Abstract:
A circuit for providing a reference voltage can be widely used in audio applications. However, at startup an abrupt start in the reference signal can cause undesirable audible artifacts. A circuit employing feedback of a reference voltage to control the charging of a capacitor which provides the reference voltage can be used to provide a smooth startup to the reference voltage. The circuit contains a differential pair for steering a fixed current source from one path to another as the reference voltage increases. The steered current can then be mirrored into one ore more current mirrors where the newly mirrored current can be squeezed to zero when the difference between a desired reference voltage and the reference voltage approaches zero. This newly mirrored current can be used to charge a capacitor which is used to provide the reference voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. Ser. No. 12/860,791, filed Aug. 20, 2010, now U.S. Pat. No. 8,212,608 entitled, “APPARATUS AND METHOD FOR A SMOOTH POWERUP OF A REFERENCE SIGNAL” which is hereby incorporated by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to audio systems and specifically with circuits and methods to generate a smooth transition from low supply voltage to a nominal reference voltage. 
     BACKGROUND 
     A pop is an undesirable audio artifact introduced in an audio system during power up. Generally, when an audio circuit is powered up there is a step in the supply and particularly to the reference voltage supplied to audio amplifiers. This discontinuity manifests itself as a pop sound which can be heard by the listener. Because a discontinuity is spectrally broad, it is especially undesirable. 
     Many techniques have been employed to address the pop problem. One method is to suppress the pop at the output, by suppressing any output while the audio system powers up. Another method is to isolate the most vulnerable portion of the audio system to pop from the output. Still others have employed charging a capacitor to generate a piecewise continuous reference voltage. 
     SUMMARY OF INVENTION 
     Embodiments of the invention provide a reference voltage, but have a smooth power up so that the reference voltage during startup does not generate discontinuities in voltage or the slope of the voltage. One embodiment is a circuit comprising a capacitor which provides the reference voltage, a differential pair with one input coupled to the reference voltage and the second input coupled to predetermined voltage. As the reference voltage increases, current is steered away from the current path controlled by the first input into the current path controlled by the second input. The circuit further comprises a current mirror which mirrors the current flowing through the current path controlled by the second input of the differential pair. The mirrored current is used to charge the capacitor completing a positive feedback loop. The output path of the current mirror is coupled to a desired reference voltage source and the reference voltage so that when the reference voltage approaches the desired reference voltage, the voltage across the output path of the current mirror approaches zero, squeezing the current flowing through the output path down to zero, thus halting the feedback. 
     The circuit can also comprise a current source which drives the differential pair. Once activated the current source begins the positive feedback through the differential pair and current mirror. Optionally, a second current mirror can be used to buffer the current between the differential pair and the current mirror coupled to the capacitor. 
     Specific implementations of the current source can comprise a field effect transistor (FET) coupled to a fixed bias voltage. Implementations of the differential pair can comprise a pair of FETs, such as p-channel FETs (PFETs). Implementations of the current mirrors can comprise a pair of FETs either n-channel FETs (NFETs) or PFETs. 
     A corresponding method of powering up a reference voltage smoothly comprises dividing a fixed current between a first current path and a second current path, directing more current to the second current path as the reference voltage increases, mirroring the current flowing through the second current path, but squeezing that current to zero as the reference voltage approaches the desired reference voltage, and charging a capacitor using the mirrored current where the capacitor provides the reference voltage. The mirroring step can also comprise first mirroring the current flowing through the second path into an intermediate current and then mirroring the intermediate current into the capacitor while squeezing that current to zero as the reference voltage approaches the desired reference voltage. 
     The reference circuit can be used as a ground reference for an audio amplifier within an audio driver. The smooth ramp up of the reference circuit prevents an audible pop in the audio driver. Such an audio driver can be an integral part of 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. 1  shows an embodiment of an audio driver comprising a two-stage amplifier; 
         FIG. 2  is a circuit that can be used to generate a reference voltage at the startup of an audio system; 
         FIG. 3  is a graph showing the reference voltage as a function of time; 
         FIG. 4  is a graph showing the reference voltage as a function of time when control switches are opened and closed too late; 
         FIG. 5  shows another embodiment of circuit used to smoothly generate a ramp signal; 
         FIG. 6  shows a time line of the reference voltage; 
         FIG. 7  illustrates an adjustable variant of a current mirror; and 
         FIG. 8  shows an alternate embodiment of the reference voltage circuit with smooth ramp up. 
     
    
    
     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. 1  shows an embodiment of an audio driver comprising a two-stage amplifier. The audio driver can also include audio processing elements and a digital to analog converter (DAC). The two-stage amplifier comprises amplifier  102  and output stage  104 . Most audio output devices expect a zero common mode voltage, that is, the audio signal swing between positive and negative voltages with a generally zero average voltage. However it is desirable for the audio driver to only use voltages between V SS  (typically ground) and V DD  so that a negative supply voltage is not needed. As a result, audio drivers usually produce an output with a common mode voltage at the midpoint between V DD  and V SS , which would allow for the maximum voltage swing in the output signal. This introduces a DC offset to the output which for prolonged use could damage a speaker which is being driven by the audio driver. For this reason often a large capacitor such as capacitor  108  is placed in series with the audio output to filter out the DC offset. 
