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LT3680 Production
The LT3680 is an adjustable frequency (200kHz to 2.4MHz) monolithic buck switching regulator that accepts input voltages up to 36V. A high efficiency 95mÎ...
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LT3680 on Analog.com

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LT3680 in inverting buck boost configuration

		
	
				

			

									
					    Niels
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Sep 27, 2018 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
		    			
    
			    
    Hello,
    

    I want to use the LT3680 in inverting buck boost topology to generate -15V@0.5A from +5V input voltage.
    

    I simulated the design with LTSpice (attached), and everything looks ok, efficiency is app. 78%.
    

    But according to some application notes, the Vin-to-Vout relationship of an inverting buck boost converter must be smaller that the efficiency. Otherwise the maximum output current will be zero.
    Thus, the maximum output current should be 0.14A (and not 0.5A) in my design.
    

    Can anybody please confirm if it is possible to generate Vout = -15V/0.5A from Vin = 5V with the LT3680?
    

    Thanks in advance, Niels
    

    LT3680_5V_IBB_-15V_500mA.zip
    
			
    
			
									
		                             
                        

    
    
                                            
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                on Oct 4, 2018 6:54 AM
            
            
						
			            		
        
		
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        Ok,
        

        as nobody has an answer to my question, I repeat it in simple words:
        

        How can I include the switcher's efficiency into the calculation of the maximum output current of the LT3680 in inverting-buck-boost (IBB) configuration?
        

        I am looking forward to any answer.
        

        Regards, Niels
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
				
				
				
		
		
        
			
		
        
	
        

        

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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Oct 4, 2018 12:08 PM
            
            
												
						in reply to Niels
					
										
			            		
        
		
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        Hello Niels,
        

        The higher the efficiency, the more output current an LT3680 inverting buck-boost circuit can deliver. What you can do is measuring the SW node peak current in your LTSpice simulation, then comparing it against the switch current limit in the datasheet. As long as the peak SW current measurement is less than the min. switch current limit. Your application is fine.
        

        To make it simpler, the LT3680 is a 3.5A buck regulator. So let's say if your measured peak current is less than 4A, you are good to go.
        

        Hope this helps.
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
						
				
				
				
		
		
        
			
		
        
	
        

        

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