Patent Abstract:
An integrated circuit aims to decrease a parasitic resistance between an input protection circuit cell and a power supply cell including a clamp circuit, restrain a size of a diode from increasing beyond ESD robustness of the diode itself in order to compensate for a decrease in the ESD robustness, and prevent high-frequency signal power from decreasing due to a large capacitance component from a diode in an input protection circuit and a parasitic resistance component from a series resistor. The input protection circuit cell includes: an input terminal coupled to a signal pin; an output terminal coupled to not only a high-frequency circuit but also the input terminal and a node; a diode that is provided between the node and VDD and makes an electric current flow from the node to VDD; another diode that is provided between the node and GND and makes an electric current flow from GND to the node; and a clamp circuit coupled between VDD and GND parallel to the diodes.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The disclosure of Japanese Patent Application No. 2008-149200 filed on Jun. 6, 2008 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an integrated circuit including an input protection circuit cell. 
       FIG. 1  shows an input protection circuit cell  70  and a power supply cell  71  according to a conventional technology. To ensure ESD robustness, the input protection circuit cell  70  according to a conventional technology uses diodes  1  and  2 . The diodes each have a capacitance of several picofarads and are reversely coupled between a power supply voltage (VDD) and a signal input line and between the signal input line and a ground (GND). A series resistor  40  is serially coupled to the signal input line. In a power supply cell  51 , as shown in  FIG. 1 , an input circuit  43  and a clamp circuit  4  are parallel coupled between VDD and GND. 
       FIG. 2  shows in more detail the inside of the input circuit  43  and the clamp circuit  4  according to the conventional technology. A diode  44  is coupled between input and output sides of the input circuit  43 . The clamp circuit  4  includes a resistor  11  and a capacitor  12  that are serially coupled between VDD and GND. The clamp circuit  4  also includes an NMOS transistor  10  that uses the source terminal for the power supply voltage and the drain terminal for the ground. The clamp circuit  4  further includes an inverter  13  that has an input section coupled to a node between the resistor  11  and the capacitor  12  and an output section coupled to the gate terminal of the NMOS transistor  10 . 
       FIG. 3  shows an example layout of the input protection circuit cell  70  and the power supply cell  71 . 
       FIG. 4  shows a chip layout of an integrated circuit  80  mounted with an input protection circuit cell and a power supply cell according to a conventional technology. As shown in  FIG. 4 , the layout is configured to include input protection circuit cells  50  and  51  and power supply cells  26  and  29 . The input protection circuit cells  50  and  51  each are equivalent to the input protection circuit cell  70 . The power supply cells  26  and  29  each are equivalent to the power supply cell  71 . The power supply cells  26  and  29  include clamp circuits  26   a  and  29   a . In the overall chip configuration, as shown in  FIG. 4 , the input protection circuit cells  50  and  51  are located adjacently to an RF input pad. The power supply cells  26  and  29  including the clamp circuit  4  are located adjacent to a VDD pad  20  or a GND pad  25 . 
     Patent Documents 1 and 2 describe the methods of decreasing parasitic capacitance in an ESD protection circuit. 
     Patent Document 1: Japanese Unexamined Patent Publication No. 2007-311813 
     Patent Document 2: Japanese Unexamined Patent Publication No. 
     SUMMARY OF THE INVENTION 
     There has been a problem that a high-frequency signal supplied to an RF input pad attenuates due to a large input capacitance from the diodes  1  and  2  of several picofarads and parasitic resistance components from the series resistor  40 . 
     The following describes why the diode capacitance increases. The input protection circuit cells  50  and  51  are routed to the power supply cells  26  and  29  via wiring metal such as discharge paths  100  and  101  in  FIG. 4 . The discharge path  100  (discharge path  9  in  FIG. 1 ) provides the shortest distance from the pulse input to the GND. The discharge path  101  includes the power supply cell  26  adjacent to the VDD pad. A clamp circuit  26   a  of the power supply cell  26  is distant from the RF input pad and the GND pad. The parasitic resistance  41  in  FIG. 1  cannot ensure a low impedance, making the ESD robustness insufficient. 
     To solve this problem, the diode needs to decrease the operating voltage when an ESD is applied. For this reason, the diode requires a larger size than is used for maintaining the specified ESD robustness. The diode capacitance increases accordingly. 
     For example, let us consider that a human body model electrostatic discharge (HBM/ESD) test is conducted on the circuit in  FIG. 1  to apply a positive surge voltage of 2000 V to the RF input pad with reference to the GND. In this case, a peak current approximates to 1.33 A. Let us suppose that the clamp circuit  4  operates on Vc (V) and that the diode  1  has a size large enough to satisfy an HBM withstand voltage of 2000 V or higher and operates on Vd (V). In this case, the voltage drops approximately by 1.33 Rp (V) when the parasitic resistance  41  is Rp (O). In total, the RF input pad is supplied with (Vc+Vd+1.33 Rp) voltages. 
