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ADP2120 Production
The ADP2119 / ADP2120 are low quiescent current, synchronous, step-down DC-to -DC regulators in a compact 3mm x3mm LFCSP_WD package. Both Devices use current...
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ADP2120 on Analog.com

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ADP2120 operating in a discontinuous mode when synchronized.

		
	
				

			

									
					    Kevin@SRS
					
							

            
            			
				
									
											
			
			
				
			
			
			    	
			                    
                
				                on Jun 4, 2013 
				
			
			
		

		
		                     
				
        		

		    
		    		    
		    			

			    
I have an ADP2120 circuit that is operating it what looks like a discontinuous mode, but is synchronized to an input clock.

 
The circuit is a 5V to 3.3 V circuit using the component values I got from ADIsimPower. The current draw on the 3.3V varies vs time. The switch waveform has an odd ring that moves around. The switch turns on (0V out) for ~200 ns, then softly releases and slews up to ~4V, then drops to 3.5V before sharply turning off (5V). The switch waveform looks a little like the DCM (discontinuous conduction mode) in the data sheet, but with only a single broad ring. The circuit is synchronized to to a 1.2MHz clock (3.3V) so it shouldn't try to go to the PFM mode.

 
 I haven't found any info about the DCM in the ADI web site yet. I have another ADP2120 operating at 5V to 1.4V that behaves nearly ideally. Any ideas or suggestions as to what this is & how to make it operate in the continuous conduction mode?

			

			
									
		                             
                        

    

                                            
                 ADP2120
                
                                                    
                 Internal Power Switch Buck Regulators
                
                                                    
                 Step-Down (Buck) Regulators
                
                                
            

    
    
		


				
				        
        		

			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		


        
        
        
    
					

			





















	

	

					



			








	
	
	
	



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                on Jun 4, 2013 7:43 PM
            
            
						
			            		
    
		
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    The circuit is synchronizing correctly. The schematic is essentially the same as the application circuit in the data sheet (figure 50) with L= 2.2uH inductor, Cin = 2x 1u, Cout = 2x 10u. The circuit appears to be operating in the Discontinuous conduction mode, not the PFM mode. I've inserted a couple scope shots of the switch waveforms.
    
 
    
 
    
 
    
 
    As is, the circuit is generating a lot of low frequency noise. I believe this is due to the varying pulse width modulation. I want to get it to operate in the Continuous conduction mode where there should be less modulation of the inductor current. I *think* that increasing the inductor value should accomplish this. I would appreciate any info or links to any ADI documents that can offer insite into this.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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                on Jun 4, 2013 7:43 PM
            
            
						
			            		
    
		
										0
					
	
    

		
		
    
		
    
                                
                                                            
    
					
    The circuit is synchronizing correctly. The schematic is essentially the same as the application circuit in the data sheet (figure 50) with L= 2.2uH inductor, Cin = 2x 1u, Cout = 2x 10u. The circuit appears to be operating in the Discontinuous conduction mode, not the PFM mode. I've inserted a couple scope shots of the switch waveforms.
    
 
    
 
    
 
    
 
    As is, the circuit is generating a lot of low frequency noise. I believe this is due to the varying pulse width modulation. I want to get it to operate in the Continuous conduction mode where there should be less modulation of the inductor current. I *think* that increasing the inductor value should accomplish this. I would appreciate any info or links to any ADI documents that can offer insite into this.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
				
				
				
		
		
    
			
		
    
	
    

    

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