     One method to obtain a common mode voltage of V REF  is to use the reference voltage, generated by reference generator  106 , as a virtual ground to amplifier  102 . If the desired reference voltage V DREF  is selected as the midpoint between V DD  and V SS , the output signal can swing between V DD  and V SS  and have a DC offset of V REF . The problem with this configuration is that when the audio driver is first powered up, the output voltage is at V SS , but ramps up to the “virtual zero” of V REF . If this transition is abrupt, it causes an audible pop. As a result, it is desirable to smoothly increase V REF  from V SS  to a desired final reference voltage V DREF . If this increase experiences discontinuities in the voltage or derivatives of the voltage, an audible pop can be heard. 
       FIG. 2  is an embodiment of a reference voltage generator. Reference voltage generator  200  comprises capacitor  202 , which can be connected through switches  204 ,  206  and  208  to three different circuits representing three different phases of the startup. The voltage across capacitor  202  furnishes the reference voltage. During the initial stage, switch  204  is closed and the capacitor is grounded (or equivalently connected to a low supply voltage V SS ). At this time any charge stored in capacitor  202  is discharged and the reference voltage is set to ground (or V SS ). At the start of power up, switch  204  is opened as switch  206  is closed. In this state, current source  210  charges capacitor  202  and the reference voltage increases linearly. When the reference voltage is near the desired reference level, switch  206  is opened and switch  208  is closed. Capacitor  202  is connected to a voltage divider comprising resistor  212  and resistor  214 . The voltage increases now as an RC circuit, that is, so the voltage now asymptotically reaches the desired reference level (shown in the example as V DREF ). 
       FIG. 3  is a graph showing the reference voltage as a function of time. At  302 , switch  204  is opened and switch  206  is closed allowing for the linear increase in the voltage. At time  304 , switch  206  is opened and switch  208  is closed so the voltage now increases asymptotically towards the desired reference level. 
     At  302  (or at  402  in  FIG. 4 ), there is a discontinuity in the first derivative of the reference voltage which can still have the undesirable spectral artifacts found in a step discontinuity. Also if the timing of the opening of switch  206  and closing of switch  208  is not precise, the slopes of the linear and the asymptotic portions of the reference voltage curve will not match, leading to another discontinuity in the first derivative as shown in  FIG. 4 . Specifically at  404 , the transition from the linear to the asymptotic portions of the voltage reference is made late. 
     Based on the current supplied by current source  210 , the capacitance of capacitor  202  and the resistances of resistor  212  and  214 , the precise voltage where the slopes of the linear portion and the asymptotic portion are equal can be determined. The reference voltage needs to be monitored and once the reference voltage reaches this determined voltage, switch  206  can be opened and switch  208  can be closed. A voltage comparator and a voltage divider can be used to make this comparison. However, due to variations due to process, voltage and temperature, the precise voltage may not be easily determined leading to a small discontinuity in the first derivative at  404 . 
       FIG. 5  shows another embodiment of a reference voltage generator. Reference voltage generator  500  comprises capacitor  502 , differential pair  510 , current source  520 , current mirror  530  and current mirror  540 , and desired reference voltage source  550  Like in reference voltage generator  200 , capacitor  502  is used to hold the reference voltage. Differential pair  510  is shown comprising PFET  512  and PFET  514 , with one input tied to ground (or V SS ). The other input is tied to the reference voltage. Current source  520  is shown comprising PFET  522  connected to a bias voltage. Current mirror  530  is shown comprising NFET  532  and NFET  534 . Current mirror  540  is shown comprising PFET  542  and PFET  544 , with an output coupled to capacitor  502 . Desired reference voltage source  550  provides a desired reference voltage. In the example shown here, desired reference voltage source  550  is shown as provided by a voltage divider having resistors  552  and  554 . However, desired reference voltage source can be a bandgap voltage, or a voltage divider having resistors and a buffer. 
     Initially, reference voltage generator  500  is activated by activating current source  520  such as by closing switch  524 . When current source  520  is activated current flows through the differential pair through PFET  512  and PFET  514 . The proportion of the current flowing through each PFET depends on the inherent resistance of each PFET and the voltage difference. In order to minimize the initial discontinuity in the slope, it is desirable for most of the current initially to flow through PFET  512 . By doing so, the amount of current used to charge capacitor  502  is small, because the current through PFET  514  is mirrored through current mirrors  530  and  540 , the latter current mirror charges capacitor  502 . The smaller the current, the smaller the initial discontinuity in slope. Several methods can be used to achieve this. 
     One method of steering current initially to PFET  512  is to bias PFET  514  to a small positive voltage rather than coupling PFET  514  to ground as shown. In this case, when the circuit is activated, the reference voltage is at the ground potential (or V SS ), so the gate of PFET  514  has a higher voltage than that of PFET  512 , thus causing more current to flow through PFET  512 . 