     A circuit to be protected is assumed to be supplied with a degradation start voltage Vic set to 11 V. The degradation start voltage Vic is expressed as follows: Vic (=11 V)&lt;Vc+Vd+1.33 Rp (=5+2.3+4=11.3 V), where Vc is set to 5 V, Vd to 2.3 V, and Rp to 3 O. The circuit to be protected is supplied with a voltage higher than or equal to the degradation start voltage. It is impossible to satisfy the condition of the HBM withstand voltage of 2000 V or higher. The diode  1  requires increasing its area so as to keep the operating voltage Vd smaller than or equal to (Vic−Vc−1.33 Rp)=11−5−4=2 V even though the diode  1  itself has the size large enough to ensure the HBM withstand voltage of 2000 V or higher. Since the discharge path causes a high resistance, the series resistor  40  is also needed to protect the internal circuit. 
     The methods described in Patent Documents 1 and 2 reduce parasitic capacitance in the ESD protection circuit. However, the methods do not decrease the resistance on the discharge path and do not solve the problem due to the high resistance. 
     The present invention has been made in consideration of the foregoing. It is therefore an object of the present invention to decrease parasitic resistance between an input protection circuit cell and a clamp circuit in an integrated circuit, restrain a diode size from becoming larger than ESD robustness of a diode itself for compensating a decrease in the ESD robustness, and prevent high-frequency signal power from attenuating due to input capacitance components from a diode in an input protection circuit and parasitic resistance components of a series resistor. 
     An integrated circuit according to an embodiment of the invention includes: a signal pin; an internal circuit including a high-frequency circuit; and an input protection circuit cell that is placed between the signal pin and the internal circuit and performs a protection operation when a signal at the signal pin is applied to the high-frequency circuit. The input protection circuit cell includes: an input terminal coupled to the signal pin; an output terminal that is coupled to the high-frequency circuit and to the input terminal via a coupling node; a first diode that is provided between the coupling node and a high-voltage power supply and makes an electric current flow from the coupling node to the high-voltage power supply; and a second diode that is provided between the coupling node and a low-voltage power supply and makes an electric current flow from the low-voltage power supply to the coupling node. The input protection circuit cell further includes: a clamp circuit that is coupled parallel to the first and second diodes between the high-voltage power supply and the low-voltage power supply. 
     The integrated circuit according to the embodiment of the invention configures the clamp circuit in the input protection circuit cell. This makes it possible to provide more discharge paths for an ESD pulse current than conventional arts and reduce the impedance. The reduction in the impedance for the discharge path can minimize the size of the diode for the input protection circuit so that the ESD robustness can be ensured. It is possible to reduce a loss in the high-frequency signal while ensuring the ESD robustness. The reduction in the impedance for the discharge path can eliminate a serially coupled resistor element and reduce a loss in the high-frequency signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit including an input protection circuit cell and a power supply cell according to a conventional technology; 
         FIG. 2  shows a circuit including an input protection circuit cell and a power supply cell according to a conventional technology; 
         FIG. 3  shows a layout including an input protection circuit cell and a power supply cell according to a conventional technology; 
         FIG. 4  shows a chip layout according to a conventional technology; 
         FIG. 5  shows a circuit of an input protection circuit cell according to the invention; 
         FIG. 6  shows a circuit of the input protection circuit cell according to the invention; 
         FIG. 7  shows a layout of the input protection circuit cell according to the invention; 
         FIG. 8  shows a circuit of the input protection circuit cell according to the invention; 
         FIG. 9  shows a layout of the input protection circuit cell according to the invention; 
         FIG. 10  shows a chip layout according to the invention; and 
         FIG. 11  shows an enlarged part of the chip layout according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes the invention with reference to the accompanying drawings showing embodiments. 
     &lt;Configuration&gt; 
       FIG. 5  shows a circuit diagram of the input protection circuit cell according to an embodiment of the invention. 
     An input protection circuit cell  70  is located between a signal pin of a mounted integrated circuit and an internal circuit including a high-frequency circuit. The input protection circuit cell  70  includes an input terminal  7  and an output terminal  8 . The input terminal  7  is coupled to the signal pin. The output terminal  8  is coupled to the high-frequency circuit as well as the input terminal  7  and a node (coupling node)  60 . A first diode  1  is provided between the node  60  and a high-voltage power supply (VDD)  5  and supplies an electric current from the node  60  to the VDD  5 . A second diode  2  is provided between the node  60  and a low-voltage power supply (GND)  6  and supplies an electric current from the GND  6  to the node  60 . Further, a clamp circuit  4  is coupled between the VDD  5  and the GND  6  parallel to the diodes  1  and  2 . Reference numeral  3  denotes parasitic resistance along the discharge path  9 . 
