Abstract:
A load driving device includes a drive control signal generation circuit generating a load drive control signal and a semiconductor buffer circuit generating an output signal in response to the load drive control signal. The buffer circuit has a pair of gate driven switching elements which are connected to each other in push-pull configuration and driven at their gate terminals by the load drive control signal. The buffer circuit has an output terminal which is connected to a connection point between ends of controlled electrodes of the gate driven switching elements, and a power source terminal and a ground connection terminal respectively connected to the remaining ends of the other controlled electrodes of the gate driven switching elements. A ground connection side element of a pair of gate driven switching elements has a set of MOS transistors which are connected across the connection point and the ground connection terminal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a load driving device, and particularly to a load driving device which protects a high withstand voltage MOS transistor from ESD (Electro Static Discharge).  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 1  of the accompanying drawings is a circuit diagram representing a conventional load driving device. The load driving device has a power source pad  1  and a ground connection pad  2 . The power source pad  1  and the ground connection pad  2  are connected to a power source line  3  and a ground connection line  4 , respectively. An internal circuit  5  i.e., a load drive signal generator that performs logical calculation operations, is connected between the power source line  3  and the ground connection line  4 . An output signal node SO of the internal circuit  5  is connected to a gate of a P-channel type MOS transistor  6  (called ‘PMOS’ hereinbelow) and a gate of an N-channel type MOS transistor  7  (called ‘NMOS’ hereinbelow) that constitutes an output buffer. The PMOS  6  and the NMOS  7  which are gate driven switching elements constitute an inverter having a push-pull configuration. Sources of the PMOS  6  and the NMOS  7  are connected to the power source line  3  and the ground connection line  4 , respectively. A drain of the PMOS  6  functioning as an output node NO is connected through a protection resistor  11  to the drain of the NMOS  7 , and connected through a protection resistor  12  to an output pad  8 .  
         [0005]     An anode and a cathode of a protection diode  13  are connected to the output pad  8  and the power source line  3 , respectively. An anode and a cathode of a protection diode  14  are connected to the ground connection line  4  and the output pad  8 , respectively. An NMOS  15  is connected between the power source line  3  and the ground connection line  4  so that the NMOS  15  performs a protection function between the power sources by its diode characteristic in a reverse direction. Such transistor is called as a power clamp transistor.  
         [0006]     When an electrostatic surge which is higher than a power source voltage VDD is supplied to the output pad  8  in the load driving device having such a protection circuit, a magnitude of electric potential of the power source line  3  rises to a potential which is roughly equal to an electric potential of the output pad  8  because of an operation of a protection diode in a forward direction. Since, at this moment, a magnitude of voltage between the output pad  8  and the power source line  3  is limited by property of the protection diode  13  in the forward direction, an electrostatic breakdown of the PMOS  6  is avoided. When a magnitude of electric potential of the output pad  8  further rises to a higher potential by the electrostatic surge, a protection diode  14  and the NMOS  15  that performs a protection operation between the power and ground lines fall into breakdown, so that a magnitude of voltage of the output pad  8  and the ground connection line  4  drops rapidly. A magnitude of electric potential of the output pad  8  rises until the protection diode  14  or the NMOS  15  falls into breakdown. However, an electrostatic breakdown of the NMOS  7  can be avoided because a current flowing through the NMOS  7  is limited by the protection resistors  11  and  12  that are inserted in series to each other between the output pad  8  and the ground connection line  4 . When an electrostatic surge that is lower than the ground potential is supplied to the output pad  8 , an electrostatic breakdown of the PMOS  6  and the NMOS  7  can be avoided by the protection diode  14  which is biased forwardly and the protection resistors  11  and  12 . Load driving devices which are known, for example, are disclosed in Japanese Laid Open No. 2004-71991, Japanese Laid Open No. H8-330521, Japanese Laid Open No. H11-274404, Japanese Patent No. 3386042, and Japanese Patent No. 3526853.  
         [0007]     When an electrostatic surge is supplied across the output pad  8  and the ground connection pad  2 , two surge current paths are formed. One of them is a path from the output pad  8  through the protection diode  13 , the power source line  3 , the NMOS  15 , and the ground connection line  4 , up to-the ground pad  2 . The other is a path from the output pad  8  via the protection resistor  12 , the protection resistor  11 , the NMOS  7  and the ground connection line  4 , up to the ground connection pad  4 . Even though, the NMOS  15  has a characteristic which is similar to the NMOS  7 , a breakdown of the NMOS  7  in the second surge current path may occur earlier, because an impedance of the power source line  3  and the ground connection line  4  in the first surge current path is large.  
