Abstract:
The invention relates to a voltage-boosting stage ( 100 ) comprising a first capacitive voltage circuit (S 1,  S 2,  S 3,  S 4,  CO, Cb) coupled to a power supply (Vs) and providing an output voltage at an output terminal. The voltage-boosting stage further comprises a second capacitive voltage circuit (S 5,  S 6,  S 7,  S 8 , C 1 , Cb) coupled to a power supply (Vs) and providing another output voltage at another output terminal the output terminal and the other terminals being coupled together and further coupled to a supply terminal of a power stage (S 9,  S 1 O) for implementing a two-level boosted power stage.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a voltage-boosting stage and to a power amplifier comprising such a stage. 
       BACKGROUND OF THE INVENTION 
       [0002]    Class D amplifiers, comprising switching power stages are used in many applications because of their efficiency. The output signal of such an amplifier is a pulse-width modulated (PWM) signal, which is low-pass filtered for obtaining the amplified analog signal. To reduce the high-frequency content, and simplify filtering, multi-level PWM signals, which more closely represent the input signal may be used. To drive the load with a multi-level PWM signal an output stage that can reproduce multiple output levels is required. The amount of power that can be delivered to the loudspeaker load depends on the available supply voltage and the loudspeaker load impedance. The maximum sinewave power that can be delivered to the load is calculated in equation 1 below, where Vs represents the supply voltage, which sets the maximum amplitude of the sinewave and Rload the load impedance. 
         [0000]    
       
         
           
             
               
