Abstract:
ESD protection circuit which can effectively protect a product with three or two leveled electrodes in any cases when an external (+) or (−) ESD charge flows into the product, including, in case of the ESD protection circuit for a product with three leveled electrodes(VP, VDD and GND), a first conduction type bipolar transistor and a second conduction type bipolar transistor connected in parallel between an input terminal and a GND, wherein the first conduction type bipolar transistor has a base terminal with a VP voltage applied thereto and the second conduction type bipolar transistor having a base terminal with a VDD voltage applied thereto, and collectors and emitters thereof connected to the input terminal or the GND.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ESD(Electro Static Discharge) protection circuit, and more particularly, to an ESD protection circuit which can effectively protect a product with three or two leveled electrodes in any cases when an external (+) or (−) ESD charge flows into the product. 
     2. Background of the Related Art 
     A related art ESD protection circuit will be explained with reference to the attached drawings. FIG. 1 illustrates a circuit diagram showing a related art ESD protection circuit. Referring to FIG. 1, the related art ESD protection circuit has two MOS transistors TR 1  and TR 2 , wherein the first MOS transistor is a field MOS transistor TR 1  and the second MOS transistor is an active MOS transistor TR 2 . The field MOS transistor TR 1  has a gate and a source connected to one pad PAD 1  in common, and the active MOS transistor TR 2  has a gate connected to the ground GND and a source connected to one pad PAD 1 . And, drains of the field MOS transistor TR 1  and the active MOS transistor TR 2  are connected in together. The unexplained ‘A’ represents a discharge path of a (+) ESD charge flowing into the PAD 1 , and ‘B’ is a discharge path of a (−) ESD charge flowing into the PAD 1 . In a case when a (+) or (−) ESD charge flows from the PAD 1  to the PAD 2 , two transistors identical to the ones connected to the PAD 1 , i.e., a field MOS transistor TR 3  and an active MOS transistor TR 4 , are provided. In the aforementioned ESD protection circuit, when a (+) ESD charge flows from PAD 1  to PAD 2 , the (+) ESD charge flows to the ground terminal GND via the field MOS transistor TR 1  having a gate and a source connected to the PAD 1 , and is discharged to the PAD 2  via the active MOS transistor TR 4  having a source connected to the PAD 2 . And, when a (−) ESD charge flows from PAD 1  to PAD 2 , the (−) ESD charge flows to the ground terminal GND via the active MOS transistor TR 2  having a source connected to the PAD 1 , and is discharged to the PAD 2  via the field MOS transistor TR 3  having a gate and a source connected to the PAD 2 . Thus, the related art ESD protection circuit is comparatively strong against (+)(−) ESD charge. 
     However, the aforementioned related art ESD protection circuit has the following problems. 
     First, application to a device with three leveled electrodes(−, +and GND), such as CCD(Charge Coupled Device), is not possible, and adjustment of a threshold voltage Vt of the field transistor to a voltage higher than 15 V is difficult for a case the ESD protection circuit should be operative at a voltage higher than 15 V. 
     Second, since an inversion layer of the MOS(Metal Oxide Semiconductor) is used, the current flows through a surface, rather than bulk. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an ESD protection circuit that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an ESD protection circuit which can protect a product with three leveled electrodes, such as a CCD, or two leveled electrodes, such as DRAM, in any case when an external (+) or (−) ESD charge flows in. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the ESD protection circuit for a product with three leveled electrodes(VP, VDD and GND) includes a first conduction type bipolar transistor and a second conduction type bipolar transistor connected in parallel between an input terminal and a GND, wherein the first conduction type bipolar transistor has a base terminal with a VP voltage applied thereto and the second conduction type bipolar transistor having a base terminal with a VDD voltage applied thereto, and collectors and emitters thereof connected to the input terminal or the GND. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 illustrates a circuit diagram showing a related art ESD protection circuit; 
     FIG. 2 illustrates a circuit diagram showing an ESD protection circuit in accordance with a preferred embodiment of the present invention; 
     FIGS. 3A and 3B illustrate ESD protection circuits between terminals each having a VP and VDD voltage applied thereto; 
     FIG. 4 illustrates a circuit diagram showing a discharge path of a (+)(−) ESD protection circuit in an ESD protection circuit of the present invention; and, 
     FIG. 5 illustrates a circuit diagram showing an ESD protection circuit in accordance with a another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. FIG. 2 illustrates a circuit diagram showing an ESD protection circuit in accordance with a preferred embodiment of the present invention. 