         [0008]     Thus, a value of the protection resistor  12  inserted between the output node NO and the output pad  8  must be so large that a current flowing through the NMOS  7  in the case of a breakdown of the NMOS  7  becomes smaller than the breakdown proof current. When, for example, a load circuit such as an organic electroluminescence display which is current drive-type circuit is connected to the conventional load driving device, a voltage drop appears across the protection resistor  12 . Therefore, it is necessary to select a high power source voltage which necessitates usage of costly semiconductor devices. Further the protection resistor  12  consumes a useless power.  
         [0009]     When a load circuit such as an organic electroluminescence display is driven at a voltage of, for example, 20 V or higher, it is necessary to use a high voltage proof MOS transistor in an output circuit. However, a magnitude of breakdown proof current against an electrostatic surge current closely relates to a gate width of the high voltage proof MOS transistor. Thus the gate width becomes huge to realize a necessary magnitude of breakdown proof current, and occupies a wide area of the element.  
       SUMMARY OF THE INVENTION  
       [0010]     An object of the present invention is to provide a load driving device which can suppress a voltage drop across a protection resistor and power consumption of the protection resistor and can efficiently protect itself from an electrostatic surge applied thereto.  
         [0011]     According to one aspect of the present invention, there is provided a load driving device that includes a drive control signal generation circuit generating a load drive control signal and a semiconductor buffer circuit generating an output signal in response to the load drive control signal.  
         [0012]     The buffer circuit has a pair of a gate driven switching elements which are connected to each other in push-pull configuration and driven at their gate terminals by the load drive control signal. The buffer circuit has an output terminal which is connected to a connection point between ends of controlled electrodes of the gate driven switching elements, and a power source terminal and a ground connection terminal respectively connected to remaining ends of the other controlled electrodes of the gate driven switching elements.  
         [0013]     A ground connection side element of the pair of gate driven switching elements has a set of MOS transistors which are connected to each other across the connection point and the ground connection terminal.  
         [0014]     The ground connection side element has a resistor element connected to the controlled electrode of the set of MOS transistor in series.  
         [0015]     The set of MOS transistors includes a plurality of N-channel type MOS transistor formed on the same substrate.  
         [0016]     The drive control signal generation circuit is connected between a power source line and a ground connection line which are connected to the power source terminal and the ground connection terminal, respectively.  
         [0017]     A load driving device further includes a power source protection circuit which is juxtaposed with the drive control signal generating circuit across the power source line and the ground connection line. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a circuit diagram showing a conventional load driving device.  
         [0019]      FIG. 2  is a circuit diagram showing a load driving device of an embodiment according to the present invention.  
         [0020]      FIG. 3A  is a layout diagram showing an output buffer illustrated in  FIG. 2 .  
         [0021]      FIG. 3B  is a cross-sectional view of the layout of FIG.  3 A taken along the line  3 B- 3 B.  
         [0022]      FIG. 3C  is a cross-sectional view of the layout of  FIG. 3A  taken along the line  3 C- 3 C. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     Drawings are used for easier understanding of the invention, and do not limit the invention.  
         [0024]      FIG. 2  is a circuit diagram showing a load driving device of the invention, and similar reference numerals and symbols are used in  FIG. 1  and  FIG. 2 .  
         [0025]     The load driving device includes a power source pad  1  and a ground connection pad  2 . These power source pad  1  and ground pad  2  are connected to a power source line  3  and a ground connection line  4 , respectively. An internal circuit  5  i.e., load drive signal generator that performs logical calculation operations which operates logical process, is connected between the power source line  3  and the ground connection line  4 . An output signal node SO of the internal circuit  5  is connected to a gate of a PMOS  6 , and gates of an NMOS  7   a  and NMOS  7   b , which constitute an output buffer. The PMOS  6 , the NMOS  7   a  and the NMOS  7   b  constitute an inverter. Sources of the PMOS  6 , NMOS  7  and NMOS  7   b  are connected to the power source line  3  and the ground connection line  4 , respectively. A drain of the PMOS  6  that is an output node NO which is connected to an output pad  8 , and also connected to a drain of the NMOS  7   a  and the NMOS  7   b  through a protection resistors  11   a  and  11   b . In this embodiment, the protection resistor  11   a  has a resistance value which is equal to a resistance value of the protection resistor  11   b.    
