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MAX20048 Production
The MAX20048 is a current‐mode buck‐boost controller. The device operates with input voltages from 4.5V to 36V while using only 55µA quiescent current...
MAX20048 on Analog.com

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Want to use the IC over the range 400mV to 30V but normally 6V-10Vin and 7Vout

		
	
				

			

									
					    andrew-lohmann@home
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Dec 4, 2025 
				
			
			
		
    
		
		                    	    
    
        	    
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                The user inquired about using the MAX20048 in a vintage car dynamo system with a 400mV input, which is below the minimum operating voltage. The support engineer confirmed that the IC can operate down to 2V after initial startup with 5V, and suggested using a mechanical relay to supply the dynamo field until 5V is available. The input current is current-limited, but the voltage rise due to the input diode is manageable. Other ICs like the LTC3789 and LM51772 were also considered, but the MAX20048 is the most suitable for the application.
                        	    
    
                
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    Category: Hardware
    
    		        		            		    
    Product Number:  MAX20048 
    
    		        		    		    
    
		    		    
		    			
    
			    
    The power input starts at 400mV and is also supplied with 5V at the IN pin to power the MAX20048 internal circuitry.
    

    https://blog.andrew-lohmann.me.uk/2025/09/vintage-and-veteran-car-dynamo-power.html
    

    So 400mV is below the minimum operating voltage so some of the functions, I guess may not work, but if enough function do for example the input current does not create an over current signal then the power supply may work fine.  Can you tell me if this is possible?
    

    The application is for a vintage or veteran car electrical system so that the dynamo can be used and more of the power generated normally not used can be used.  It would be for 12V or 6V car.
    
			
    
			
									
		                             
                        

    
    
                                            
                 hardware
                
                                                    
                 External Power Switch Buck-Boost Controllers
                
                                                    
                 Buck-boost_lower_Vin
                
                                                    
                 MAX20048
                
                                                    
                 Buck-Boost Regulators
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                                                                    
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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                on Dec 17, 2025 9:12 AM
            
            
						
			            		
        
		
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        I found a model in the LTspice library.  It works well from 200mV or less input, but I see that consequently CSN1 and CSP1 are operating above IN.  If I place a diode to IN then IN becomes 500mV below CS_1 plus 100R series resistors is still out of spec which is 300mV.

        LTC3789 also works on LTspice.  In that case Vin and EXTVcc are connected to a 5V axillary, this works.

        LM51772 made by ti.com also looks like it will work, but using it within spec. Though testing it is outside their modelling tools, voltage range.  Some Renasas part looked good, but they tell me it won't work with such a low voltage input and an axillary 5V power supply.                             
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
				
				
				
		
		
        
			
		
        
	
        

        

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                on Jan 7, 2026 7:36 AM
            
            
						
			            		
          
		
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          It turns out that this IC if the simulator is correct, once started up by nearly 5V it then continues to operate down to 2V.  This works, and I can use a mechanical relay to supply the field from the dynamo until there is at least 5V to start it initially.  After that it runs as I want it to 2.5V to 30Vin.
          

          The supply is current limited, though, which means that the brief 2mS of high current at start up is limited.  But the input voltage by the LTspice model does rise because of the input diode turns out to not be too severe.
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
				
						
				
				
				
		
		
          
			
		
          
	
          

          

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