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I'm working on a 5V buck converter circuit. Below is the schematics:

enter image description here

The circuit is designed using the LM43603-Q1.

I put in an input voltage of 8.4V into the PCM_IN pin. I used a variable load to test the circuit. The problem is that as soon I withdraw 200mA from the circuit, the output voltage drops from 5V to 3.8V. Any suggestion for what could be causing the problem?

Thanks.

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    \$\begingroup\$ Please post your layout. \$\endgroup\$
    – Navaro
    Commented Dec 30, 2020 at 6:13
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    \$\begingroup\$ Try to reduce your feedback by a factor of 5-10. What kind of capacitor is Cboot? Is it a ceramic? \$\endgroup\$
    – Navaro
    Commented Dec 30, 2020 at 6:14
  • \$\begingroup\$ The capacitor is a ceramic capacitor. The size is 0603. I would post the layout for the board, but it's a big board. I'm not sure if a picture of the layout only would be helpful. In any event, I will take your advice and lower the value of the feedback resistors. Thank you \$\endgroup\$
    – Analog
    Commented Dec 30, 2020 at 19:31
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    \$\begingroup\$ Just show the layout of the converter itself; we don't need the entire board. \$\endgroup\$
    – Hearth
    Commented Jan 1, 2021 at 22:00
  • \$\begingroup\$ What’s the current rating on your inductor? \$\endgroup\$
    – winny
    Commented Jan 1, 2021 at 22:04

1 Answer 1

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As already mentioned in the comments and stated in the datasheet of this converter, a possible problem would be the feedback network. You should reduce the feedback resistance, or make sure that the layout accounts for undesirable effects.

According to the datasheet:

The choice of the RFBT depends on the application. RFBT in the range from 10kΩ to 100 kΩ is recommended for most applications. A lower RFBT value can be used if static loading is desired to reduce VOUT offset in PFM operation. Lower RFBT will reduce efficiency at very light load. Less static current goes through a larger RFBT and might be more desirable when light load efficiency is critical. But RFBT larger than 1MΩ is not recommended because it makes the feedback path more susceptible to noise. Larger RFBT value requires more carefully designed feedback path on the PCB....

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