Patent Publication Number: US-9407084-B2

Title: Over-voltage protection circuit

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to over-voltage protection circuits suitable and, more particularly, to over-voltage protection of analog functional modules. 
     Analog functional modules, for example, an analog-to-digital converter (ADC) of a system on chip (SOC) device sometimes need to handle input voltages that exceed the voltage rating of the devices constituting the analog functional module. One known way of handling high analog input voltage ranges on a SOC incorporating devices with a lower rating employs a resistor divider network connected at the front end of the analog module. However, the resistor divider network constitutes a continuous load on the input, which results in inefficiency as, in the particular example of a multi-channel ADC, one of the channels may be idle for some considerable time. Cutting-off the continuous loading of the divider network can overcome the inefficiency problem. However, in so doing, the functional module may see a high voltage at its input, which can affect reliability. Using comparatively high resistor values for the divider network will reduce the loading but, disadvantageously, increase RC time delays in the input path. 
     Hence it would be advantageous to provide a front-end circuit that can eliminate input loading without the drawbacks of the known arrangements while also providing input over-voltage protection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic circuit diagram of an input circuit including over-voltage protection circuitry in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practised. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. Furthermore, terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that module, circuit, device components, structures and method steps that comprises a list of elements or steps does not include only those elements but may include other elements or steps not expressly listed or inherent to such module, circuit, device components or steps. An element or step proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements or steps that comprises the element or step. 
     In one embodiment, the present invention provides an over-voltage protection circuit having an input, an output, a potential divider connected to the input, a first switch arrangement connected to the potential divider and having a select input for receiving a select signal, and a second switch arrangement connected between the potential divider at a first node and the output and having a control input connected to the potential divider and the first switch arrangement at a second node. In a first mode of operation, the first switch arrangement enables the potential divider in response to receipt of a select signal and closes the second switch arrangement, thereby permitting a voltage at the first node to be transferred to the output at a third node. In a second mode of operation, in response to the select signal, the first switch arrangement disables the potential divider. 
     Referring now to  FIG. 1 , a circuit having an input  100  and which includes an over-voltage protection circuit  101  including a potential divider comprising first and second resistors  102 ,  103 , is shown. An output of the over-voltage protection circuit  101  is coupled to a sampling switching circuit  104 . A sampling capacitor  105  is connected between an output of the sampling switching circuit  104  and ground. In one embodiment, the circuitry  101 - 105  forms a front end of an Analog to Digital Converter (ADC) with the output of the sampling switching circuit  104  being fed to further conventional ADC circuitry  106 . In one embodiment, the over-voltage protection circuit  101  is implemented in a SOC device that typically includes a plurality of other functional modules. 
     The over-voltage protection circuit  101  ensures that the input to the sampling switching circuit  104 , and hence the ADC circuitry  106  is limited to a supply voltage VDD. The over-voltage protection circuit  101  also has the capability to disable the potential divider  102 ,  103  during an ‘off’ condition to be explained below. 
     The over-voltage protection circuit  101  includes a first switch arrangement including first and second switches, which in this example comprise first and second N channel metal oxide silicon field effect transistors (NMOSFET)  107  and  108  respectively, connected in series between the potential divider  102 ,  103  and ground serve to either connect or disconnect the potential divider to or from ground in response to a first select signal SELA. A second switch arrangement comprising third and fourth switches, which in this example comprise a third NMOSFET  109  and a P channel metal oxide silicon field effect transistor (PMOSFET)  110 , connected in parallel with one another, serve to protect the input of the sampling switching circuit  104  from an over-voltage at the input  100 . A third switch arrangement comprises fifth and sixth switches, which in this example comprise fourth and fifth NMOSFETs  111 ,  112  respectively, connected in series, that serve as additional protection devices to ensure none of the transistors comprising the over-voltage protection circuit  101  is subjected to voltages higher than its rating. 
     The sampling switching circuit  104  comprises seventh and eighth switches, which in this example comprise sixth and seventh MOSFETs  113 ,  114  connected in parallel and controlled by first and second select signals SELA and SELB respectively (where SELB is the opposite of SELA). The sixth MOSFET  113  is an NMOSFET and the seventh MOSFET  114  is a PMOSFET. 
