching control signal of the switching control circuit SCLx is L level (=0 V), the corresponding output transistor Tx turns on. 
     In this case, the switching control circuits SCL 1  to SCLm are made of holding circuits that hold data (e.g. a 1 bit static type memory), and hold display data for controlling the lighting of vacuum fluorescent displays VFD 1  to VFDm in this embodiment. The drains d 1  to dm of the output transistors T 1  to Tm are connected to the output terminals P 1  to Pm, respectively. 
     The output terminals P 1  to Pm are connected to the anodes of the vacuum fluorescent displays VFD 1  to VFDm provided outside the semiconductor die  2 , respectively. A negative high voltage VPP (e.g. −80 V) is applied to the cathodes of the vacuum fluorescent displays VFD 1  to VFDm. 
     The diodes DO 1  to DOm are made of high breakdown voltage P-channel type MOS transistors of which the source and gate are commonly connected. In this case, the drain serves as an anode and the source serves as a cathode. The anodes of the diodes DO 1  to DOm are connected to the drains d 1  to dm of the corresponding output terminals P 1  to Pm, respectively. The cathodes of the diodes DO 1  to DOm are commonly connected to the control terminal PX through a wiring  3 . 
     Providing the diodes DO 1  to DOm in this manner prevents a short circuit between the output terminals P 1  to Pm. The reason why the diodes DO 1  to DOm are made of high breakdown voltage P-channel type MOS transistors is to prevent breakdown when the negative high voltage VPP is applied to the drains d 1  to dm of the output terminals P 1  to Pm and the anodes of the diodes DO 1  to DOm, in other words, when the diodes DO 1  to DOm are reverse-biased. 
     The control terminal PX is connected to a voltage application circuit  4 , and receives a voltage from the voltage application circuit  4 . The voltage application circuit  4  is provided outside or inside the semiconductor die  2 . 
     The drive circuit  1  of the embodiment enables testing the breakdown voltage characteristics of the output transistors T 1  to Tm in a batch by using the control terminal PX. In detail, while the supply voltage VDD is supplied, all the output transistors T 1  to Tm are turned off by the switching control circuits SCL 1  to SCLm, and the negative high voltage VPP is applied to the control terminal PX by the voltage application circuit  4 . 
     At this time, the diodes DO 1  to DOm connected to the output transistors T 1  to Tm are forward-biased, and the negative high voltage VPP is applied to the drains d 1  to dm of the output transistors T 1  to Tm. It is noted that this is in the case of neglecting the forward voltage of the diodes DO 1  to DOm (=the threshold of the P-channel type MOS transistor). Then since the supply voltage VDD is applied to the sources s 1  to sm of the output transistors T 1  to Tm, a potential difference of VDD+VPP occurs between the source and drain if the output transistors T 1  to Tm are normal. 
     Then a leakage current flowing from a supply voltage VDD application terminal (not shown) to the control terminal PX through the output transistors T 1  to Tm is measured by an ammeter provided outside the semiconductor die  2 . 
     When the measured leakage current is higher than a judgment reference value, the drive circuit  1  is judged as a defective. In this case, it is conceivable that there may be one among the output transistors T 1  to Tm that is broken or deteriorated by stress by the application of the high voltage VPP, or some process error may have occurred in the wafer process or the like. When the measured leakage current is lower than the judgment reference value, the drive circuit  1  is judged as a normal. 
     Accordingly, the drive circuit  1  of the embodiment enables testing the breakdown voltage characteristics of the output transistors T 1  to Tm in a batch without connection of probe needles to the output terminals P 1  to Pm in the breakdown voltage test when the wafer is completed. This is effective when there are a large number of output terminals P 1  to Pm, like several hundreds or thousands of output terminals P 1  to Pm. Such a case causes a problem like a shortage of probe needles of a prober, a long measurement time and so on. 
     This breakdown voltage test also enables a short circuit test between the wiring  3  connected to the control terminal PX and an inner wiring adjacent to the wiring  3  in the drive circuit  1  as well as the breakdown voltage test for the output transistors T 1  to Tm. For example, when there is an obstacle between the wiring  3  and the inner wiring, this is measurable since an electric current flows between these by applying the high voltage VPP to the control terminal PX. 
     A normal operation is as follows. In the operation, the supply voltage VDD is supplied, and the output transistors T 1  to Tm, the switching control circuits SCL 1  to SCLm and the voltage application circuit  4  are in operation. The control terminal PX is set at the supply voltage VDD by the voltage application circuit  4 . 