                 
                   Pload 
                   = 
                   
                     
                       V 
                       s 
                       2 
                     
                     
                       2 
                        
                       Rload 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0003]    To increase the amount of power that can be delivered to the load the supply voltage can be increased or the load impedance lowered or a combination of both options can be used. The loudspeaker load impedance cannot be made infinitely small and the available supply voltage is often fixed. This especially holds for portable devices where the supply is a battery with a given cell voltage. To increase the amount of power that can be delivered to the load the supply voltage must be boosted. The switching power-stage of a class-D amplifier is then operated from the boosted power supply with a voltage value of NVs as shown in  FIG. 1 . When the supply voltage is e.g. doubled i.e. N=2, then the maximum power that can be delivered to the loudspeaker load is quadrupled. A common way to boost the supply voltage is to use an inductive DC/DC converter. But a coil is a highly undesirable component. It is expensive and has a large footprint. A capacitive DC/DC converter to boost the supply voltage is therefore preferable. 
         [0004]    A known circuit using capacitors for boosting the supply voltage is a charge-pump. This capacitive voltage-doubler circuit has two states and is continuously switched between both states by operating the switches. In a first state, a capacitor C 0  is charged to the supply voltage Vs. While capacitor C 0  is charged, a buffer capacitor Cb delivers current to a load and it is discharged. Because a capacitor in parallel with the load Cb is discharged the output voltage drops. After the output voltage has dropped to a certain value, the circuit is switched to a second state. Capacitor C 0  is connected in series with the supply voltage. Because of some charge redistribution between C 0  and Cb the output voltage will be lower than 2Vs. After switching both capacitors C 0  and Cb are discharged and the output voltage drops. After the output voltage has dropped below a certain value, the circuit is switched again to the previous state. Two different slopes in the output voltage can be distinguished. In the first state only buffer capacitor Cb supplies all the load current resulting in a faster drop in the output voltage compared to the second state where the load current is supplied by capacitor Cb and C 0  in parallel. A resistance in series with capacitor Cb would cause an additional voltage step in the output voltage when the capacitive voltage-doubler circuit is switched from the second state to the first state caused by the additional load current which capacitor Cb has to deliver in the first state. 
         [0005]    To avoid this problem a double-phase capacitive circuit as is shown in  FIG. 2  may be used. In  FIG. 2 , the above-mentioned notations can be easily observed. The double-phase capacitive circuit has also two states and it is continuously switched between both states by operating the switches. In the double-phase capacitive voltage-doubler circuit two capacitors, C 0  and C 1  are switched. In the first state the capacitor C 0  is charged to the supply voltage and capacitor C 1  is connected in series with the supply and in parallel with capacitor Cb. In the second state the function of capacitor C 0  and C 1  are interchanged. Now capacitor C 1  is charged to the supply and capacitor C 0  is connected in parallel with capacitor Cb. In both states there is a capacitor in parallel with Cb and the slope of the output voltage drop is equal in both states. 
       SUMMARY OF THE INVENTION 
       [0006]    Hence, it is a need to provide a voltage-boosting stage having a reduced number of components and no inductors. The invention is defined by the independent claims. Dependent claims define advantageous embodiments. 
         [0007]    This object is defined by a voltage-boosting stage comprising: 
         [0008]    a first capacitive circuit coupled to a power supply and providing an output voltage at an output terminal, 
         [0009]    a second capacitive circuit coupled to a power supply and providing another output voltage at another output terminal, and 
         [0010]    the output terminal and the other terminal being coupled together and further coupled to a supply terminal of a half bridge power stage for implementing a two-level boosted power stage. 
         [0011]    For reducing the number of components the capacitor Cb is omitted. In an embodiment of the invention each of the capacitive circuits comprises a first switch having a respective first terminal coupled to a negative terminal of the power supply and a respective second terminal coupled to a second switch, the second switch having a third terminal coupled to the respective second terminal of the respective first switch and further coupled to a first terminal of a respective capacitor, a second terminal of said capacitor being coupled to a fourth terminal of a respective third switch and to a fifth terminal of a respective fourth switch, the fourth switch being further coupled to respective output terminals of the respective capacitive circuit. 
         [0012]    In another embodiment of the invention the voltage-boosting stage further comprises a fifth switch coupled between the output node and a positive terminal of the power supply, a sixth switch coupled between the output node and the negative terminal of the power supply and a seventh switch connected between the output terminals of the capacitive circuits and the output node for implementing a three-level supply-boosted power stage. 
         [0013]    In a preferred embodiment, the power stage only comprises two capacitive circuits and the sixth switch. This has the advantage of reducing the number of switches of the three—level power stage without affecting the performance of the circuit. 
         [0014]    In another embodiment of the invention one of the fourth switches comprises at least two switches having different equivalent resistances such that a balance between the equivalent resistances in different states of the circuit is obtained. 
         [0015]    In an embodiment of the invention the bridge-tied load supply-boosted power stage comprises two capacitive circuits. Furthermore each of the capacitive circuits comprises at least a fourth switch, each comprising a plurality of switches having different areas. The fourth switch parts i.e. some of the plurality of switches of each of the voltage boosting stages are differently connected to each other in use for obtaining an equal output resistance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The above and other advantages will be apparent from the exemplary description of the accompanying drawings in which: 
           [0017]      FIG. 1  depicts a supply-boosted class-D amplifier, 
           [0018]      FIG. 2  depicts a prior art double-phase capacitive voltage doubler, 
           [0019]      FIG. 3  depicts a two-level supply-boosted power stage, according to the invention, 
           [0020]      FIG. 4  depicts a three-level supply-boosted power stage, according to an embodiment of the invention, 
           [0021]      FIG. 5  depicts another embodiment of a three-level supply-boosted power stage, according to the invention, 
           [0022]      FIG. 6  depicts another embodiment of a three-level boosted power stage, according to the invention, 
           [0023]      FIG. 7  depicts a five-level supply-boosted power stage, according to the invention, 
           [0024]      FIG. 8  depicts an embodiment of a five-level supply-boosted power stage, according to the invention, and 
           [0025]      FIG. 9  depicts a bridge-tied load supply-boosted power stage, according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0026]    A way to implement the voltage boosting stage is shown in  FIG. 3 , wherein the circuit in the dashed box generates a voltage equal to double the supply voltage at node Vb. The capacitive voltage-doubler circuit is switching continuously. The output may now be switched to ground level  0  by turning switch S 9  ON and to a level equal to double the supply voltage 2Vs by turning switch S 10  ON. The supply voltage Vs may also be used to generate an additional output level to realize a three-level power-stage. 
         [0027]    To generate an output level equal to Vs an additional switch is required as shown in  FIG. 4 . The output may now be switched to ground level  0  by turning switch S 9  ON, to a level equal to the supply voltage Vs by turning switch S 11  ON and to a level equal to double the supply voltage 2Vs by turning switch S 10  ON. For a linear transfer of the three-level power-stage it is important that the voltage level steps are equidistant and that the output impedance is equal for each voltage level. The output impedance for the low ground level and the Vs level are determined respectively by the switch impedance S 9  and S 11 . The output impedance for the 2Vs level is set by the series connection of the capacitive voltage-doubler output impedance and the impedance of the switch S 10 . When the capacitor value of C 0 , C 1  and the switch resistance are fixed the capacitive voltage-doubler impedance depends on the switching frequency. The output impedance is inversely proportional to fC when the switching time period is larger than a time constant τ, set by the capacitor and switch resistance. The output impedance is equal to four times a single switch impedance Rs when the switching time period is smaller than the time constant τ, assuming the impedances of the switches S 1  to S 8  are equal to each other. The lower bound of the capacitive voltage-doubler output impedance is 4Rs. Assuming that the switches S 1  to S 8  and S 10  have the same impedance then this requires to have a single-switch impedance that is five times lower than that of the switches S 9  and S 11 . The switches are normally implemented with transistors but also may be implemented with e.g. MEMS. The series connection of transistors increases the ohmic resistance. At the expense of an increase in transistor area the ohmic resistance of the transistors in series can be made equal to a single transistor. But the increased area results in larger switching losses. 
         [0028]    An improved implementation of the power-stage where the output impedance of the 2Vs level is only determined by the capacitive voltage-doubler impedance is shown in  FIG. 5 . The capacitive voltage-doubler is now only operated when an output level of double the supply voltage needs to be generated eliminating the need for an additional series switch. When a level is generated of 0 or Vs the switches S 1  to S 8  are turned off and the capacitors C 0  and C 1  are kept floating. To generate an output level of Vs the switch pairs S 2 , S 4  or S 6 , S 8  can be used reducing the switch count even further. The output impedance of the level Vs is now not matched anymore. Assume that the switches S 1  to S 8  are equal and scaled to get an output impedance R for the 2Vs output level. Furthermore the switching time period is chosen to be much smaller than τ. The impedance of each of the switches S 1  to S 8  is then ¼R. Using either of the switch pairs S 2 , S 4  or S 6 , S 8  to generate the level of Vs now results in an output impedance of ½R. To obtain a matched output impedance one of the switches in the pairs S 2 , S 4  or S 6 , S 8  can be split-up in two parallel switches. 
         [0029]    This is shown in  FIG. 6  for the case where the switch pair S 2 , S 4  is used and S 4  is split-up into two parallel switches S 4   a  and S 4   b.  The switch S 4   a  is used to generate an output level of Vs. To get an output impedance match for the level Vs the switch S 4   a  must be made equal to ¾R, three times as large as the original impedance of S 4 . To generate the level 2Vs both switches S 4   a  and S 4   b  are used in parallel. The parallel combination of S 4   a  and S 4   b  must have an impedance of ¼R, the impedance of S 4   b  must thus be made one and a half times larger than the original switch impedance of S 4 . The impedance of the level 2Vs is then also R and matches the impedance of the other levels. The impedance of a MOS transistor used as switch is inversely proportional to the area, assuming the length is fixed and only the width is scaled. An increase in impedance results in a decrease of area. Assuming switch S 4  occupies an area of A then the switches S 4   a  respectively S 4   b  occupy an area of ⅓ A respectively ⅔ A. The total area occupied by the parallel combination of the switches S 4   a  and S 4   b  is equal to the area of the original switch S 4 . 
         [0030]    A five-level supply-boosted power-stage may be built using two three-level supply-boosted power-stages in a bridge-tied-load (BTL) configuration as shown in  FIG. 9 . In  FIG. 9 , a load Z 1  is coupled to each of its end to an output of a power amplifier  200 , each power amplifier  200  being a supply-boosted class-D amplifier. Each power amplifier comprises a voltage multiplier  100  coupled to a half-bridge power-stage  1 . Two voltage-doublers, each comprising two capacitive circuits are used. An optimized five-level supply-boosted power stage using only a single voltage-doubler in shown in  FIG. 7 . The parallel switches S 4   n , S 4   p  respectively S 8   n ,S 8   p  allow the doubled supply voltage to be switched to the outputs Vo,p and Vo,n. Adding a common voltage to the outputs Vo,p and Vo,n does not change the differential output. The nine possible ways to generate the differential five-level output signal are shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Possible ways to generate the differential five level output signal 
               
