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I soldered up a LM2678 for a 5V 1A output with a 12 volt input (Input voltage and current from (12v supply @2A) like shown here. http://www.ti.com/lit/ds/symlink/lm2678.pdf

It outputs a solid 5 volts under low < 50mA loads but experiences significant output voltage drop to ~3.6 volts under loads as small as 400mA. Any ideas what is causing this? It is soldered on perfboard with small distances between components. Mylar and Electrolytic capacitors are used for decoupling are are close to the input leads on the LM2678.

Any ideas?

Thank you

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  • \$\begingroup\$ What are you using for an inductor? What is its DC resistance? \$\endgroup\$ – WhatRoughBeast Jan 16 '15 at 2:15
  • \$\begingroup\$ A self wound toroidal inductor wound with 18 gauge wire. I don't have any tools to measure the resistance that low but it is < .2 ohm at worst. \$\endgroup\$ – Andrew Cragg Jan 16 '15 at 2:18
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    \$\begingroup\$ And what is the inductance? \$\endgroup\$ – WhatRoughBeast Jan 16 '15 at 3:45
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    \$\begingroup\$ around 20uh from what i measured \$\endgroup\$ – Andrew Cragg Jan 16 '15 at 4:46
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    \$\begingroup\$ Measured how? At what frequency? \$\endgroup\$ – WhatRoughBeast Jan 16 '15 at 5:41
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At the higher current point use a scope to check if the circuit is oscillating or ringing on the input or output pins.

Be sure the feedback line is far enough away from the toroid so as not to pick up noise from it, (per the data sheet). While a good commercially made toroid should not radiate much magnetic noise a hand made part may not be so perfect.

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