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Resistors in Switching Regulator design...

Hi all,

I have a switching regulator design using National's LM2675, adjustable version. I have input, ouput caps (Low ESR, 10uF input, 100uF output) inductor (100 uH) and the feedback resistors to adjust the output voltage. (Also the diode and the boost cap. as shown in datasheet)

As the datasheet suggests, I have placed 1.5k and 4.75k respectively. When I measure the resistors when the inductor is not mounted on the circuit I can read correct values, but when i solder the inductor and then measure the resistances, I always get a lower value (such as 1.3k and 2.2k instead of 1.5k and 4.7k)

And the output voltage is not 5V and instead something very low. Around

1.5 - 2 Volt.

I have reviewed my board layout and schematics and they seem to be 100% same as what the datasheet suggests... Does anyone have any ideas ?

Thanks, Antonis

Reply to
antonis_konstantinos
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Perhaps a silly question, but have you reversed the two resistors? The

4K75 should go to the output and the 1K5 to ground.

---- indeed silly, silly me !

I checked once again and you are definitely right! Now it works great !

Thanks for reminding..

Antonis

Reply to
antonis_konstantinos

Perhaps a silly question, but have you reversed the two resistors? The

4K75 should go to the output and the 1K5 to ground.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

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