             
          
           
               
                   
                 Level 
                 V o, p   
                 V o, n   
                 V o   
               
               
                   
                   
               
               
                   
                 −1   
                 0 
                 2 V s   
                 −2 V s   
               
               
                   
                 −½ 
                 0 
                 V s   
                 −V s   
               
               
                   
                 0 
                 V s   
                 V s   
                 0 
               
               
                   
                   ½ 
                 V s   
                 0 
                   V s   
               
               
                   
                 1 
                 2 V s   
                 0 
                   2 V s   
               
               
                   
                   
               
             
          
         
       
     
         [0031]    In this circuit the output impedance of the capacitive voltage-doubler for a 0 level output with a common signal of 2Vs can not be made equal to the output impedance of the other levels and should be avoided. For the five-level power-stage it is also possible to reuse the switches of the capacitive circuits. 
         [0032]    This is shown in  FIG. 8 . The switches S 4   n  and S 8   p  are split-up to ensure equal output impedance for the different levels. The output-level combinations in table 1 where on one output a level of Vs is needed and on the other output a level of 2Vs cannot be realized. A level of ±Vs is generated with a combination of a Vs level on one output and a 0 level on the other output. 
         [0033]    It is remarked that the scope of protection of the invention is not restricted to the embodiments described herein. Neither is the scope of protection of the invention restricted by the reference numerals in the claims. The word “comprising” does not exclude other parts than those mentioned in the claims. The word “a(n)” preceding an element does not exclude a plurality of those elements. Means forming part of the invention may both be implemented in the form of dedicated hardware or in the form of a programmed purpose processor. The invention resides in each new feature or combination of features.