         [0026]     The NMOS  7   a  and  7   b  may be high withstand voltage transistors each having an active layer of offset construction which is formed on an N+ diffusion area in an N− diffusion layer.  
         [0027]     It can be said that a conventional circuit constituted by the NMOS  7  and the resistor  11  in the circuit  FIG. 1  is substituted for a new circuit including a first series of NMOS  7   a  and a resistor  11   a  and a second series of NMOS  7   b  and a resistor  11   b . If the conventional circuit and the new circuit are equivalent to each other, each of the NMOSs  7   a  and  7   b  have a gate width which is a half of that of NMOS  7  and furthermore the resistor  11   a  and  11   b  have the same resistance value which is a double of that of the resistor  11 . In this embodiment, the NMOS  7   a  has a gate width equal to a gate width of the NMOS  7   b.    
         [0028]     An anode and a cathode of a protection diode  13  are connected to the output pad  8  and the power source line  3 , respectively. An anode and a cathode of a protection diode  14  are connected to the ground connection line  4  and the output pad  8 , respectively.  
         [0029]     A power source protection circuit  20  (also called as a power clamp circuit) is connected between the power source line  3  and the ground connection line  4 , which protects these circuits by a thyristor switching characteristic realized by a set of a thyristor  21  and a PMOS  22 .  
         [0030]     Several embodiments of the power source protection circuit  20  are further described in commonly assigned U.S. patent application Ser. No. 110/355,190 filed Jan. 31, 2003 entitled, “Electrostatic Breakdown Preventing Circuit for Semiconductor Device” by Fukuda, which is incorporated herein by reference. This U.S.. patent application is Japanese counterpart patent application of Japanese Patent No. 3526853 as described above.  
         [0031]      FIG. 3A  is a layout diagram showing a layout example formed in a semiconductor substrate of the output buffer illustrated in  FIG. 2 .  FIG. 3A  is plan view of the layout, and  FIGS. 3B , and  3 C are cross-sectional views taken along the lines  3 B- 3 B and  3 C- 3 C, respectively.  
         [0032]     The output buffer is, for example, configured as an IC mounted on a semiconductor substrate as seen from  FIG. 3A . This IC includes two series circuits which are connected between the output terminal NO and the ground connection line  4  and have respectively series connections of the NMOS  7   a ,  7   b  and the protection resistors  11   a ,  11   b . A transistor formation area and a resistor formation area are respectively disposed at left and right sides of the substrate as seen from  FIG. 3A .  
         [0033]     As shown in  FIG. 3B , the NMOS  7   a  and the NMOS  7   b  are formed in a p-well area defined in a silicon substrate Si. A drain D, a source S and a gate of the NMOS  7   a  and the NMOS  7   b  are partitioned by field oxide films F. The drain D and the source S i.e., active areas, have respectively offset constructions each of which is formed by an N+ diffusion area within a deep N− diffusion area. The NMOS  7   a  and the NMOS  7   b  have high withstand voltage characteristics because of depletion layers extending along boundaries between N− diffusion areas and p-well areas. The gate D is formed on a thick gate oxide film Go which extends on the p-well and between the drain D and the source S.  
         [0034]     An interconnect insulation film I is formed on the drain D, the source S and the gate G. A wiring pattern layer M is formed on the interconnect insulation film I. The N+ diffusion areas of the drain D and the source S are electrically connected to the wiring pattern layer M via contacts Cd and Cs respectively which pass at pre-selected positions through the interconnect insulation film I.  
         [0035]     As shown in  FIG. 3C , the protection resistors  11   a  and  11   b , are formed by mounting a polysilicon film P on the field oxide film F which is formed on the silicon substrate Si. The interconnect insulation film I which covers the transistor formation area further covers the polysilicon film P. The metallic wiring pattern M on the interconnect insulation film I overlies the film P. The polysilicon film P which includes the protection resistors  11   a  and  11   b  is electrically connected to the wiring pattern M via a contact Cc which passes at a pre-selected position through the interconnect insulation film I.  
