Abstract:
An audio amplifier output stage layout technique achieves minimum cross coupling between audio amplifier channels. Regarding TDAA output stages, the typical TDAA includes two demodulation inductors per audio channel. The two pair of demodulation inductors associated with the TDAA are arranged to form an X-pattern to simultaneously minimize cross coupling between audio amplifier channels and reduce PCB layout size.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to audio amplifiers, and more particularly to a channel-to-channel separation technique to resolve cross coupling problems associated with multi-channel audio amplifier layouts.  
           [0003]    2. Description of the Prior Art  
           [0004]    Typical true digital audio amplifier (TDAA) systems include two demodulation inductors per audio channel. These demodulation inductors tend to couple in multi-channel boards if they are placed too close to one another; and this characteristic causes inferior channel-to-channel separation and other undesirable artifacts. This cross coupling is particularly problematic when downsizing printed circuit boards (PCBs), since inductors will be then be placed even closer to one another.  
           [0005]    Two common techniques have been employed to resolve inductor coupling problems. One technique includes use of fully encapsulated demodulation inductors (in contrast to bar-type, high air-gap inductors). This technique, however, has negative affects on audio performance since it causes and increase in THD. Another technique includes use of large physical PCB spacing between audio channels. This technique is also problematic however, since it has a negative influence on total PCB size, i.e., the amplifier size/power ratio goes up.  
           [0006]    In view of the foregoing, it would both desirable and advantageous to provide an inductor layout technique that minimizes or eliminates cross coupling between audio amplifier channels caused by demodulation inductors spacing. It would be further advantageous if the layout technique provided a way to further minimize printed circuit board sizes associated with multi-channel audio amplifiers such as TDAAs.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is directed to audio amplifier output stage layout techniques to achieve minimum cross coupling between audio amplifier channels. Regarding TDAA output stages, the typical TDAA includes two demodulation inductors per audio channel. The two pair of demodulation inductors associated with the TDAA are arranged to form an X-pattern to simultaneously minimize cross coupling between audio amplifier channels and reduce PCB layout size.  
           [0008]    According to one aspect of the invention, an audio amplifier output stage layout technique is implemented to reduce the amplifier size/power ratio.  
           [0009]    In yet another aspect of the invention, an audio amplifier output stage layout technique is implemented to improve audio performance, i.e. minimize channel-to-channel cross coupling.  
           [0010]    In still another aspect of the invention, an audio amplifier output stage layout technique is implemented to improve EMI performance due to minimal capacitive and inductive coupling between PCB tracks.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    Other aspects, features and attendant advantages of the present invention will be readily appreciated as the present invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:  
         [0012]    [0012]FIG. 1 is a pictorial illustrating a PCB layout having parallel tracks to position closely spaced inductors associated with different channels;  
         [0013]    [0013]FIG. 2 is a pictorial illustrating a PCB layout having tracks positioned with a 90° angle between different channels having closely spaced inductors;  
         [0014]    [0014]FIG. 3 is a pictorial illustrating the stray emission field caused by a single vertical bar-type inductor;  
         [0015]    [0015]FIG. 4 is a pictorial illustrating the stray emission field caused by a pair of vertical bar-type inductors placed close to one another and coupled out of phase; and  
         [0016]    [0016]FIG. 5 is a pictorial illustrating the stray emission field caused by two pairs of vertical bar-type inductors placed close to one another in an X-pattern. 
     
    
       [0017]    While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Cross coupling between channels is a problem associated with audio amplifiers that employ conventional H-bridge output stage layouts for placing demodulation inductors. Unshielded demodulation inductors produce large stray fields requiring large physical channel-to-channel spacing on a PCB to avoid channel cross coupling. While this technique provides good audio performance with low THD, it also undesirably requires a large PCB area.  
         [0019]    Shielded demodulation inductors produce very small stray fields allowing the inductors to be placed close to one another without cross coupling making such inductors desirable when PCB size must be reduced. Such inductors have a low saturation current level however, such that the THD is high at rising power levels, resulting in poor audio performance due to non-linear characteristics.  
         [0020]    [0020]FIG. 1 is a pictorial illustrating a PCB layout  100  having parallel tracks  102  to position closely spaced inductors  104  associated with different channels  106 ,  108 , associated with conventional H-bridge output stage routing. All power and output lines are routed via parallel tracks  102  on a narrow PCB area. Identical channels  106 ,  108  are placed side by side. PCB layout  100  undesirably produces high inductive and capacitive coupling due to parallel tracks  102 , resulting in poor channel separation with respect to audio performance, among other problems. Initiatives to solve these problems have always resulted in additional components, driving up the cost, and/or larger PCB areas.  
         [0021]    [0021]FIG. 2 is a pictorial illustrating a PCB layout  200  according to one embodiment of the present invention, and having tracks  202  positioned with a 90° angle between different channels  204 ,  206  having closely spaced inductors  208 . PCB layout  200  can be very compact, even using open inductors; since inductive and capacitive coupling between tracks  202  (channel  204 -to-channel  206 ) is minimized. The present inventor found that optimal inductor  208  placement is actually as close together as possible. PCB layout  200  was also found to provide better routing space to separate power tracks and output tracks within one channel. Those skilled in the art will appreciate that only two routing layers are sufficient to implement PCB layout  200 .  
         [0022]    The present invention is better understood by the following discussion regarding stray fields associated with inductors and with reference to FIGS.  3 - 5 , wherein FIG. 3 is a pictorial illustrating the stray emission field  300  caused by a single vertical bar-type inductor  302 . The stray field  300 , when viewed from the top of the inductor  302 , has a shape that appears like the letter ‘O’.  
         [0023]    [0023]FIG. 4 is a pictorial illustrating the stray emission field  400  caused by a pair of vertical bar-type inductors  302  placed close to one another and coupled out of phase. The stray field  400 , when viewed from the top of the inductors  302 , has a shape that appears like the number ‘8’. When used in association with a BTL true digital audio amplifier (TDAA), the two demodulation inductors  302  will typically carry the same current.  
         [0024]    [0024]FIG. 5 is a pictorial illustrating the stray emission field  500  when viewed from the top, caused by two pairs of vertical bar-type inductors  302  placed close to one another in an X-pattern. It is easy to see that channel  304 -to-channel  306  coupling can be minimized if the 2×2 inductors  302  in a 2-channel output stage are placed in a pattern having a shape that appears like the letter ‘X’. This is due to the reduction in overlap between stray emission fields  500  caused by each inductor  302 .  
         [0025]    In view of the above, it can be seen the present invention presents a significant advancement in the art of PCB layout associated with multi-channel audio output stages to minimize coupling between the output stages. This invention has been described in considerable detail in order to provide those skilled in the PCB layout art with the information need to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims that follow.