Patent Abstract:
A grounding switch is described which operates properly even in the presence of negative voltages on a signal line. The grounding switch uses isolated field effect transistors that have their substrates tied to different voltages. The isolated field effect transistor has a gate voltage and substrate voltage which can be pulled down to a negative voltage when the signal line has a negative voltage allowing the switch to remain open even with a negative voltage.

Full Description:
TECHNICAL FIELD 
     This invention relates generally to the electronic switches and specifically to grounding switches that operate in the presence of negative voltage signals. 
     BACKGROUND ART 
     Typically, grounding switches, often comprising a single field effect transistor (FET), are used to ground an input or an output to a zero or ground voltage. In audio applications this can be used to prevent audio artifacts such as audible pops. For example, to prevent an undesirable audible pop a grounding switch can be used to ground an input to an audio circuit during an initialization phase before the audio circuit is equipped to handle an input signal. The switch can release the input from ground once the audio circuit has been initialized. Another application is where a grounding switch is used to ground an output during the startup of an audio circuit, when the audio circuit may produce glitches in the output leading to an audible pop. 
     SUMMARY OF INVENTION 
     A grounding switch is described which operates properly in the presence of negative voltages on a signal line. In one embodiment, the switch comprises an n-channel field effect transistor (NFET) with an isolated substrate which allows the substrate near the NFET to have a different potential than the substrate around the other circuitry in the grounding switch. A pull down element is used to turn this NFET off. 
     In one embodiment, the switch comprises a pull up circuit and the substrate of the NFET is coupled to a negative supply voltage. When the control signal is low, the pull up circuit is inactive and the pull down element pulls the gate of the NFET down to the negative supply voltage causing the NFET to turn off even in the presence of a signal with a negative voltage on its source or drain. A second NFET, which does not have to have an isolated substrate, can be added in series with the first NFET to prevent damage to the first NFET due to large voltage swings. In one variant, the pull down element comprises a single resistor. 
     In another embodiment where a negative supply voltage is not available, the substrate of the isolated NFET is tied to the signal line. The switch comprises a second NFET, which need not have a separate substrate connection, in series with the isolated NFET. When the control input is high, both NFETs turn on, turning the switch on. When the control input is low, the isolated NFET switches off when the signal has a negative voltage and the second NFET switches off when the signal has a positive voltage, thus switching the grounding switch off regardless of the signal voltage. 
     In one embodiment the pull down element comprises a circuit having another isolated NFET where the drain and substrate are connected to the signal. In another embodiment, the pull-up circuit comprises a p-channel field effect transistor (PFET), optionally a second PFET, and an inverter. 
     In another embodiment, the grounding switch is a circuit comprising two transistors in series operable to turn on when the control input is high. In operation, the first transistor is turned off when the control input is low and the signal voltage is positive, and the second transistor is turned off when the control input is low and the signal voltage is negative. The second transistor can be turned on by pulling the gate of the transistor to the positive supply voltage and can be turned off by pulling the gate down to the negative supply voltage while maintaining its substrate at the negative supply voltage. Alternately, the second transistor can be turned off when the signal voltage is negative by pulling the gate down to the signal voltage while maintaining its substrate at the signal voltage. 
     In one embodiment, the grounding switch is used in an audio driver to suppress audible pops. The audio driver can be used to suppress an undesired audio artifact in many electronic devices including but not limited to personal computer sound cards, voice-over-IP telephones, cellular telephones, digital picture frames, universal serial bus headsets, televisions, video game consoles, MP3 players and Bluetooth headsets. 
     Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1A  shows an embodiment of a system employing a grounding switch to tie a single ended input to ground; 
         FIG. 1B  shows an embodiment of a system employing a grounding switch to tie a single ended output to ground; 
         FIGS. 1C and 1D  shows differential analogs of the systems described in  FIGS. 1A and 1B ; 
         FIG. 2  shows an embodiment of a grounding switch comprising a single NFET; 
         FIG. 3  shows an embodiment of a grounding switch which can tolerate negative voltages seen at the signal line; 
         FIG. 4  shows an embodiment of a grounding switch which can tolerate negative voltages on a signal line without a negative reference voltage; 
         FIG. 5  shows an embodiment of a grounding switch; 
         FIG. 6  shows another embodiment of a grounding switch; and 
         FIG. 7  an embodiment of audio driver employing a grounding switch to suppress audible pop during the power up and power down of the audio driver. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of embodiments of the present invention is presented below. While the disclosure will be described in connection with these drawings, there is no intent to limit it to the embodiment or embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications and equivalents included within the spirit and scope of the disclosure as defined by the appended claims. 
       FIG. 1A  shows an embodiment of a system employing a grounding switch to tie a single ended input to ground. System  102  can be any system which receives a single ended input signal. Grounding switch  104  ties the input to ground when closed. An example of such a system is an audio driver which employs grounding switch  104  to zero the signal during power up and power down to prevent the occurrence of a pop sound. 
