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LTC3803 soft start issue



		

			

									
					    Kencheng
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Sep 9, 2019 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi ADI Experts
    

    I am using LTC3803 for POE at application. but I have some inrush current issue that needs to be reduced.
    

    since the SS is internally controlled by LTC3803, May I know if it is possible to build a external SS circuit on Ith pin?
    

    Does ADI/LTC have reference circuit for external soft start for LTC3803?
    

    

    thanks
    

    Ken
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	
    
		
	
    















    
	
    
	
    
					



			








	
	
	
	



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													+1
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
			                            
                        
                on Sep 9, 2019 2:22 PM
            
            
					
      
	
      

		
		
      
					
      Hi Ken:
      

      If this is isolated application and you are using OPTO feedback circuit the soft start should be implemented on the secondary side. We have LT4430 part that addresses that issue.
      

      The internal soft-start should be adequate for non-isolated application. If not, there is a circuit in LTC1871 data sheet that shows how to add external soft-start in non-isolated application.
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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													0
							
			
				
									
							        
			
			
			
        
            
			                            
                        
                on Sep 10, 2019 5:26 AM
            
            
												
						in reply to GPerica
					
									
        
	
        

		
		
        
					
        HI Gperica
        

        thanks for your prompt reply.  My application is an isolated power supply but I used a Aux winding for FB
        

        I checked the reference schematicn of  LTC1871 with external SS and ran a simple simulation(added the circuit at the output of the Aux winding),
        

        I think the circuit is work!~
        

        

        thanks for the suggestion
        

        Ken 
        
			
        

		
		
		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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													0
							
			
				
									
							        
			
			
			
          
            
			                            
                        
                on Sep 10, 2019 5:26 AM
            
            
												
						in reply to GPerica
					
									
          
	
          

		
		
          
					
          HI Gperica
          

          thanks for your prompt reply.  My application is an isolated power supply but I used a Aux winding for FB
          

          I checked the reference schematicn of  LTC1871 with external SS and ran a simple simulation(added the circuit at the output of the Aux winding),
          

          I think the circuit is work!~
          

          

          thanks for the suggestion
          

          Ken 
          
			
          

		
		
		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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