     The circuitry of  FIG. 1  will now be described in greater detail. A first terminal of the first resistor  102  receives an input voltage on line  100 . A second terminal of the first resistor  102  is connected to a first terminal of the second resistor  103  at a first node  115 . A second terminal of the second resistor  103  is connected to a first terminal of a delay circuit  116 , a drain terminal of the first NMOSFET  107  and a gate terminal of the fourth NMOSFET  111  at a second node  117 . In one example, the delay circuit  116  is a simple RC circuit. A second terminal of the delay circuit  116  is connected to a gate terminal of the PMOSFET  110 . A source terminal of the PMOSFET  110  and a drain terminal of the third NMOSFET  109  are both connected to the first node  115 . A gate terminal of the third NMOSFET  109  is connected to the supply voltage VDD. A source terminal of the third NMOSFET and a drain terminal of the PMOSFET  110  are both connected to a third node  118 . Thus, a divided-down input voltage (appearing at the first node  115 ) is transferred to the second node  118 , which serves as the input to the sampling switching circuit, by the NMOSFET-PMOSFET pair  109 ,  110  when the circuit of  FIG. 1  in an ‘on’ condition. 
     A gate terminal of the first NMOSFET  107  is connected to the power supply VDD. A source terminal of the first NMOSFET  107  is connected to a drain terminal of the second NMOSFET  108  and a source terminal of the fourth NMOSFET  111  at a fourth nodal pint  119 . A gate terminal of the second NMOSFET receives the first select signal SELA on line  120  which is generated elsewhere on the SOC. The SELA select signal can be generated by any appropriate logic function and SELA will be set high when it is required that a sample of the input voltage is to be taken, otherwise it will be set low. A source terminal of the second NMOSFET  108  is connected to ground. A drain terminal of the fourth NMOSFET  111  and a drain terminal of the fifth NMOSFET  112  are connected to the supply voltage VDD. A gate terminal of the fifth NMOSFET  112  is connected to the first node  115  and a source terminal of the fifth NMOSFET  112  is connected to the third node  118 . 
     Also connected to the third node  118  are a drain terminal of the sixth MOSFET  113  and a source terminal of the seventh MOSFET  114 . A source terminal of the sixth MOSFET  113  and drain terminal of the seventh MOSFET  114  are connected together and to the first terminal of the sampling capacitor  105  and to the ADC circuitry  106 . A gate terminal of the sixth MOSFET  113  receives the first select signal SELA. A gate terminal of the seventh MOSFET  114  receives the second select signal SELB which is generated elsewhere on the SOC. The SELB select signal is the logical inverse of SELA and can be derived from the SELA select signal by simple logic circuitry. A body terminal of the sixth NMOSFET  113  is connected to ground. A body terminal of the seventh MOSFET  114  is connected to the supply voltage VDD. 
     In an ‘on’ operating condition where a voltage on input line  100  is required to be sampled, the select input SELA is set high (that is, at VDD) and the select input SELB is set low (that is, at ground potential). Hence the first and second NMOSFETs  107 ,  108  are both ON, discharging the second node  117  to ground. Thus, a path exists between the input line  100  and ground through the potential divider  102 ,  103  and a divided voltage appears at the first node  115 . The setting of the input SELA to VDD also means that the fourth and fifth NMOSFETs  111 ,  112  are OFF. Also, the gate of the PMOSFET  110  is pulled down, thereby turning the PMOSFET  110  ON. (It will be noted that the gate of the third NMOSFET  109  is always pulled up to VDD). So with both the third NMOSFET  109  and the PMOSFET  110  conducting, the input signal appearing at the first node  115  is transferred to the third node  118  via the PMOSFET  110  and the third NMOSFET  109 . The third NMOS FET  109  ensures efficient transfer of low voltages. PMOSFETs are, in general, not so reliable in such cases. The setting of the select input SELA to VDD and the select input SELB to ground switches on both the sixth and seventh MOSFETs  113 ,  114  which comprise the sampling switching circuit  104 . Thus, the divided input voltage is further transferred to sampling capacitor  105  (and the ADC circuitry  106 ). Preferably, the third NMOSFET  109 , the PMOSFET  110 , the sixth MOSFET  113  and the seventh MOSFET  114  (which is a P channel MOSFET) are sized to ensure that the (dynamic) voltage appearing at the first node  115  is precisely transferred to the sampling capacitor  105 . 