       FIG. 1B  shows an embodiment of a system employing a grounding switch to tie a single ended output to ground. System  102  can be any system which produces a single ended output signal. Grounding switch  106  ties the output to ground when closed. As an example, an audio driver can employ grounding switch  106  to zero the output signal during power up and power down to prevent the occurrence of a pop sound. 
       FIGS. 1C and 1D  shows differential analogs of the systems described in  FIGS. 1A and 1B . In particular,  FIG. 1C  shows an embodiment of system  112  employing switch  114  to tie the two differential inputs together. Though the switch does not specifically tie a signal to ground, for the purposes of this disclosure, a grounding switch can also be used to zero a differential signal by tying the differential signal lines together. System  112  has a differential input which is zeroed by switch  114  when switch  114  is closed. Similarly,  FIG. 1D  shows an embodiment of system  116  employing switch  118  to tie the two differential outputs together. System  116  has a differential output which is zeroed by switch  118  when switch  118  is closed. 
       FIG. 2  shows an embodiment of a grounding switch comprising a single NFET. The gate of NFET  202  is connected to a control input, the drain of NFET  202  is connected to signal line  204  and the source is connected to a ground potential. If the switch is used to zero a differential input, signal line  204  is one of the signal lines (e.g., the positive signal line) and the drain is connected to the other signal line (e.g., the negative signal line). When the control input is tied to the positive supply voltage the switch turns on. When the control input is tied to ground, NFET  202  acts as a reverse bias diode and the switch turns off. However, if signal line  204  permits a negative voltage, and NFET  202  is a standard NFET that has a grounded substrate, NFET  202  acts as a forward bias diode when the gate is grounded causing the switch to conduct even though it is supposed to be off. For this reason use of a single NFET is undesirable in many applications, such as audio applications where the voltage can swing in both a positive and negative direction. 
       FIG. 3  shows an embodiment of a grounding switch which can tolerate negative voltages seen at signal line  204 . Grounding switch  300  comprises controllable pull up circuit  304 , isolated NFET  306 , optional NFET  308 , pull down element  310 , and isolated NFET  312  Controllable pull-up circuit  304  is responsive to a control signal and pulls the voltage of the gate of isolated NFET  306  to the positive supply voltage (shown as V DD ) when the control signal is high and provides high impedance when the control signal is low. Isolated NFET  306  is electrically isolated from the substrate and more specifically, its p-well is isolated. This allows the p-well surrounding the NFET  306  to be tied to a different “substrate voltage” from the rest of the circuitry. In this case, NFET  306  has an “isolated substrate connection” tied to a negative supply voltage (for example −V DD  shown in the figure), represented by a fourth connection to the usually three connection NFET symbol. Many techniques exist to fabricate isolated FETs including deep n-well fabrication. Finally pull down element  310  which can be a resistor is tied to the negative supply voltage. 
     When the control input to grounding switch  300  is tied to the supply voltage, NFET  308  is turned on. In addition, pull up circuit  304  pulls up the gate voltage of NFET  306  so it turns on as well, thus turning switch  300  on. This pulls the voltage on signal line  204  to ground. 
     When the control input to grounding switch  300  is tied to ground, pull-up circuit  304  is deactivated and pull-down element  310  can pull the voltage down to the negative supply voltage which causes NFET  306  to turn off, even in the presence of a negative voltage on signal line  204 . However, if the voltage on signal line  204  is positive such as V DD  the gate to drain voltage of NFET  306  as a result of the pull down element would be 2 V DD , which can exceed the tolerance of NFET  306 . Therefore, NFET  312  is included to protect NFET  306 . Because the gate of NFET  306  does not need to pull down to −V DD , NFET  312  is used to counteract pull down element  310 . In fact, when the voltage on signal line  204  is positive, NFET  312  permits a current to flow which allows the voltage on the gate to NFET  306  to rise so that the gate to drain voltage can be within the tolerance of the technology. Because of the potential for a negative source voltage on NFET  312 , NFET  312  has a substrate voltage coupled to the negative supply line. NFET  312  Optional NFET  308  can be included to protect NFET  306  from excessive voltages that can occur especially if the signal line swings between the extreme positive and negative voltages, when complementary metal-oxide-semiconductor (CMOS) technology is used. In other technologies or even other CMOS technologies with different design rules, NFET  308  can be omitted. In the present embodiment, the switch operates only so long as the voltage on signal line  204  remains greater than the negative supply voltage. 
       FIG. 4  shows an embodiment of a grounding switch which can tolerate negative voltages on a signal line without the need for a negative supply voltage. Grounding switch  400  comprises controllable pull-up circuit  402 , pull-down element  404 , isolated NFET  406  and NFET  408 . Controllable pull-up circuit  402  functions in a manner similar to that describe for circuit  302  above. NFET  406  is isolated in the same manner as that described for NFET  306  above however the substrate voltage is tied to the drain voltage. 
