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LTM4646 - power outputs shorted to GND net



		

			

									
						EHarshman
					
							

			
				
									
											
							 		
		        	            							
			
			    	
			                    
                
				                on Nov 27, 2019 
				
			
			
		


				
		

		    		    
		    
		    	
			
Hello,


I made a PCB using an LTM4646 chip to generate 1.5V and 3.3V from a 12V supply.


All three of the power nets, 12V, 3.3V and 1.5V are shorted to the GND net in the finished board.The PCB manufacturer checked them out with an X-Ray to verify the issue was not with the manufacturing of the board, so I wanted to see if my design looked correct to this community.


Here is my setup in LTSPice, which is the design I used on my PCB. It simulated the correct voltages just fine.




 

								


				
				
		

			
						
									
									
						
						
			
			
			
			
		


					

			





	














	

	

					



			








	
	
	
	



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                on Feb 25, 2020 5:06 AM
            
            
					
    
	
    

		
		
    
					
    I just wanted to follow up on this.
    

    The reason the nets were shorted was that R3 and R6 on my PCB were milliohm values. They were defined with the lower-case 'm' in the LTSPICE demo file, and so I ordered them that way. After I realized those resistors were creating a rather effective short, I removed them from the PCB. LTSpice lets you use a lowercase 'm' for a Megohm resistor. Lesson learned.
    

    To avoid damaging the chip due to the 6V RUN pins, I also lowered the input voltage of the shown setup to 5.5 VDC. At 12VDC I was blowing a 2 amp fuse on my PCB. 
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
						
				
				
		
		
    
			
		
    
	
    

    

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													0
							
			
				
									
							        
            
                                    			
			                            
                        
                on Feb 25, 2020 5:06 AM
            
            
					
    
	
    

		
		
    
					
    I just wanted to follow up on this.
    

    The reason the nets were shorted was that R3 and R6 on my PCB were milliohm values. They were defined with the lower-case 'm' in the LTSPICE demo file, and so I ordered them that way. After I realized those resistors were creating a rather effective short, I removed them from the PCB. LTSpice lets you use a lowercase 'm' for a Megohm resistor. Lesson learned.
    

    To avoid damaging the chip due to the 6V RUN pins, I also lowered the input voltage of the shown setup to 5.5 VDC. At 12VDC I was blowing a 2 amp fuse on my PCB. 
    
			
    

		
		
		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
						
				
				
		
		
    
			
		
    
	
    

    

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