         [0036]     The load driving device of this embodiment operates in response to an electrostatic surge which is supplied to an output pad  8  as will be mentioned in the following.  
         [0037]     When a positive electrostatic surge which is positive relative to that at the ground connection pad  2  is supplied to the output pad  8 , the positive electrostatic surge is supplied to series circuits each including a protection resistor  11   x , and a NMOS  7   x  (where, x represents a, b) through a node NO. The positive electrostatic surge is further supplied to the power source protection circuit  20  via the power source line  3  and the protection diode  13 , which is forward driven by the surge.  
         [0038]     When the positive electrostatic surge rises up, the power source protection circuit  20  breaks down earlier than the NMOS  7   x . In other words, when a breakdown of the PMOS  22  occurs, a potential of a gate of the thyristor  21  becomes equal to the ground potential so that this thyristor  21  becomes ‘on’. The positive electrostatic surge does not cause any problem if a magnitude of voltage across the power source line  3  and the ground connection line  4  decreases. It is natural to design that the power source protection circuit  20  breaks down earlier than the NMOS  7   x . The breakdown of the power source protection circuit  20  may not always occur before breakdown of the NMOS  7   x , because an impedance of the power source line  3  and the ground connection line  4  up to the power source protection circuit  20  depends on a position of the power source protection circuit  20 .  
         [0039]     When a breakdown of the NMOS  7   x  occurs earlier than a breakdown of the power source protection circuit  20 , a positive electrostatic surge current branches into the NMOS  7   a  and the NMOS  7   b . The NMOS  7   a  and the NMOS  7   b  are respectively connected to the protection resistors  11   a  and  11   b  in series each other which have relatively large resistance. A current flowing in each of the NMOS  7   a  and the NMOS  7   b  is relatively small. Thus, the current flowing through the NMOS  7   a  and NMOS  7   b  is about one half of a current flowing through the NMOS  7  in  FIG. 1 , the NMOS  7   a  and NMOS  7   b  are not broken immediately, and a magnitude of potential of the node NO rises up by a voltage drop which depends on the protection resistors  11   a  and  11   b.    
         [0040]     If a breakdown of the power source protection circuit  20  occurs earlier than a breakdown of the NMOS  7   a  and the NMOS  7   b , a magnitude of voltage across the power source line  3  and the ground connection line  4  lowers rapidly, and the NMOS  7   a  and the NMOS  7   b  are protected or free from the electrostatic surge. Therefore, the load drive device has an improved electrostatic surge proof characteristic in comparison with the circuit of  FIG. 1  and lower possibility of breakdown of the NMOS  7   a  and the NMOS  7   b.    
         [0041]     In an operation of the NMOS  7   a  and the NMOS  7   b  as the output buffer in the load driving device according to the invention, the NMOS  7   a  and the NMOS  7   b  having gates with a relatively small gate width are coupled between the output node NO and the ground connection line  4 . Furthermore, the protection resistors  11   a  and  11   b  having relatively large resistances are connected between the output node NO and the ground connection line  4  through the NMOS  7   a  and the NMOS  7   b  in parallel. Therefore, the circuit of the invention has a driving ability which is similar to a conventional circuit. Since the load driving device of the invention does not need a protection resistor between a node NO and the output pad  8 , the circuit of the invention can suppress voltage drop and power consumption of a protection resistor. In the embodiment described above, the load driving circuit according to the invention does not include a resistor which is connected to the output pad  8  in series, but it is obvious that a relatively small resistor may be connected to the output pad  8 , if necessary.  
         [0042]     The present invention is not be limited to the embodiment described above, but covers various modifications thereof. For example:  
         [0043]     (1) In the illustrated embodiment the output buffer includes the NMOS  7   a  and the NMOS  7   b  which are juxtaposed, but the load driving device according to the invention may have more than three NMOSs which are connected in parallel with each other.  
         [0044]     (2) The load driving device according to the present invention has a thyristor  21  and a PMOS  22  which controls a gate of the thyristor  21 , but the load driving device according to the invention may have thyristor having an element which controls a gate of other thyristor.  
         [0045]     This application is based on Japanese Patent Application No. 2005-260053 filed on Sep. 8, 2005, and the entire disclosure thereof is incorporated herein by reference.