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LTC4079 I_Leak and I_FB help



		

			

									
						m3atwad
					
							

			
				
									
											
							 		
		        	            							
			
			    	
			                    
                
				                on Oct 8, 2019 
				
			
			
		


				
		

		    		    
		    
		    	
			
Hello,




I'm designing the LTC4079 into a product and am confused regarding the voltage divider equation in the "Feedback Divider Selection" section.  At the end of the Vchg equation there is the term (I_FB + I_LEAK).  The paragraph under the equation is not clear.  How does one figure out these terms and what is the difference between the two.  Does the attached plot describe the leakage current through the feedback pin due to the 160mR FBG pin?  Should I just estimate this to be 1nA at 25C?  If so, how does one come up with the I_LEAK value?

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	










	

	

					



			


				
	









	
    
	

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                                 Analog Employees 
                                
                            
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                on Oct 8, 2019 3:18 PM
            
            
					
    
	
    

		
		
    
					
    Hello,
    

    I agree that this is made more confusing than it needs to be. ILEAK is defined as parasitic leakage external to the IC, so perhaps leakage of a capacitor being used for feedback compensation, or any other leakage that you may have introduced. My recommendation is to just ignore it as it will make a negligible impact on the calculation anyway; even the example shows far less than the tolerance of a 1% resistor.
    

    As for IFB, yes, you can use 1nA at 25degC. Again, it is arguably negligible. Here's the plot:
    

     
    

    Id' say the bottom line here is to use FB resistor values that are not so high that these currents matter. Of course, you don't want to go so low that there is significant drain on your battery. The resistor value ranges shown in the datasheet applications are a good starting point.
    

    

    Regards,
    

    Zack
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
						
				
				
		
		
    
			
		
    
	
    

    

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													0
							
			
				
									
							        
            
                                    			
			                            
                        
                on Oct 20, 2019 11:10 PM
            
            
												
						in reply to Zack
					
									
    
	
    

		
		
    
					
    Ok thanks for the direction.  That is what I ended up doing!
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    

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