     Another method of steering current initially to PFET  512  is to fabricate PFET  514  with greater resistance than PFET  512 . For example, PFET  512  can be fabricated as a larger PFET than PFET  514  thus creating a PFET with lower resistance than PFET  514 . Additionally, both approaches can be combined by supplying a smaller PFET  514  with a small positive voltage as described above. 
     Once the circuit is activated, a positive feedback loop is established, because as the voltage to the gate of PFET  512  increases, more current is drawn through PFET  514  which is mirrored by current mirrors  530  and  540  into capacitor  502  causing the reference voltage to increase. By increasing the reference voltage even more current is drawn through PFET  514  as the positive feedback continues. 
     Once the positive feedback commences, current mirror  540  under the control of current mirror  530  and differential pair  510  charge up capacitor  502  thus increasing the reference voltage. Initially, this begins slowly, but as the current increases, the reference voltage increases more rapidly. Eventually, the reference voltage causes the current through differential pair  510  to essentially flow through the second current path (e.g., through PFET  514 ) which feeds current mirror  530 , at this point the current used to charge capacitor  502  has reached its maximum. Current mirror  540  receives a current in its input path (i.e., through PFET  542 ) and provides a mirrored current in its output path (i.e., through PFET  544 ). However, because the voltage across the output path is the difference between the desired reference voltage and the reference voltage, as the reference voltage increases, the voltage across the output path approaches zero. The effect of the decrease in voltage is that the mirrored current gets “squeezed” to a zero current. 
     More specifically, current mirror  540  functions because PFET  544  is operating in the saturation region, but as the drain to source voltage decreases, which occurs as the reference voltage approaches the desired reference voltage, PFET  544  begins to operate in the linear region. In the linear region, the current provided by PFET  544  becomes proportional to the drain to source voltage which decreases as the reference voltage increases leading to less current flowing to capacitor  502  and slowing down the increase to the reference voltage. Eventually, the reference voltage achieves the desired reference voltage resulting in PFET  544  shutting off completely as the drain and source voltage becomes the same. 
       FIG. 6  shows a time line of the reference voltage. At  602 , current source  520  is activated resulting in possibly a very small jump. Since this jump is very small, the effect is a near smooth increase in the reference voltage eliminating the corner shown at  302  in  FIG. 3 . The reference voltage continues to increase, but begins to slow down at  604  where the reference voltage begins to approach the desired reference voltage. At  606 , the desired reference voltage is attained. It should also be noted that slope at  608  is determined by the amount of current flowing through capacitor  502  once the differential pair has steered essentially all of the current through its second current path. The greater the current, the steeper the slope and the faster the rise time to reach the desired reference voltage. 
     In another embodiment, the slope and hence the rise time can be made adjustable. One method is to use an adjustable current source for current source  510 . The more current the current source permits, the greater the current used to charge the capacitor and hence the faster the rise time to reach the desired reference voltage. Another method is to use an adjustable current mirror for either current mirror  530  or current mirror  540 . 
       FIG. 7  illustrates an adjustable variant of a current mirror. Like current mirror  540  shown in  FIG. 5 , the current comprises PFET  542  with a source connected to ground. However PFET  544  is replaced by a plurality of PFETs connected in parallel, shown as four PFETs namely, PFETs  702 ,  704 ,  706  and  708 . The current mirror can be programmed by setting switches  712 ,  714 ,  716  and  718 . PFETs  702 ,  704 ,  706  and  708  can be of different sizes so they have different impedances. In this way, I OUT  is proportional to I IN  rather than equal to I IN  with a proportion set by switches  712 ,  714 ,  716  and  718 . Because the proportion of current that is mirrored by this current mirror can be chosen, the rise time of the reference voltage can be chosen by programming setting switches  712 ,  714 ,  716  and  718  as desired. 
     Reference voltage generator  500  has many advantages including the elimination of corners at startup. In addition, there are no switches so the transition between the positive feedback and the shutting down of current mirror  540  is performed smoothly without the need for additional circuitry to monitor the voltage. However, for noise or other considerations, it may be desirable once the desired reference voltage is attained to “switch over” directly to the desired reference voltage source. 
       FIG. 8  shows an alternate embodiment of the reference voltage generator. The circuit is similar to reference voltage generator  500 , but further comprises switch  802  and  804 . Prior to ramp up, switch  802  is closed and switch  804  is open. In this configuration, the circuit functions in a similar manner as described for reference voltage generator  500 . Once reference voltage generator  500  has attained the desired reference voltage, switch  802  can be opened and switch  804  can closed so that desired reference voltage source  550  rather than capacitor  502  provides the reference voltage. 
     The reduction or elimination of pop eliminates a chief nuisance facing a listener of audio either in the form of voice or music. Audio drivers such as that described in  FIG. 1  can benefit greatly from the inclusion of the reference circuit generator. These audio drivers 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.