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LTC4267-1 Recommended for New Designs
The LTC4267-1 combines an IEEE 802.3af compliant Powered Device (PD) interface with a current mode switching regulator, providing a complete power solution...
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LTC4267-1 on Analog.com

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POE with LTC4267-1



		

			

									
					    andyp
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Apr 2, 2021 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi all
    

    I have a problem with LTC4267-1. I try to use it for an application but when I connect it to the switch the output voltage instead of being 12V as foreseen by the diagram oscillates between 12V and 0V
    

    I attach the schematics
    

    PDF
    

    

    Thanks for help!
    

    Andrea
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 PoE Powered Device (PD)
                
                                                    
                 LTC4267-1
                
                                                    
                 power by linear
                
                                                    
                 Power-over-Ethernet (PoE) Interface Controllers
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Apr 7, 2021 8:24 PM
            
            
					
      
	
      

		
		
      
					
      Hi Andrea,
      

      Can you try switching out the MOC211M to a part with a specified minimum and maximum CTR? MOC207M would be a better choice with a CTR between 100% and 200%. You might want to try bumping up C314 to 22nF or 33nF. You may also want to confirm the PVCC voltage is steady and above 7V but less than 11V.
      

      Best Regards,
      

      Eric
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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                on Jul 30, 2021 4:04 AM
            
            
					
        
	
        

		
		
        
					
        Hi Eric 
        

        Thanks for your reply.
        

        PDF
        

        Unfortunately i can't find the MOC211M ...
        During this time I redesigned the board and set the output voltage to 5V.
        I have some improvements there but the circuit still doesn't work.
        In input I have the correct voltage coming from the ethernet switch (the switch LED indicates a connected POE device) but PVCC is equal to 0.6V and the Q1 mosfet is short-circuited DRAIN and SOURCE.
        I have already tried to replace LTC4267, Si3440DV and PA1137 but as soon as I connect to the switch the situation reappears.
        

        PS I find the same behavior if I change the transformer with a PA1138 and if I change the resistors R9 and R10 to have an output voltage of 12V
        

        

        thanks for yours help
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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