       FIG. 6  shows a circuit diagram of the input protection circuit cell  70  including the clamp circuit  4  whose inside is revealed. The clamp circuit  4  includes an NMOS transistor  10  having a source terminal coupled to the VDD  5  and gate and drain terminals coupled to the GND  6 . 
       FIG. 7  illustrates a layout example of the input protection circuit cell  70  shown in  FIG. 6 . The input protection circuit cell  70  includes a VDD ring  30  (first wiring) and a GND ring  5   b  (second wiring). The VDD ring  30  supplies VDD. The GND ring  5   b  is placed parallel to the VDD ring  30  and supplies GND. The input protection circuit cell  70  further includes diodes  1  and  2 . The diode  1  is placed between the VDD ring  30  and GND ring  5   b  near the VDD ring  30 . The diode  2  is placed between the VDD ring  30  and GND ring  5   b  near the GND ring  5   b . The input protection circuit cell  70  furthermore includes the NMOS transistor  10 , an output terminal  8   a , and an input terminal  7   a . The NMOS transistor  10  is placed outside the GND ring  5   b . The output terminal  8   a  is placed outside the VDD ring  30 . The input terminal  7   a  is placed outside the NMOS transistor  10 . 
     Compared to a conventional technology, the embodiment reduces the size of the diodes  1  and  2  by reducing the impedance of the discharge path to be described later. Since the NMOS transistor  10  is used as a clamp circuit, it is possible to reduce the layout area in comparison with the clamp circuit  4  in  FIG. 2  according to the conventional technology. The clamp circuit  4  can be contained in the input protection circuit cell  70 . The embodiment eliminates the series resistor  40  that is used for the conventional technology. 
       FIG. 8  shows a circuit diagram of the input protection circuit cell  70  including the clamp circuit  4  according to another mode. The clamp circuit  4  includes the resistor  11  and the capacitor  12 . The resistor  11  is coupled between the VDD  5  and a node  61  (first node). The capacitor  12  is coupled between the node  61  and the GND  6 . The clamp circuit  4  also includes the inverter  13  and the NMOS transistor  10 . The inverter  13  is coupled between the node  61  and a node  62  (second node). The inverter has an input side corresponding to the node  61  and an output side corresponding to the node  62 . The NMOS transistor  10  has a gate terminal coupled to the node  62 , a source terminal coupled to the VDD  5 , and a drain terminal coupled to the GND  6 . 
       FIG. 9  shows a layout example of the input protection circuit cell  70  shown in  FIG. 8 . The input protection circuit cell  70  includes: the VDD ring  30  that supplies VDD; a GND ring  31  placed parallel to the VDD ring  3  and supplies GND; the diode  1  (first diode) placed under the VDD ring  30 ; and the diode  2  (second diode) placed between the VDD ring  30  and the GND ring  31  beside the diode  1 . The input protection circuit cell  70  further includes: the resistor  11  placed between the VDD ring  30  and the GND ring  31  beside the diode  2 ; the capacitor  12  placed between the VDD ring  30  and the GND ring  31  beside the resistor  11 ; the inverter  13  placed under the GND ring  31 ; the NMOS transistor  10  placed outside the GND ring  31 ; an output terminal  8   a  placed outside the VDD ring  30 ; and an input terminal  7   a  placed outside the NMOS transistor  10 . 
     Compared to a conventional technology, the embodiment reduces the size of the diodes  1  and  2  by reducing the impedance of the discharge path to be described later. The input protection circuit cell  70  in  FIG. 8  uses a clamp circuit equivalent to the clamp circuit  4  in  FIG. 2  according to the conventional technology. The circuit design is changed to halve the capacitor  12  and double the resistor  11 . The clamp circuit  4  can be contained in the input protection circuit cell  70  by matching the CR time constant and reducing the layout area. The embodiment eliminates the series resistor  40  that is used for the conventional technology. 
       FIG. 10  shows a chip layout of the integrated circuit  80  according to the embodiment. The integrated circuit  80  contains a high-frequency circuit  81  and a low-frequency circuit  82 . Near the high-frequency circuit  81 , there are provided the VDD pad  20 , a control signal pad  21 , an RF input pad  22 , an RF input pad  23 , a control signal pad  24 , and the GND pad  25 . The VDD pad  20  and the GND pad  25  contain power supply cells  26  and  29  (equivalent to the power supply cell  71  in  FIGS. 1 and 2 ) that further contain clamp circuits  26   a  and  29   a . The RF input pads  22  and  23  contain input protection circuit cells  27  and  28  (equivalent to the input protection circuit cell  70  in  FIGS. 1 and 2 ). 
