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LT8357 Recommended for New Designs
The LT8357 is a wide input range, current mode, DC/DC controller which can be configured as a boost, SEPIC or flyback converter. The LT8357 drives a low...
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LT8357 on Analog.com

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LT8357 Sepic - not working at low voltage input

		
	
				

			

									
					    ohadm
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jan 22, 2023 
				
			
			
		
    
		
		                    	    
    
        	    
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                The user designed a SEPIC converter using the LT8357 with a 9V-36V input and 15V, 3A output. The issue is that the output voltage drops to 5V when the input voltage is below 10.8V. The final solution involves using a 60V FET and a 4.7mOhm sense resistor, as the 40V FET and 8mOhm resistor limit the output power at lower input voltages. Additional troubleshooting steps included adjusting the EN/UVLO resistors and removing capacitors in parallel with the feedback dividers.
                        	    
    
                
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    Category: Hardware
    
    		        		            		    
    Product Number:  LT8357 
    
    		        		    		    
    
		    		    
		    			
    
			    
    Hi,

    I have designed a SEPIC converter, based on LT8357. I used the LT Power CAD and verified the design with excel sheet based on the datasheet.
    

    The design spec is 9V-36V input, 15V output, 3A. 

    The problem is: When input voltage in <10.8V, output voltage drops to 5V. 
    

    My load is ~2A at 15V.
    

    

     
    

    

    Any idea?
    

    

    Ohad.
    
			
    
			
									
		                             
                        

    
    
                                            
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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 3, 2023 5:22 AM
            
            
						
			            		
      
		
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      Hello,
      

      1. You have used a 40V FET -> in a SEPIC the Switch does see the sum of VIN and VOUT, hence 51V at 36V Vin -> You need a 60V FET. And the diode has to be a 60V type as well.
      

      2 With my approach LTpowerCAD suggests a Sense resistor of 4.7mOhm. You have chosen 8mOhm, hence you limit the possible output power at lower VIN.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
				
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 3, 2023 5:22 AM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      Hello,
      

      1. You have used a 40V FET -> in a SEPIC the Switch does see the sum of VIN and VOUT, hence 51V at 36V Vin -> You need a 60V FET. And the diode has to be a 60V type as well.
      

      2 With my approach LTpowerCAD suggests a Sense resistor of 4.7mOhm. You have chosen 8mOhm, hence you limit the possible output power at lower VIN.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
				
				
				
		
		
      
			
		
      
	
      

      

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