     When a positive supply voltage is applied to the control signal, NFET  404  turns on. In addition, pull-up circuit  402  pulls up the gate voltage on NFET  406  causing NFET  406  to turn on, thus turning the switch on. When the control signal is grounded and the voltage on signal line  204  is positive, NFET  408  is turned off. Since NFET  408  is in series with NFET  406 , the switch is turned off. When the control signal is grounded and the voltage on signal line  204  is negative, pull-up circuit  402  is left in a high impedance state, allowing pull-down element  404  to pull down the gate voltage of NFET  406  down to the voltage of signal line  204  which is also the drain voltage of NFET  406 , causing NFET  406  to turn off. Since NFET  408  and NFET  406  are in series, the switch is turned off. 
       FIG. 5  shows an embodiment of grounding switch  500 . Pull up circuit  402  includes an inverter  502  and PFET  504 , and pull-down element  404  includes isolated NFET  506 . When a control signal is high, inverter  502  grounds the gate of PFET  504  which turns on PFET  504 , causing a positive gate voltage at NFET  406  which turns NFET  406  on. Because both NFET  406  and NFET  408  are turned on, the switch is turned on. When the control signal is grounded, inverter  502  imposes a positive supply voltage on the gate of PFET  504  turning the PFET off effectively disconnecting the pull-up circuit  402  from NFET  406 . With the pull-up circuit disconnected, NFET  506  can manipulate the voltage on the gate of NFET  406 . When the voltage on signal line  204  is positive, the drain and substrate of NFET  506  is positive, NFET  506  acts as a forward biased diode between the source and the substrate connections. As a result, NFET  506  pulls up the gate of NFET  406 . Likewise, because the drain and substrate of NFET  406  is also positive, NFET  406  acts as a forward biased diode and pulls up on the drain of NFET  408 . However, because the control line is low, NFET  408  is turned off, so the switch is turned off. When the voltage on signal line  204  is negative, the role of the source and drain are essentially reversed. The drain to gate voltage is negative, but can be viewed as a positive gate-to-source voltage of NFET  506  where the source and drain are reversed. This causes NFET  506  to turn on which pulls down (because the signal line is negative it is a pull down rather than a pull up) the gate voltage of NFET  406  to the signal line  204  voltage. Because the drain (which now functions as a source) voltage and the gate voltage are made equal by NFET  506 , NFET  406  turns off, turning the switch off. One advantage in this embodiment of NFET  506  as the pull-up element over a resistor described in  FIG. 3  is that NFET  506  only draws current in the specific case where the voltage on signal line  204  is negative, whereas a resistor would draw current all the time. 
       FIG. 6  shows another embodiment of a grounding switch. In this embodiment, the voltage of the signal line swings between V DD  and −V DD , Thus, the total voltage between the source and drain of PFET  502  could be 2V DD  which can be outside the specifications of the transistor technology. This can potentially cause PFET  502  to operate out of specification, which can damage PFET  502 . To address this problem PFET  602  is added to pull up circuit  402 , by adding PFET  602  to grounding switch  600 , the worst case voltage between the source and drain of each transistor is V DD . 
       FIG. 7  an embodiment of audio driver employing a grounding switch to suppress audible pop during the power up and power down of the audio driver. The audio driver is shown comprising a two stage audio amplifier. For a digital audio driver, it can further comprise a digital to analog converter (not shown) as well as other audio processing components. In the example shown, the two stage audio amplifier comprises amplifier stage  702 , and output stage  720 . Output stage  720  comprises output driver  708 , capacitor  704 , resistor  706  and grounding switch  600 . Capacitor  704  and resistor  706  are used to provide stability to the two stage amplifier. Grounding switch  600  is used to ground the output of output driver  708  during power up and power down so that an audible pop is not heard by the listener. Control signal ctrl is set high during power up and power down so grounding switch  600  is closed. Once the amplifier stage powers up and has settled into an operational mode. The grounding switch is opened by setting ctrl low and the audio signal is allowed to pass externally where a listener can hear it. It should be noted that while grounding switch  600  is used as an example, other embodiments, including any of the grounding switches described above, can also be used. Furthermore, since voltages are relative, the grounding switches described herein can also be used to tie differential inputs or outputs together. 
     Audio drivers such as that described are integral to a wide variety of electronic devices including but not limited to personal computer sound cards, voice-over-IP telephones, cellular telephones, digital picture frames, universal serial bus headsets, televisions, video game consoles, MP3 players and Bluetooth headsets. 
     It should be emphasized that the above-described embodiments are merely examples of possible implementations. Many variations and modifications may be made to the above-described embodiments without departing from the principles of the present disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Technology Classification (CPC): 7