     The input protection circuit cells  27  and  28  are placed between the RF input pads  22  and  23  coupled to signal pins of the integrated circuit  80  and the internal circuit containing the high-frequency circuit  81 . The input terminal  7  in  FIG. 5  is coupled to the RF input pads  22  and  23 . The output terminal  8  is coupled to the high-frequency circuit  81 . 
     The VDD ring  30  and the GND ring  31  are wired so as to be common to the entire chip.  FIG. 11  shows an enlarged view around the cells in  FIG. 10 . 
     &lt;Operations&gt; 
     The following describes operations. First, clamp circuit operations will be described. 
     The clamp circuit  4  in  FIGS. 6 and 7  uses characteristics of an NMOS transistor that the NMOS transistor causes a parasitic bipolar operation outside a normal operation range. During a normal operation, the NMOS transistor  10  hardly makes an electric current flow because the drain is supplied with a specified voltage. When a surge is applied, the drain is supplied with a large voltage. The NMOS transistor  10  causes a parasitic bipolar operation to makes an electric current flow through the discharge path  9 . 
     During a normal operation, the clamp circuit in  FIGS. 8 and 9  allows the inverter  13  to be supplied with an H level voltage and output an L level voltage. The NMOS transistor  10  turns off. Immediately after a surge is applied, the source of a PMOS transistor (not shown) for the inverter  13  is supplied with the applied surge voltage. The gate of the PMOS transistor for the inverter  13  maintains a voltage before the surge is applied. A voltage difference between the source and the gate turns on the PMOS transistor for the inverter  13 . Turning on the PMOS transistor increases a gate potential of the NMOS transistor  10  to turn on the NMOS transistor  10 . The current caused by the applied surge flows through the discharge path  9  to the GND. A delay circuit including the resistor  11  and the capacitor  12  propagates the applied surge voltage. The PMOS transistor for the inverter  13  turns off. The NMOS transistor  10  turns off to terminate the discharge. 
     The clamp circuit  4  is not limited to the configurations as shown in  FIGS. 6 ,  7 ,  8 , and  9 . The clamp circuit  4  may be configured not to apply the current to the discharge path  9  during a normal operation and to apply the current to the discharge path  9  when a surge is applied. 
       FIG. 11  shows the discharge path in the integrated circuit  80 . The conventional technology uses only the discharge paths  100  and  101  in consideration for application of a surge to the RF input pad  22 . According to the embodiment, the input protection circuit cells  27  and  28  contain the clamp circuits having discharge paths  200  and  201  shorter than the discharge path  101 . 
     Similarly to the conventional technology, let us examine a case where the human body model electrostatic discharge (HBM/ESD) test applies a positive surge of 2000 V from the RF input pad to the input terminal  7  with reference to the GND. The increased paths decrease the parasitic resistance for the discharge path in the entire chip in comparison with the conventional technology that uses the clamp circuits only in the power supply cell. It is assumed that the parasitic resistance becomes 0.3 O while the conventional technology indicates 3 O. 
     The peak current approximates to 1.33 A. The clamp circuit  4  is assumed to operate on Vc (V). The diode itself is assumed to be so sized as to satisfy the HBM withstand voltage of 2000 V or higher and is assumed to operate on Vd (V). The voltage drops by 1.33 Rpp (V) when the parasitic resistance  3  is assumed to be Rpp (O). In total, the RF input pad is supplied with (Vc+Vd+1.33 Rpp) (V). 
     Let us assume that Vic is set to 11 V as a degradation start voltage for the circuit to be protected. The voltage is expressed as: Vic (=11V)&gt;Vc+Vd+0.133 Rp (=5+2.3+0.4=7.7 V), where Vc=5 V, Vd=2.3 V, and Rpp=0.3 O. These networks do not reach the degradation start voltage for the circuit to be protected and are capable of discharging an ESD pulse current caused by the HBM test. 
     &lt;Effects&gt; 
     The discharge path can be increased as shown in  FIG. 11  because the clamp circuit  4  is contained in the input protection circuit cell  70 . The discharge path  200  can ensure the shortest distance from the GND because the clamp circuit is always available adjacently to the input pad. It is possible to use the discharge path  201  provided by the clamp circuit in the other input protection circuit. The conventional discharge paths  100  and  101  are also available. That is, it is possible to increase the discharge paths for an ESD pulse current, maintain a low impedance, and ensure the number of clamp circuits needed in the chip. Since the discharge path ensures a low impedance, it is possible to minimize the size of the diodes  1  and  2  in the input protection circuit cell  70  so that the ESD robustness is ensured. It is possible to reduce a loss in the high-frequency signal while the ESD robustness is ensured. Since the discharge path ensures a low impedance, it is possible to eliminate the serially coupled series resistor  4  and reduce a loss in the high-frequency signal. 
     The embodiment of the invention can be applied to RFICs for wireless LAN and the other high-frequency integrated circuits in general.

Technology Classification (CPC): 7