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LTC3355 Recommended for New Designs
The LTC3355 is a complete input power interrupt ride-through DC/DC system. The part charges a supercapacitor while delivering load current to VOUT, and...
Datasheet
LTC3355 on Analog.com

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How do I use an LTC3355 with a LiFePO4 battery?

		
	
				

			

									
					    bowzee
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jul 22, 2025 
				
			
			
		

		
		                     
				
        		

		    
		    		    

		      		            		        
Category: Hardware

    		        		            		    
Product Number:  LTC3355  

    		        		    		    

		    		    
		    			

			    
How do I use an LTC3355 with a LiFePO4 battery?


I tried using the last figure in the datasheet called "NiMH Trickle Charger and Ride-Through Backup Supply" modified like this:


- Vout (pin 15) is 5.00V using appropriate voltage divider


- L1, the main inductor, is 6.8uH (pin 7)


- Vcap (pin 14) is 3.58V using appropriate voltage divider


- Ichg (pin 12) set to 1A using RIchg = 60.4kohm


I tried varying the (undocumented?) resistor on Vcap (pin 14), but the voltage on the battery never goes beyond 3.3V.


Any ideas?


 

			

			
									
		                             
                        

    

                                            
                 power management
                
                                                    
                 LTC3355
                
                                                    
                 hardware
                
                                                    
                 Supercapacitor Chargers
                
                                                    
                 lifepo4
                
                                        
                                            
                
                
                                
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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Jul 23, 2025 8:24 AM
            
            
						
			            		
    
		
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    Hello,
    

    With VOUT set to 5V the charger should be able to charge up close to 5V providing the input current limit is not reached while doing so. The input currents sense resistor should be set to limit the input current based on the input voltage and the output voltage. IIN should be about VOUT*IOUT/(Efficiency*VIN). When the input current * RS = about 40mV then the charger is disabled. This should be reduced.
    

     
    

    Lithium batteries are a much different battery than NiMH batteries. Care must be made not to over charge or discharge them. The LTC3355 does not have a built-in algorithm to control the charging of the battery. It also does not have the ability to prevent the battery from being deeply discharged. The LTC3355 does have the ability to disable the charger but not to disconnect the battery and prevent deep discharge.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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                on Jul 24, 2025 8:53 AM
            
            
												
						in reply to MartyM
					
										
			            		
    
		
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     Thanks for the reply. It does not seem like the input voltage and/or current are an issue, but after your suggestion, I will check this. About the LiFePO4, for me it is a very new technology. I was able to figure out that, although the so called "nominal voltage" is 3.2V, any charging voltage smaller than 3.65V should be OK and that discharging below 2.5V will damage the battery. Right now my focus is on charging it up to 3.58V ~ 3.6V using the LTC3355, assuming that this is possible.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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