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I have a personal project to make an adjustable switch mode lab power supply. I've used the LT1074HV switcher in standard buck regulator configuration. The end result mostly works except that with a 9V output, at 4A draw it drops down to 8.7V.

schematic assembly

The inductor L1 is 330uH Bourns 2318-V-RC http://nz.element14.com/bourns/2318-v-rc/inductor-toroid-v-330uh-15-5-2a/dp/1929761?Ntt=2318-V-rc

The output cap C1 is Rubycon 470uF/63V YXF a low ESR variety.

The switch diode MBR20100CT is a dual centre tapped diode but I'm only using one side (it was the item in stock at the time)

The regulator FB and Gnd pins are connected by separate wires to the output, so there is low current flowing.

It's only assembled on veroboard, not a PCB. I've tried to be tidy but distances are not as short as they could be on a PCB. Having checked all the other things I can, it seems like it must be due to construction issues, but I didn't think load regulation would be related to that, more like stability or transient response...

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    \$\begingroup\$ Are you measuring the output voltage right at the point where the FB divider connects to the output, with your ground lead right at the ground of the IC? Have you tried measuring the FB pin voltage between the FB and GND pins while you see the voltage drop? \$\endgroup\$
    – John D
    Commented Sep 30, 2017 at 20:22
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    \$\begingroup\$ I'd try making the VSW and V+ traces (wires?) as thick as possible, and I second John D's comment. Try putting the pot as close to the V+ terminal as possible. \$\endgroup\$
    – calcium3000
    Commented Sep 30, 2017 at 23:00
  • \$\begingroup\$ @JohnD I haven't done those exact measurements, I will now, thanks. I'll also try shortening pot wires thanks @ \$\endgroup\$
    – O'Rooney
    Commented Oct 1, 2017 at 18:42
  • \$\begingroup\$ The voltage right at the IC is 2.13v under 4A load, 2.19v when unloaded. Voltage output at the board is 8.78v vs 8.73v at the output wires. The pot is on quite long wires, maybe 20cm. \$\endgroup\$
    – O'Rooney
    Commented Oct 2, 2017 at 7:07
  • \$\begingroup\$ Input voltage drops to 40v when the 4A load is on \$\endgroup\$
    – O'Rooney
    Commented Oct 2, 2017 at 7:15

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