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lm1085 Vout issue.

Sandesh N J
Prodigy 30 points

Hi

    i am using LM1085IT-ADJ/NOPB IC in my pcb, I am using this to generate 8V from 9.5V. When the board is powerd on it is drawing 0.3A(28V) current for entire board, where the tab (Vout) is not connected to copper below it. Now when i short tab to the copper the current consumption is increasing to 0.7A(28V).

                            My question is

 1. Why this increase in current consumption after connecting tab to copper?

2. What is this Vout weather it is for thermal dissipation?

  • 0
    TI__Genius 12630 points

    Hello,

    1. It is important that if you connect the tab to copper that it is connected to the same copper as the Vout pin (pin #2) since these are both Vout. See the attached layout for an example of how to connect this part into a pcb. If the Vout traces are both connected and short, wide traces, there should not be additional current drawn.

    2. Yes, the Vout tab is used for heat dissipation.

    I am not sure what you mean when you say it is drawing current and then you say 28V. What is this 28V value you are referring to? And is there anything else connected to the copper when you short it to Vout tab?

  • 0 in reply to Michael_DeSando
    Prodigy 30 points
    Thanks for ur replay,
    Sir i have used this regulator in on of the PCB, whose input voltage is 28V. Also i have grounded this tab thinking it is for thermal dissipation in my schematic. During assembly this pad is not soldered to the copper below it. At this condition if i power on the board, current drawn is 0.3A, and if i solder this pad to copper the current drawn is increased to 0.6A.
    Sir My only dought is, i have to modify the schematic for further use

    Weather i should ground this pin or should i connect it to the output pin?
    Or else i can leave it unused?
  • 0 in reply to Sandesh N J
    TI__Genius 12630 points

    The tab must be connected to Vout (the output pin #2) or leave it unused. Do not connect the tab to ground.