     For example, when the output transistor T 1  is in the on state, the supply voltage VDD is applied to the anode of the vacuum fluorescent display VFD 1  through the output terminal P 1 , thermoelectrons flow in the vacuum fluorescent display VFD 1 , and the vacuum fluorescent display VFD 1  lights. When the output transistor T 2  is in the off state, the voltage of the anode of the vacuum fluorescent display VFD 2  becomes VPP, thermoelectrons do not flow in the vacuum fluorescent display VFD 2 , and the vacuum fluorescent display VFD 2  does not light. 
     At this time, since both the voltages of the anode and cathode of the diode DO 1  connected to the output transistor T 1  are VDD and the diode DO 2  connected to the output transistor T 2  are reverse-biased, a short circuit between the output transistors T 1 , T 2  is prevented. In other words, by providing the diodes DO 1  to DOm and setting the voltage of the control terminal PX at VDD, the output transistors T 1  to Tm are electrically isolated to achieve the normal operation. 
     Furthermore, the control terminal PX is usable to initialize the voltages of the drains d 1  to dm of the output transistors T 1  to Tm when the drive circuit  1  is started or the like. For example, when the drive circuit  1  is started (when the supply voltage VDD and the high voltage VPP are supplied), the voltages of the drains d 1  to dm of the output transistors T 1  to Tm, i.e., the voltages of the output terminals P 1  to Pm are not fixed. 
     Then when the drive circuit  1  is started, a potential difference may occur between the anode and cathode of the vacuum fluorescent displays VFD 1  to VFDm to cause an undesirable flash of the vacuum fluorescent displays VFD 1  to VFDm. In order to prevent this, the potential difference between the anode and cathode of each of the vacuum fluorescent displays VFD 1  to VFDm is cancelled by applying the high voltage VPP to the control terminal PX by the voltage application circuit  4  when the drive circuit  1  is started or the like. This prevents unnecessary lighting of the vacuum fluorescent displays VFD 1  to VFDm. 
     Next, a drive circuit  1  of a second embodiment of the invention will be described referring to  FIGS. 2 and 3 . In the embodiment,  FIG. 2  shows a circuit structure of the drive circuit  1  and also a positional relation in a plan view. As shown in the figure, output transistors T 1  to Tm, diodes DO 1  to DOm and switching control circuits SCL 1  to SCLm are disposed under the corresponding output terminals P 1  to Pm. 
     The other structure is the same as in the first embodiment, and the drains d 1  to dm of the output transistors T 1  to Tm are connected to the corresponding output terminals P 1  to Pm. In this case where the output transistors T 1  to Tm and so on are disposed under the output terminals P 1  to Pm in this manner, when a probe needle is connected to each of the output terminals P 1  to Pm for measurement in a breakdown voltage test when the wafer is completed, the mechanical impact of the probe needle when connected causes the output transistors T 1  to Tm and so on to be broken or the electric characteristics to be degraded. 
     Therefore, the method of testing the breakdown voltage characteristics of the output transistors T 1  to Tm in a batch by using the control terminal PX as described above is effective particularly in this embodiment. 
     This case includes a case where the output transistors T 1  to Tm, the diodes DO 1  to DOm and the switching control circuits SCL 1  to SCLm are partially or completely disposed under the corresponding output terminals P 1  to Pm. For example, the invention is also effective in the case where only the output transistors T 1  to Tm are disposed under the corresponding output terminals P 1  to Pm, and also in the case where the output transistors T 1  to Tm are partially disposed under the corresponding output terminals P 1  to Pm. 
       FIG. 3  is a cross-sectional view of the output transistor T 1 . The output transistor T 1  is formed on an N type semiconductor substrate  50  and covered by the output terminal P 1  through an interlayer insulation film  51 . The drain d 1  of the output transistor T 1  is connected to the output terminal P 1  disposed above through a contact hole  52  formed in the interlayer insulation film  51 . The output transistor T 1  may be formed on an N type well formed in the front surface of a P type semiconductor substrate. 
     Alternatively, in order to decrease the packaging area of the display device including the drive circuit  1  and the vacuum fluorescent displays VFD 1  to VFDm, the vacuum fluorescent displays VFD 1  to VFDm may be disposed on the corresponding output terminals P 1  to Pm. 
     Although the drive circuit  1  drives the vacuum fluorescent displays VFD 1  to VFDm in the embodiments described above, the drive circuit  1  of the invention is not limited to this and may be used to drive other display element needing a high voltage. 
     The drive circuit of the invention enables testing the breakdown voltage characteristics of the plurality of output transistors T 1  to Tm in a batch by using the control terminal commonly connected to the drains of the output transistors without using probe needles. 
     Furthermore, by applying a predetermined voltage to the control terminal, the voltages of the drains of the output transistors T 1  to Tm are initialized when the drive circuit is started or the like. 
     Furthermore, by measuring a leakage current by applying a voltage to the control terminal, a short circuit test for an inner wiring of the drive circuit is achieved.