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LTC3805-5 Production
The LTC3805-5 is a current mode DC/DC controller designed to drive an N-channel MOSFET in flyback, boost and SEPIC converter applications. Operating frequency...
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LTC3805-5 on Analog.com

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LTC3805-5 min duty cycle question



		

			

									
					    bytewise
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Jun 22, 2021 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hello,
    

    I'm building a flyback using LTC3805-5 and my input voltage conditions are 30V - 300V and have 3 outputs 18, 12 and 5.5V. The total load on my power supply is about 16W.
    

    I wanted to know the what would be the implications of having a duty cycle less than 6% as my initial tests have suggested that the duty cycle could be as low as 1.5%. I've simulated by circuit in LTSpice & already seeing that some Gate Drive signals are being skipped/Blanks - will this be a concern?
    

    Lpri - 56uH
    

    Lsec - 27.7uH
    

    Laux - 13uH
    

    Lter - 2.7uH
    

    Thanks,
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 SEPIC Regulators
                
                                                    
                 ltc3805-5
                
                                                    
                 switching regulators
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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      Hibytewise,
      

      Quite a wide range of input voltage there. When the part runs into minimum ON time limit of 6%, it will skip pulses. It is normal & nothing that can be done about that. In pulse skipping converter operation, output voltage ripple will be higher than when not pulse skipping. Therefore, you may need much bigger output caps. This is one tradeoff you should consider.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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      Hibytewise,
      

      Quite a wide range of input voltage there. When the part runs into minimum ON time limit of 6%, it will skip pulses. It is normal & nothing that can be done about that. In pulse skipping converter operation, output voltage ripple will be higher than when not pulse skipping. Therefore, you may need much bigger output caps. This is one tradeoff you should consider.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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						in reply to Fil
					
									
        
	
        

		
		
        
					
        Hi, Thanks for the reply - Can you also comment on the equation (on page 9 of datasheet) to select the Rfs (resistor to determine switching frequency)? It seems that the equation don't match up with the resistor values as mentioned in graph ( Oscillator Frequency vs RFS)...
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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