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Powering Circuit Q's



		

			

									
					    dlh
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 3, 2010 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hello,
    
 
    I am have some questions regarding the Schematics of the BF548 ezkit (attached). Specifically
    
 
    the powering sections.
    
 
    Firstly, a basic questions, what is the difference between the filled square and hollow 
    
 
    circle schematic symbol for the power buses? The page Power Page 1, for example,
    
 
    on the bottom right shows the circuitry for the VDDEXT and 3.3V, shown as filled square
    
 
    and hollow circle respectively. They are both 3.3V so what is the difference?
    
 
    Looking now at Power Page 2, I don't really understand the power supply for VDDDR and 
    
 
    5V. What does the adp1823 chip setup do? I looked briefly at its datasheet but didn't
    
 
    understand how it relates to the regulation of VDDDR and 5V (again the symbols are different?).
    
 
    On the bottom left of this page there are two outputs for 5V_USB? Why?
    
 
    Also, on the bottom right there are these symbols (MH4, MH5, etc) but they aren't connected
    
 
    to anything? What are they for?
    
 
    It's pretty evident there are lots of things that I'm confused with - the power circuitry has been
    
 
    a bit of a hurdle. Is there an easier design for the power supply? Would really appreciate
    
 
    any reference designs that have a simpler powering system.
    
 
    Thanks to anyone who can be bothered to try and help. I know it is not exactly 'fun', but it 
    
 
    will be much appreciated.
    
 
    Cheers...
    
									
			        
		
    

				
							A0206-2006_CUST.pdf
							
        

    
    Tags: 
                                            
                 External Power Switch Buck Controllers
                
                                                    
                 ADP1823
                
                                                    
                 Step-Down (Buck) Regulators
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 26, 2010 8:06 PM
            
            
					
      
	
      

		
		
      
					
      Hi D H,
      
 
      A close examination of the tolerance specification for the BF54x USB pins shows that they are only 5volt tolerant if VDDUSB is at 3V.  The specification may be violated if your device is powered off but you are connected to a live USB host or hub.  If this condition occurs for a long time, the processor may fail.  I can’t be explicit here but my belief is that it would take many months of elevated temperature to cause a failure.
      
 
      Therefore we suggest that for applications like PC peripherals that can expect to be powered off for very long periods, that you find another way to power VDDUSB.  That is the purpose of these diodes, when local power is on; the 3.6V supply through D2 puts 3.3V on VDDUSB.  When there is no local power, the VBUS pin caries 5V from the host and will provide about 3V to VDDUSB through D3 and D7.
      
 
      This extra circuitry is only required for the BF54x family.  There are also many BF54x applications that do not require this circuit.  USB powered devices are an example of a designs that do not require the additional diodes.
      
 
      This and other hardware topics are discussed in EE-281.
      
 
      _BobK
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 26, 2010 8:06 PM
            
            
					
      
	
      

		
		
      
					
      Hi D H,
      
 
      A close examination of the tolerance specification for the BF54x USB pins shows that they are only 5volt tolerant if VDDUSB is at 3V.  The specification may be violated if your device is powered off but you are connected to a live USB host or hub.  If this condition occurs for a long time, the processor may fail.  I can’t be explicit here but my belief is that it would take many months of elevated temperature to cause a failure.
      
 
      Therefore we suggest that for applications like PC peripherals that can expect to be powered off for very long periods, that you find another way to power VDDUSB.  That is the purpose of these diodes, when local power is on; the 3.6V supply through D2 puts 3.3V on VDDUSB.  When there is no local power, the VBUS pin caries 5V from the host and will provide about 3V to VDDUSB through D3 and D7.
      
 
      This extra circuitry is only required for the BF54x family.  There are also many BF54x applications that do not require this circuit.  USB powered devices are an example of a designs that do not require the additional diodes.
      
 
      This and other hardware topics are discussed in EE-281.
      
 
      _BobK
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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