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PCB:

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My name is Ertuğrul. I made a regulation board with an LM2678 but I have a problem: I provide 12V input but I can not get 5V output.

I only get 5V with the on/off pin enabled, but the datasheet says that I can get a 5V output with the on/off pin not active.

I have to give 1.4-6V to the on/off pin because otherwise it does not work.

I tried it on a breadboard but it did not change. I used Webench schematic design.

Do you know about LM2678? Can you help me ?

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ocrdu
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Ertugrul Sahin
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  • You're right, you should be able to leave the ON/OFF pin not connected. Where did you source the parts? Are you sure they're genuine ? Perhaps add a picture of the PCB, there might be an error that you overlooked. – Unimportant Dec 28 '20 at 21:20
  • Nothing to do with the problem, but I would change your diode symbol from Zener to Schottky. I had to go look the part up to verify it was a Schottky. – GB - AE7OO Dec 29 '20 at 07:20
  • I add new picture about my pcb.Also my diode is schottky.I made this circuit on breadboard.the result did not change – Ertugrul Sahin Dec 29 '20 at 11:28
  • Your layout is not optimal. You have a big loop in your input capacitors (e.g. C3 and C4). The leads of C3 and C4 must be connected to the input pins as short as possible. This could cause the regulator to behave strange. You might want to follow the layout of the official LM2678 eval board. – Stefan Wyss Dec 29 '20 at 12:44

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