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LTC3871 Recommended for New Designs
The LTC3871/LTC3871-1 is a high performance bidirectional buck or boost switching regulator controller that operates in either buck or boost mode on demand...
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LTC3871 on Analog.com

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[LTC3871] SETCUR Voltage



		

			

									
					    T_yoshi
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Nov 16, 2022 
				
			
			
		
    

				
		
    
		    		    
		    
		    		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  LTC3871 
    
    		        		    		    	
			
    Hi team,
    

    

    My customer is trying to set the SETCUR voltage with a resistive voltage divider by connecting a resistor not only to GND but also to a separate 3.3V supply to set the SETCUR.
    

    However, when they actually measure the SETCUR voltage, it is much lower than the design value.
    

    Is it not possible to use it in a resistor divider network like this?
    

    Or does the pullup need to be 5V?
    

    

    I have one more question.
    

    They say the voltage on SETCUR is 0V at light load.
    

    I have read the datasheet and understand that this is not the case, is this understanding correct?
    

    

    Best regards,
    

    T-yoshi
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 hardware
                
                                                    
                 High Input Voltage Buck Regulators
                
                                                    
                 LTC3871
                
                                                    
                 Step-Down (Buck) Regulators
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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							FormerMember
					
      
		
      
			
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									FormerMember
							        
			
			
			
    
            
						
                            
            
            				
            
                on Nov 16, 2022 7:02 PM
            
            
					
    
	
    

		
		
    
					
    Hi T-Yoshi,
    

    T_yoshi said:
    They say the voltage on SETCUR is 0V at light load.
    

    I have read the datasheet and understand that this is not the case, is this understanding correct?
    

    

    During at light load / no load, SETCUR voltage should be around 1.25V.
    

    

    T_yoshi said:
    My customer is trying to set the SETCUR voltage with a resistive voltage divider by connecting a resistor not only to GND but also to a separate 3.3V supply to set the SETCUR.
    

    However, when they actually measure the SETCUR voltage, it is much lower than the design value.
    

    Is it not possible to use it in a resistor divider network like this?
    

    

    

    SETCUR voltage divider can be connected to an input supply of 3.3V as long as it satisfies the calculations (Rsense & current limit).
    

    I do, however, have seen during bench testing that the Vsetcur voltage is either lower or higher than expected calculation and would adjust the divider resistors accordingly.
    

    

    Regards,
    

    Jhun
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
				
						
				
				
		
		
    
			
		
    
	
    

    


	

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													0
							
			
				
									
							        
			
			
			
      
            
						
                            
            
            				
            
                on Nov 17, 2022 2:35 AM
            
            
												
						in reply to FormerMember
					
									
      
	
      

		
		
      
					
      Hi Jhun-san,
      

      

      Thank you for your response.
      

      As additional information, SETCUR goes to 0V at start-up and at light loads.
      

      When the load current increases after startup, the constant current source turns ON, and after that the constant current source keeps turning ON.
      

      As far as I can see from the datasheet, the constant current source is driven by V5, so I think that 7.5uA will be output immediately when V5 starts.
      

      Is it correct?
      

      

      Best regards,
      

      T-yoshi
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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							FormerMember
					
      
		
      
			
													0
							
			
				
									FormerMember
							        
			
			
			
            
						
                            
            
            				
            
                on Nov 17, 2022 9:22 PM
            
            
												
						in reply to T_yoshi
					
									
      
	
      

		
		
      
					
      I think I misunderstood the question. Let me correct myself.
      

      What you will see on the IMON at no load/light load is 1.25V.
      

      When using the SETCUR with voltage divider, what you will see on the pin voltage should be the voltage drop across the low side resistor, this is also be affected by the 7.5uA sourced by the pin, regardless of the load.
      

      See snip below. Please make sure that the voltage divider satisfies the condition below.
      

       
      

      

      T_yoshi said:
      As far as I can see from the datasheet, the constant current source is driven by V5, so I think that 7.5uA will be output immediately when V5 starts.
      

      This is my understanding as well. 
      

      Regards,
      

      Jhun
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      

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							FormerMember
					
        
		
        
			
													0
							
			
				
									FormerMember
							        
			
			
			
      
            
						
                            
            
            				
            
                on Nov 17, 2022 9:22 PM
            
            
												
						in reply to T_yoshi
					
									
      
	
      

		
		
      
					
      I think I misunderstood the question. Let me correct myself.
      

      What you will see on the IMON at no load/light load is 1.25V.
      

      When using the SETCUR with voltage divider, what you will see on the pin voltage should be the voltage drop across the low side resistor, this is also be affected by the 7.5uA sourced by the pin, regardless of the load.
      

      See snip below. Please make sure that the voltage divider satisfies the condition below.
      

       
      

      

      T_yoshi said:
      As far as I can see from the datasheet, the constant current source is driven by V5, so I think that 7.5uA will be output immediately when V5 starts.
      

      This is my understanding as well. 
      

      Regards,
      

      Jhun
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
						
				
				
		
		
      
			
		
      
	
      

      





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