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LTC2980 Recommended for New Designs
The LTC2980 is a 16-channel Power System Manager used to sequence, trim (servo), margin, supervise, manage faults, provide telemetry and create fault logs...
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LTC2977 Recommended for New Designs
The LTC2977 is an 8-channel Power System Manager used to sequence, trim (servo), margin, supervise, manage faults, provide telemetry and create fault logs...
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LTC2977 on Analog.com

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LTC2980 VPWR Unexpected Voltage drop on input



		

			

									
					    ja424fs
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Feb 10, 2022 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hello, I have an LTC2980 Power Sequencer with the input power connected similarly to shown in Figure 30 of the LTC2977 datasheet.
    

    The difference is that I have a Schottky Diode between the system 5V and the LTC2980 VPWR inputs.
    

    I am seeing a 1V drop across the Schottky Diode that I'm not expecting.  In other words the VPWR inputs are measuring 4.0V. 
    

    This tells me that the current draw on these pins (A_VPWR and B_VPWR) are many Amps.
    

     
    

    Any ideas why I would be seeing this voltage drop?  Thanks.
    
										

    
				Corrected part numbers.
    
[edited by: ja424fs at 8:49 PM (GMT -5) on 10 Feb 2022]
				
    
						
			        
		
    

				
						
        

    
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                 ltc2977
                
                                                    
                 ltc2980
                
                                                    
                 Digital Power System Managers
                
                                                    
                 Digital Power System Management
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on Feb 10, 2022 4:31 PM
            
            
					
      
	
      

		
		
      
					
      First, let me ask why you need to add a schottky to VPWR.  The VPWR pins may go to GND while VDD33 is applied.  That is, there is no internal diode between VDD33 and VPWR.  An external diode is not necessary.
      

      A BAS70 is rated for 70mA max. It may fail open or resistive if it sees high current.
      

      The 2980 has two 2977s inside, and the pinout allows you to treat each 2977 as a separate chip.  Have you tied the two VPWR pins together?  Each 2977 draws ~10mA, so 20mA total.  If the schottky is dropping 1V, then it will be obvious whether the 2980 is drawing lots of current by feeling the package to see if it gets hot. 
      

      Did you leave the DNC pins floating?  
      

      Regards
      

      Mike
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
						
				
				
		
		
      
			
		
      
	
      

      

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                on Feb 10, 2022 4:54 PM
            
            
												
						in reply to mpeters
					
									
        
	
        

		
		
        
					
        I don't know why the Schottky diode is there as I "inherited" the design.  There may have been some concern that current would pass through the LTC2980 when powered by VDD33 for programming.
        

          

        • The A_VPWR and B_VPWR pins are tied togther.
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        • The Schottky diode is not hot.
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        • The A_DNC and B_DNC pins are floating.
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        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Feb 10, 2022 5:49 PM
            
            
												
						in reply to ja424fs
					
									
        
	
        

		
		
        
					
        OK, I see.  The purpose of the PFET is to shut off when Vin is applied, the gate pulls high.  This allows the internal 3.3V LDO to operate.  If Vin is 5V, I suggest removing the gate divider and tie the gate directly to VPWR/5V.  We added the divider to the datasheet since most apps use 12V for Vin and some PFETs in the past had 8V limit to Vgs.  When Vin is powered off, the gate goes to GND, and the PFET is fully on, connecting the external 3.3V supply to VDD33.
        

        I suggest you replace the diode with a zero ohm SMD jumper.
        

        Mike
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
						
						
				
				
		
		
        
			
		
        
	
        

        

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													0
							
			
				
									
							        
			
			
			
        •  Analog Employees  
        
            
						
                            
            
            				
            
                on Feb 10, 2022 5:49 PM
            
            
												
						in reply to ja424fs
					
									
        
	
        

		
		
        
					
        OK, I see.  The purpose of the PFET is to shut off when Vin is applied, the gate pulls high.  This allows the internal 3.3V LDO to operate.  If Vin is 5V, I suggest removing the gate divider and tie the gate directly to VPWR/5V.  We added the divider to the datasheet since most apps use 12V for Vin and some PFETs in the past had 8V limit to Vgs.  When Vin is powered off, the gate goes to GND, and the PFET is fully on, connecting the external 3.3V supply to VDD33.
        

        I suggest you replace the diode with a zero ohm SMD jumper.
        

        Mike
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
						
						
				
				
		
		
        
			
		
        
	
        

        

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