Abstract:
A switching amplifier has a network including current sources and resistors connected to the two output terminals of the H-bridge of the switching amplifier, to provide a small current to the load connected between the two output terminals at zero input, whereby the common mode voltage bouncing is reduced and the switching amplifier has less power consumption and reduced electro-magnetic interference.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to switching amplifiers and, more particularly, to an apparatus and method for reducing the power consumption and electro-magnetic interference (EMI) of a switching amplifier. 
       BACKGROUND OF THE INVENTION 
       [0002]    Audio power amplifiers of conventional design suffer from low efficiency, and this causes these designs to generate heat that must be removed by large heat sinks, causing the physical amplifier designs to be quite large. In order to make amplifiers smaller, high-efficiency designs have been proposed. Switching amplifiers, also known as class-D amplifiers, are analogous to switching regulators, and so have similar advantage, when compared to class-AB amplifiers, in efficiency and its derivatives, i.e., lower thermal dissipation, battery life, smaller power supplies, size, weight, etc. These amplifiers work by converting the analog or digital-input signal into a 2-level output signal using a high-frequency modulation process. This 2-level signal is then fed to a power stage to switch the power switches of either a full H-bridge or a half H-bridge. These power switches operate with low switching loss, and thus significantly improve the efficiency of the amplifiers. For switching amplifiers, most prior-art systems employ a pulse width modulation (PWM) scheme. A typical audio PWM amplifier can work at a switching frequency of between 100 KHz and 500 KHz. Higher switching frequencies will reduce distortion but also result in lower efficiency due to the extra transitions in the output waveform. 
         [0003]      FIG. 1  is a circuit diagram of a conventional audio PWM switching amplifier, which includes a full H-bridge  10  and a controller  12 . The H-bridge  10  is constructed by four power switches M 1 -M 4  connected between a power supply Vdd and a ground terminal GND. The controller  12  provides PWM signals PWM_P and PWM_N according to an input signal Vin, to switch the power switches M 1 -M 4  so as to generate a differential output voltage OUT=OUTP−OUTN between two output terminals OUTP and OUTN, which is filtered by a low-pass filter (LPF)  14  to filter out audio components contained therein before being applied to a load  16 . In further detail,  FIG. 2  is a circuit diagram of a conventional PWM generator, in which a comparator CMP compares the input signal Vin with a trianglewave signal Vt to generate the PWM signals PWM_P and PWM_N in opposite phases to each other. Hence, as shown in  FIG. 3 , the voltage at the positive output terminal OUTP switches between 0 and Vdd, the voltage at the negative output terminal OUTN is in opposite phase to OUTP, and the differential output voltage OUT switches between Vdd and −Vdd. The load current IL decays in the inductors L 1  and L 2  during the time period of the differential output voltage OUT at −Vdd, and re-establishes in the opposite direction during the period of the differential output voltage OUT at Vdd. Since the output squarewave OUT has the amplitude of 2Vdd and has the duty of 50% when the input signal Vin is zero, the load current IL will have a great ripple and the equivalent series resistance (ESR) of the LPF  14  will cause a great power consumption. 
         [0004]    On the other hand, the output filter  14  will reduce the efficiency of the switching amplifier and typically includes external inductors L 1 , L 2  and capacitors C 1 -C 3  which are expensive and consume undesirable amounts of space. Therefore, filterless switching amplifiers have been proposed. However, this will require that the load  16  be inductive. Considering a pure resistive load  16 , switching the H-bridge  10  in a binary fashion would place the power supply voltage Vdd across the load  16 . Unlike the current waveform IL shown in  FIG. 3 , the resulting load current IL would be a squarewave with a magnitude equal to the power supply voltage Vdd divided by the resistance of the load  16 , and this is with no signal. Although the electrical equivalent of a speaker is somewhere between purely resistive and purely inductive, this would still prevent filterless switching amplifiers in audio applications as the main benefit of efficiency is lost. Even in case of inductive loads, for operation near zero crossing, or no audio signal (Vin=0), the majority of the load current IL is wasted, and is a drop in efficiency, in addition that high electro-magnetic interference (EMI) is produced. Disclosed in U.S. Pat. Nos. 6,262,632 and 6,211,728 is a filterless switching amplifier having a ternary modulation scheme implemented in an H-bridge configuration to eliminate the zero-input load current IL, which operates the H-bridge  10  with a common mode for zero crossing state, by which the two opposite terminals OUTP, OUTN of the load  16  are simultaneously switched between the power supply Vdd and the ground terminal GND. In further detail, at zero input, the H-bridge  10  is switched between two states, one is that the transistors M 1 , M 3  are both turned on to apply the power supply voltage Vdd to the output terminals OUTN, OUTP, and the other is that the transistors M 2 , M 4  are both turned on to ground the output terminals OUTN, OUTP. Consequently, there is no current wasted at zero input. However, the common mode voltage bounces between 0 and Vdd, and this would still cause severe EMI. 