     Referring to FIG. 2, for an ESD protection circuit for a product with three leveled electrodes(VP, VDD, GND), the present invention suggests to provide one pair of an NPN bipolar transistor TR 1  and a PNP bipolar transistor TR 2  connected in parallel between an input terminal PAD 1  and a ground terminal GND. VP is provided to a base of the NPN bipolar transistor TR 1  as a lowest negative voltage(VP=−9 V in a case of the CCD), VDD is provided to a base of the PNP bipolar transistor TR 2  as a highest positive voltage(VDD=15 V in a case of the CCD), and an emitter and a collector are connected to the PAD 1  and the GND, respectively. One pair of an NPN bipolar transistor TR 3  and a PNP bipolar transistor TR 4  are connected in parallel between an input terminal PAD 2  and a ground terminal GND. VP is provided to a base of the NPN bipolar transistor TR 3  as a lowest negative voltage(VP=−9 V in a case of the CCD), VDD is provided to a base of the PNP bipolar transistor TR 5  as a highest positive voltage(VDD=15 V in a case of the CCD), and an emitter and a collector are connected to the PAD 2  and the GND, respectively. And, an emitter or collector is connected between the PAD 1 , PAD 2  and GND, the PNP bipolar transistor TR 5  and the NPN bipolar transistor TR 6  having VP or VDD applied thereto are provided between respective base 4 s and emitters or collectors. The bipolar transistors are vertical or horizontal type. And, in an ESD protection circuit for a product with two leveled electrodes(VDD, VSS), an NPN bipolar transistor TR 1  and a PNP bipolar transistor TR 2  are connected in parallel between a PAD and a ground terminal GND, collectors or emitters of the NPN bipolar transistor TR 1  and the PNP bipolar transistor are respectively connected to input terminals or ground terminals, a base terminal of the NPN bipolar transistor TR 1  is connected to a ground terminal, and a base of the PNP bipolar transistor TR 2  is connected to VDD voltage. The unexplained ‘C’ denotes a discharge path of the (+) ESD charge flowing to the PAD 1 , ‘D’ denotes a discharge path of (−) ESD charge flowing to the PAD 1 . 
     FIGS. 3A and 3B illustrate ESD protection circuits between terminals each having a VP and VDD voltage applied thereto. 
     Referring to FIG. 3A, the ESD protection circuit having an emitter or collector terminal with a VDD voltage applied thereto may include only one NPN bipolar transistor  7  having a base with a VP applied thereto, or an NPN bipolar transistor TR 7  having a base with VP applied thereto and a PNP bipolar transistor TR 8  having a collector(or emitter) and a base both with VDD applied thereto. And, the collector or emitter terminal having VDD voltage applied thereto is connected to relevant PAD with a large sized diode, no separate protection circuit is required at times. And, as shown in FIG. 3B, the ESD protection circuit having a collector or emitter terminal with VP voltage applied thereto may includes only one PNP bipolar transistor TR 9  having a base with VDD applied thereto, or an NPN bipolar transistor TR 10  having a collector and a base both with VDD applied thereto and a PNP bipolar transistor TR 9  having a collector(or emitter) and a base connected to VP together. And, the collector or emitter terminal having VP voltage applied thereto is connected to relevant PAD with a large sized diode, no separate protection circuit is required at times. 
     The operation of the aforementioned ESD protection circuit of the present invention will be explained. FIG. 4 illustrates a circuit diagram showing a discharge path of a (+)(−) ESD protection circuit in an ESD protection circuit of the present invention. 
     Referring to FIG. 4, when a (+) ESD charge flowed into one PAD 1  is discharged through one PAD 2 , the (+) ESD charge from the PAD 1  is discharged to GND via the PNP bipolar transistor TR 2  exhibiting forward characteristics to the (+) ESD charge, and is discharged again from the GND via the NPN bipolar transistor TR 6  also exhibiting forward characteristics. And, when a (−) ESD charge flowed into one PAD 1  is discharged through one PAD 2 , the (−) ESD to charge from the PAD 1  is discharged to GND via the NPN bipolar transistor TR 1  exhibiting forward characteristics to the (+) ESD charge, and is discharged again from the GND via the NPN bipolar transistor TR 6  also exhibiting forward characteristics. When a (+) or (−) ESD charge flowed into the PAD 1  is discharged through VDD terminal, the (+) ESD charge is discharged with easy by a PN diode in the PNP bipolar transistor TR 2  of forward characteristics, the (−) ESD charge, as explained before, is discharged to GND via the NPN bipolar transistor TR 1 , and discharged again to the VDD terminal via the NPN bipolar transistor TR 6 . And, in a case when the discharge is made from PAD 2  to VP, (−) ESD charge is discharged with easy by an NP diode in the NPN bipolar TR 3  of forward characteristics, and (+) ESD charge, as explained before, is discharged to GND via the PNP bipolar transistor TR 4 , and discharged to VP again via the PNP bipolar transistor TR 5 . The unexplained ‘E’ denotes a discharge path of (+) ESD charge having forward characteristics, ‘F’ is a discharge path of (−) ESD charge having forward characteristics. 
     FIG. 5 illustrates a circuit diagram showing an ESD protection circuit in accordance with a another preferred embodiment of the present invention. 
     Referring to FIG. 5, in order to induce an effective discharge between VDD and VP, a bipolar transistor TR 11  is provided between VDD and VP. An emitter or collector of the bipolar transistor TR 11  is connected to the VDD, and a base and collector or emitter are connected to VP in common. 
     As has been explained, the ESD protection circuit of the present invention always facilitates bipolar action for an external ESD charge in a product with three leveled electrodes, such as a CCD, or two leveled electrodes, such as a DRAM by means of forward diodes, thereby improving a discharge efficiency, significantly. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the ESD protection circuit of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.