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LT4276 Recommended for New Designs
The LT4276 is a pin-for-pin compatible family of IEEE 802.3 and LTPoE++ Powered Device (PD) controllers. It includes an isolated switching regulator controller...
LT4276 on Analog.com

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LT4276 handshake failure

		
	
				

			

									
					    twilkers1976
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 18, 2025 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
    
		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  LT4276  
    
    		        		    		    
    
		    		    
		    			
    
			    
    Bringing up our version of POE copied from the eval board schematic DC2046AC version, but output doesn't turn on.  I'm using a POE+ switch as input and the attached pics are monitoring the VPORTP and VPORTN inputs. We don't have any additional loads on the input as the other post.  I currently have no load on the circuit output.  RCLASS++ is open and RCLASS is 82 ohms.  It seems that the handshake is trying, but not satisfied.  Would you have any suggestions for next steps?
    

      
    
			
    
			
									
		                             
                        

    
    
                                            
                 PoE Powered Device (PD)
                
                                                    
                 hardware
                
                                                    
                 LT4276
                
                                                    
                 Power-over-Ethernet (PoE) Interface Controllers
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                                                                    
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Jan 6, 2026 8:00 PM
            
            
						
			            		
      
		
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      Hello,
      

      Sorry for the delayed response but I was out-of-office for the holidays. Have you tested the this POE+ switch with other PDs or the DC2046A-C demo board itself?
      

      From the waveform, it looks like the first step, Detection is failing. Detection can be corrupted by AC noise in the system, which appears present but not excessive. Can you double check all capacitance between VPORTP and VPORTN? Too much differential capacitance can exceed IEEE requirements. Are you using differential scope probes and if not does the issue change if you use differential probes or disconnect the scope from VPORTP and VPORTN? 
      

      Can you also confirm you are building a Class 2 PD (6.49W Max)?
      

      

      Best Regards,
      

      Eric
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
				
				
				
		
		
      
			
		
      
	
      

      

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