         [0005]    Disclosed in U.S. Pat. Nos. 6,847,257 and 6,970,123 are apparatus and methods to reduce the output voltage amplitude and the common mode voltage difference in filterless switching amplifiers.  FIG. 4  is a circuit diagram showing the PWM generator used therein, in which, in addition to a comparator CMP to compare the input signal Vin with a trianglewave signal Vt to generate the positive PWM signal PWM_P, another comparator CMP 1  is used to compare a signal Vin 1  which is an inverse to the input signal Vin, with the same trianglewave signal Vt to generate the negative PWM signal PWM_N.  FIG. 5 . is waveform diagram showing the differential pair Vin, Vin 1  and illustrating how they determine the waveform of the PWM signals OUTP, OUTN. This method reduces the output waveform amplitude OUT of the switching amplifier to Vdd and thus reduces the EMI. However, a minimum pulse width is required for the common mode operation, in order to maintain a non-zero load current at zero input. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to reduce the power consumption of a switching amplifier at zero input. 
         [0007]    Another object of the present invention is to reduce the EMI of a switching amplifier at zero input. 
         [0008]    It is the common mode voltage bouncing between two values to cause EMI when a switching amplifier is at zero input. To eliminate this EMI, therefore, a small current may be provided to flow through the load of a switching amplifier at zero input, to thereby maintain a constant common mode voltage. 
         [0009]    According to the present invention, to reduce the power consumption and EMI of a switching amplifier, a first current source and a first resistor are connected in series between a power input terminal and a first output terminal, and a second current source and a second resistor are connected in series between a second output terminal and a ground terminal. This circuit will supply a small current to the load of the switching amplifier at zero input and thus eliminate the common mode voltage bouncing. 
         [0010]    According to the present invention, a method for reducing the power consumption and EMI of a switching amplifier includes connecting a network having current sources and resistors to the two output terminals of the switching amplifier, and providing a small current by the network to the load of the switching amplifier at zero input to eliminate the common mode voltage bouncing. 
         [0011]    According to the present invention, a switching amplifier includes an H-bridge and two output terminals drawn therefrom, a controller to switch the H-bridge according to an input signal to generate a differential output voltage between the two output terminals, and a network having current sources and resistors connected to the two output terminals. In zero input operation, a small current is provided to the load between the two output terminals to eliminate the common mode voltage bouncing. 
         [0012]    According to the present invention, a control method for a switching amplifier includes switching an H-bridge according to an input signal to generate a differential output voltage between two output terminals in a first state, switching the H-bridge to generate a common mode voltage across the two output terminals according to the input signal in a second state, and providing a small current to the load between the two output terminals to eliminate the common mode voltage bouncing. 
