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LTC3350 Recommended for New Designs
The LTC3350 is a backup power controller that can charge and monitor a series stack of one to four supercapacitors. The LTC3350’s synchronous step-down...
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LTC3350 on Analog.com

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LTC3350 refuses to charge

		
	
				

			

									
					    EricChilds
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 5, 2026 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
    
		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  LTC3350  
    
    		        		        		        
    Software Version: Not Applicable
    
    		    		    
    
		    		    
		    			
    
			    
    I have a circuit using LTC3350 that is showing odd behavior and wonder if anyone could help me resolve the issue?
    

    When Vin is increased from 0V to 12V the capacitors do not start charging. This behavior is consistent across all boards. The prototype worked reliably, but maybe this was a fluke?
    

    When probing the design to try to find the issue I have found that touching the PFI pin sometimes kicks the design into life and the capacitors then charge perfectly.
    

    Looking at the SW node, as I gradually increase Vin from 0V to 12V the SW node starts switching when I reach ~5V (at 500KHz as expected). However when I get past 11V it suddenly stops and the charger sits there doing nothing (see images)
    

    I've run out of things to try! I can't see anything wrong with the schematic, maybe I'm missing something?
    

    

    The design intent is as follows:
    

    1/ Vcap ~10V
    

    2/ Switches over to back-up mode when Vin falls below ~9.9V
    

    3/ Charge current limit set to 4A, but total current limited to 640mA (expectation is ~150mA load and 500mA left over for charging)
    

    4/ Backup voltage is 12V
    

    

    

     
    

     
    

    
			
    
			
									
		                             
                        

    
    
                                            
                 power management
                
                                                    
                 hardware
                
                                                    
                 Supercapacitor Chargers
                
                                                    
                 Not applicable
                
                                                    
                 LTC3350
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                                                                    
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 10, 2026 7:59 AM
            
            
						
			            		
      
		
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      Hello,
      

      I am glad progress is being made on this. I have not seen the RSNSC affect the ability to charge like this before. This might have to do with the fact that RSNSC adjusts the peak current to provide an average charge current in charge mode. With the peak current much higher than the RSNSI limit, I expect the peak current limit is being reached before the RSNSI has a chance to react. That is my theory but will need to investigate. Most typical designs have an input current limit 1A or greater. 
      

      The other possibility is that the larger peak current creates more noise and is causing noise on the VCC2P5 pin. I have seen issues with noise on the VCC2P5 regulator causing odd behavior like this before.  One way to test this out is to remove C11, place it on top of the IC and connect it directly between the SGND and VCC2P5 pins.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
				
				
				
		
		
      
			
		
      
	
      

      

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 10, 2026 7:59 AM
            
            
						
			            		
      
		
										0
					
	
      

		
		
      
		
      
                                
                                                            
      
					
      Hello,
      

      I am glad progress is being made on this. I have not seen the RSNSC affect the ability to charge like this before. This might have to do with the fact that RSNSC adjusts the peak current to provide an average charge current in charge mode. With the peak current much higher than the RSNSI limit, I expect the peak current limit is being reached before the RSNSI has a chance to react. That is my theory but will need to investigate. Most typical designs have an input current limit 1A or greater. 
      

      The other possibility is that the larger peak current creates more noise and is causing noise on the VCC2P5 pin. I have seen issues with noise on the VCC2P5 regulator causing odd behavior like this before.  One way to test this out is to remove C11, place it on top of the IC and connect it directly between the SGND and VCC2P5 pins.
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
				
				
				
		
		
      
			
		
      
	
      

      

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