     In an alternative operating condition where it is required that the potential divider  102 ,  103  be disconnected, (that is, an ADC ‘off’ condition) the first select signal SELA is set to ground and the second select signal SELB is set to VDD. Once SELA goes low and SELB goes high, the second NMOSFET  108  is turned OFF, removing the potential divider&#39;s link to ground. Also when the circuit is in the ‘off’ condition, as the PMOSFET  110  is OFF and the third NMOSFET  109  has its gate terminal fixed at VDD, no voltage greater than VDD can be transferred from the first node  115  to the third node  118 . Also in the ‘off’ condition, if the input on line  100  rises above VDD then the fifth NMOSFET  112  turns ON, limiting the voltage at the third node  118  to VDD. In addition, both sixth and seventh MOSFETs  113 ,  114  comprising the sampling switching circuit  104  are turned OFF by SELA going low and SELB going high. So the potential divider  102 ,  103  and the over-voltage protection circuit  101  are cut off from the ADC circuitry  106 . However, the change in state of SELA causes the fourth and fifth NMOSFETs  111 ,  112  to turn ON. As a consequence, the third and fourth nodes  118 ,  119  are forced to a predetermined voltage which is limited to VDD. Forcing the third and fourth nodes  118 ,  119  to a predetermined voltage during this ‘off’ mode of operation, ensures that none of the NMOSFETs or the PMOSFET comprising the over-voltage protection circuit  101  is subjected to a voltage higher than its rating. 
     The first and second nodes  115 ,  117  rise to the level of the input voltage (on line  100 ). This latter occurrence however can adversely affect reliability of the first and third NMOSFETS  107 ,  109  and the PMOSFET  110 . In order to obviate this, the first NMOSFET  107  has its gate terminal connected to VDD and its source terminal is also connected to VDD through the fourth NMOSFET  111 . Thus the first NMOSTFET  107  is protected from any over-voltage appearing at the input on line  100  up to twice its rating. The third NMOSFET  109  is similarly protected as its gate terminal is also connected to VDD and its source terminal is connected to VDD through the fifth NMOSFET  112 . To protect the PMOSFET  110  from over-voltage conditions, its gate terminal is arranged to be connected to the input  100  when the circuit is in the ‘off’ condition (that is, the potential divider is disconnected) and connected to ground when in the input circuit  100  is in the ‘on’ condition, (that is when the potential divider is connected). As the gate terminal of the P NOSFET  110  is connected (via the delay circuit  116 ) to the second node  117 , this gate terminal will receive the voltage appearing on the input line  100  when the input circuit is in the ‘off’ condition. The source terminal of the PMOSFET  110  also receives the voltage appearing on the input line  100  in the ‘off’ condition and so the source-to-gate voltage is zero. When the select input SELA on line  120  goes from low to high (and the potential divider  102 ,  103  is connected), the circuit is in the ‘on’ condition and the second node  117  is pulled to ground as is the gate terminal of the PMOSFET  110  via the first and second NMOSFETs  107 ,  108 . However, the source terminal of the P NMOSFET  110  (which is connected to the first node  115 ) will remain at the input level for some period of time while the potential divider  102 ,  103  settles. This can cause a source-to-gate voltage spike which could go above the rating of the PMOSFET  110 . The provision of a delay (which can be chosen to be small in terms of MOSFET parameters) between the gate terminal of the PMOSFET  110  and the second node  117  prevents any spike being generated. 
     It will be understood that while the example of  FIG. 1  describes an analog-to-digital function, the over-voltage protection circuit  101  may be used with other analog functional modules. 
     Advantageously, the present invention enables shut-down of front-end resistor divider loading while protecting front-end devices from over-voltage and without losing operating condition precision. 
     The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals. 
     Although specific conductivity types or polarity of potentials have been described in the examples, it will be appreciated that conductivity types and polarities of potentials may be reversed. 
     The description of the preferred embodiments of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.