         [0013]    It is therefore offered a solution by adding current sources to a switching amplifier, to reduce not only the power consumption at zero input but also the EMI effect by keeping the common mode voltage constant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a circuit diagram of a conventional audio PWM switching amplifier; 
           [0016]      FIG. 2  is a circuit diagram of a conventional PWM generator; 
           [0017]      FIG. 3  is a waveform diagram of the PWM signals of a conventional switching amplifier; 
           [0018]      FIG. 4  is a circuit diagram of a conventional differential PWM generator; 
           [0019]      FIG. 5  is a waveform diagram of the PWM signals of a conventional differential switching amplifier; 
           [0020]      FIG. 6  is a circuit diagram of a first embodiment according to the present invention at non-zero input; 
           [0021]      FIG. 7  is a circuit diagram of the first embodiment according to the present invention at zero input; 
           [0022]      FIG. 8  is a circuit diagram of a second embodiment according to the present invention; 
           [0023]      FIG. 9  a circuit diagram of a third embodiment according to the present invention; and 
           [0024]      FIG. 10  a waveform diagram showing the output waveform of a conventional switching amplifier, a general ternary modulation switching amplifier with a minimum pulse width, and a switching amplifier according to the present invention at zero-input, respectively. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]      FIG. 6  is a circuit diagram of a first embodiment according to the present invention at non-zero input, in which the controller and input signal of the switching amplifier are omitted from depiction. A load  20  is connected between output terminals OUTP and OUTN, a current source  22  and a resistor  24  are connected in series between a power supply Vdd and a node Lx 1 , and a resistor  26  and a current source  28  are connected in series between a node Lx 2  and a ground terminal GND. When the input signal Vin has a positive or negative value, the output stage works as a normal ternary modulation switching amplifier. In this case, the output current IL is mainly supplied from the power switches of the H-bridge during the period, and the additional sourcing current would not affect the function. In normal operation, the current supplied by the current sources  22  and  28  is much less than the current IL supplied by the H-bridge to the load  20 . However, as shown in  FIG. 7 , as the input signal Vin approaches zero, the current sources  22  and  28  would dominate and supply a small current I 0  to introduce a small value voltage drop across the load  20 . Therefore, the current across the load  16  during the zero input is small and the power consumption is reduced. Unlike prior arts, which are necessary to generate a minimum pulse width to maintain a small current at zero input, the current I 0  in this embodiment is offered by the additional current sources  22  and  28 . This small current I 0  reduces the common mode voltage bouncing at zero input, and thus reduce the EMI effect. In this embodiment, the small current I 0  is less than 10 μA. 
         [0026]      FIG. 8  is a circuit diagram of a second embodiment according to the present invention, in which a current source  32  and a resistor  34  are connected in series between a power supply Vdd and a node Lx 2 , and a resistor  36  and a current source  38  are connected in series between a node Lx 1  and a ground terminal GND, by which a small current I 0  is provided to generate a small voltage across the load  30  between the nodes Lx 1  and Lx 2  at zero input. This circuit operates identically to that of  FIG. 6 . 
         [0027]      FIG. 9  is a circuit diagram of a third embodiment according to the present invention, in which a current source  42  and a resistor  44  are connected in series between a power supply Vdd and a node Lx 1 , a resistor  46  and a current source  48  are connected in series between the node Lx 1  and a ground terminal GND, a current source  52  and a resistor  54  are connected in series between the power supply Vdd and a node Lx 2 , and a resistor  56  and a current source  58  are connected in series between the node Lx 2  and the ground terminal GND. Thereby, a small voltage will be present across the load  50  between the nodes Lx 1  and Lx 2  at zero input and reduces or eliminates the common mode voltage bouncing. 
         [0028]    There is an additional benefit for solving EMI problem of the output stage. If the source currents are designed properly, the nodes Lx 1  and Lx 2  would be kept at a constant voltage, usually half of the supply voltage Vdd, overall period. The common mode voltage of the output then is kept constant without variation so that the EMI issue due to the unstable common mode voltage can be avoided. 
         [0029]      FIG. 10  is a waveform diagram of the output currents IL at zero input (Vin=0) of (a) a conventional switching amplifier, (b) a general ternary modulation switching amplifier with minimum pulse width, and (c) a switching amplifier according to the present invention. As shown in  FIG. 10(   a ), the conventional switching amplifier has a large load current  60  and an output voltage  62  switched between two levels. The general ternary modulation switching amplifier with minimum pulse width, as shown in  FIG. 10(   b ), has a load current  64  slightly smaller than that of  FIG. 10(   a ), and an output voltage  66  staying at zero most of the time but having a minimum pulse width. As shown in  FIG. 10(   c ), the switching amplifier according to the present invention has a very small load current  68  and a constant output voltage  70 . 
         [0030]    As illustrated by the above embodiments, by adding proper current source(s), the power dissipation can be reduced without the need to generate a minimum pulse width to offer the output current at zero input. In addition, the output common voltage would be kept at a specific voltage to avoid the EMI affection due to the variable common mode voltage at the output. 
         [0031]    While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation to encompass all such modifications and similar arrangements.