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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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                on Feb 8, 2026 7:36 AM
            
            
						
			            		
        
		
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        Hi again,
        

        I have made progress. Firstly the 2V5 supply looked clean even with the cap a little distance from the pin. I measured about 60mVpp of clock noise on this supply, but I think my oscilloscope was picking that up from the general setup. I did the capacitor mod and with the cap in the optimal location and I still see the same result.
        

        However, I randomly changed Rsnsc from 8mR to 50mR (4A current limit reduced to 640mA) as I don't need anywhere near 4A in backup mode. My understanding from the datasheet was that this should have no effect upon charging as the charge limit is set by Rsnsi which I have set to 50mR (640mA total input current limit).
        

        With, this change in Rsnsc the issue seems to be resolved. I have done this to 5 boards now and they all start charging as expected when I either slowly ramp up the input voltage or hit them with with the default 12V Vin. When I apply Vin I see the current smoothly ramp up and drop off again when the capacitors are fully charged.
        

        Have you seen the Rsnsc resistor having any effect like this before? Maybe the difference between Rsnsi and Rsnsc shouldn't be too great?
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
				
				
				
		
		
        
			
		
        
	
        

        

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                on Feb 8, 2026 7:36 AM
            
            
						
			            		
          
		
										0
					
	
          

		
		
          
		
          
                                
                                                            
          
					
          Hi again,
          

          I have made progress. Firstly the 2V5 supply looked clean even with the cap a little distance from the pin. I measured about 60mVpp of clock noise on this supply, but I think my oscilloscope was picking that up from the general setup. I did the capacitor mod and with the cap in the optimal location and I still see the same result.
          

          However, I randomly changed Rsnsc from 8mR to 50mR (4A current limit reduced to 640mA) as I don't need anywhere near 4A in backup mode. My understanding from the datasheet was that this should have no effect upon charging as the charge limit is set by Rsnsi which I have set to 50mR (640mA total input current limit).
          

          With, this change in Rsnsc the issue seems to be resolved. I have done this to 5 boards now and they all start charging as expected when I either slowly ramp up the input voltage or hit them with with the default 12V Vin. When I apply Vin I see the current smoothly ramp up and drop off again when the capacitors are fully charged.
          

          Have you seen the Rsnsc resistor having any effect like this before? Maybe the difference between Rsnsi and Rsnsc shouldn't be too great?
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
				
				
				
		
		
          
			
		
          
	
          

          

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