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LTC7821 Recommended for New Designs
The LTC7821 uses a proprietary architecture that merges a soft switching charge pump topology with a synchronous step-down converter to provide superior...
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LTC7821 on Analog.com

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Low-frequency oscillations on inductor current and erratic switching behavior

		
	
				

			

									
					    SFU
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Mar 11, 2026 
				
			
			
		

		
		                    	    

        	    
Thread Summary

                The user is experiencing different behaviors in a 24V to 5.2V step-down converter using the LTC7821 with DCR sensing. The final answer suggests that the oscillation may be related to the ITH pin and slope compensation. The user is advised to optimize the compensation components and the RC placement for DCR sensing, and to use LTSpice to simulate and optimize the design.
                        	    

                
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Category: Hardware

    		        		            		    
Product Number:  LTC7821  

    		        		    		    

		    		    
		    			

			    
Hello everybody,


I am encountering a problem in a design with the LTC7821 working as a 24V to 5.2V step-down controller.


The current sensing is implemented using the inductor DCR (with a Würth 7443551111 inductor).


I am observing very different behavior between light load conditions (~800mA mean inductor current) and higher load (~1.6A inductor current).


Please see the attached oscilloscope screenshots (the inductor current is shown inverted). I cannot quite pinpoint where the inductor current oscillation is coming from, and also don't understand the switching behavior at ~270us.


Does somebody better understand what kind of effect is occurring here?


Best regards
Simon


high-load condition:


High Load condition


high-load condition, detail on switching behavior:


high-load condition, zoom


low-load condition:


low-load condition

			

			
									    

			        
Edit Notes

    				update images with higher resolution

[edited by: SFU at 4:20 PM (GMT -4) on 11 Mar 2026]
				

						
		                             
                        

    

                                            
                 hardware
                
                                                    
                 High Input Voltage Buck Regulators
                
                                                    
                 LTC7821
                
                                                    
                 switching regulators
                
                                                    
                 Step-Down (Buck) Regulators
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			


				
	









	
    
	

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    •  Analog Employees  
    
            
						
                            
            
            				
            
                on Mar 31, 2026 5:29 AM
            
            
						
			            		
    
		
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    Hello,
    The oscillation you are seeing can be associated to the compensation pin ITH and slope compensation as discussed in the datasheet. If you have followed the layout suggestions from kelvin connections to grounding, you can start optimizing the components related to compensation. You can also start re-creating your circuit design with LTSpice as this controller spice model is available. From there  you can play around  the values of the compensation and get the optimized initial value for your design. DCR sensing can be tricky, the RC placement and value is critical. Try to optimized its value as well. Hope this provide some insights.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
				
				
				
		
		
    
			
		
    
	
    

    

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                on Mar 31, 2026 10:21 AM
            
            
						
			            		
    
		
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    Hello tonie,
    

    thanks for your answer! I could indeed trace back the problem to the inductor compensation (the inductor has most probably been exchanged at some point in the design process, but the compensation components have not). The optimum solution over the full temperature and power range was to choose a slightly smaller RC time constant than the inductor time constant, since the coil will have higher DCR and lower inductance at high currents and temperatures.
    

    Best regards
    Simon
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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