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LTC3226 transition



		

			

									
					    JPK007
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 25, 2021 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    The datasheet for the LTC3226 states in the "Programming the Input Voltage Threshold for the Power-Fail Comparator" section on page 11:
    

    For a smooth transition from normal to back-up mode, the PFI threshold should be set 50mV to 100mV above the programmed LDO output voltage, VOUT.
    

    However, three out of the four design examples I have found do not do this.
    

    


    
Example
Data Sheet page 1
Data Sheet page 16
Design Note 498
Demo Manual DC1735A

    

    
Vout
3.331
3.331
3.331
3.331

    

    
Vpfi
3.144
3.392
3.094
3.283

    

    
Delta
-187 mV
61 mV
-237 mV
-48 mV

    
    

    

    As you can see, the only example that has the PFI threshold greater than Vout is the example on page 16 of the datasheet.
    

    

    What happens if the PFI threshold is not set between 50 mv to 100 mV above Vout?
    

    How weird is a "non-smooth" transition from normal to backup mode?
    
								
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	















    
	
    
	
    
					



			








	
	
	
	



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													+1
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
			                            
                        
                on Jun 9, 2021 7:05 PM
            
            
					
      
	
      

		
		
      
					
      First, let me say I appreciate your attention to detail and your efforts which have uncovered these inconsistencies.  
      

      Obviously, the statement, "For a smooth transition from normal to back-up mode, the PFI threshold should be set 50mV to 100mV above the programmed LDO output voltage, VOUT," is not a quantitative design requirement.  In the context, "smooth" should be interpreted as "does not interfere with operation of the load."  Choosing PFI and LDO resistive dividers such that the PFI threshold is between 50-100mV greater than the output voltage should be employed as a design guideline or perhaps a design starting point.  Validation of any power-fail switchover circuit must include lab evaluation of switchover behavior, and the PFI divider ratio for a particular output voltage should be selected to ensure that operation of the load is not impaired by switchover in either direction.
      

      Still, our documentation should show better consistency.
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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													+1
							
			
				
									
							        
			
			
			
      •  Analog Employees  
      
            
			                            
                        
                on Jun 9, 2021 7:05 PM
            
            
					
      
	
      

		
		
      
					
      First, let me say I appreciate your attention to detail and your efforts which have uncovered these inconsistencies.  
      

      Obviously, the statement, "For a smooth transition from normal to back-up mode, the PFI threshold should be set 50mV to 100mV above the programmed LDO output voltage, VOUT," is not a quantitative design requirement.  In the context, "smooth" should be interpreted as "does not interfere with operation of the load."  Choosing PFI and LDO resistive dividers such that the PFI threshold is between 50-100mV greater than the output voltage should be employed as a design guideline or perhaps a design starting point.  Validation of any power-fail switchover circuit must include lab evaluation of switchover behavior, and the PFI divider ratio for a particular output voltage should be selected to ensure that operation of the load is not impaired by switchover in either direction.
      

      Still, our documentation should show better consistency.
      
			
      

		
		
		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
				
						
						
				
				
		
		
      
			
		
      
	
      

      

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