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I have a LTspice design issue where I encounter the same problem, even when I use other available simulations.

Whenever I increase the load, there is a significant drop in voltage, despite having considered the current requirements in my design.

Here's what I'm attempting to simulate: Vin = 5 V, Vout = 3.3 V for a maximum current of 8A at a frequency of 550 kHz.

LTSPICE

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The simulation works well when the load is less than 300 mA. However, when I set the load to 0.8 Ω to draw a current of around 4.5 A, I only get 1.5 V instead of the expected 3.3 V at the output. I tried to place a diode instead of the lower switch, but nothing changed. Please advise.

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1 Answer 1

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The "on" resistance of your switches is 1 ohm. You don't have a control loop around your buck to regulate the output voltage, so your output will drop in proportion to the effective source resistance looking back into the output.

You could add a control loop to keep the output voltage constant, or you could reduce the on resistance of your switches to be << than the load resistance.

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  • \$\begingroup\$ Thank you. I understand that I do not have a feedback loop. However, I was under the impression that since the input voltage and all other conditions remain constant, a feedback loop might not be necessary. \$\endgroup\$
    – chami
    Commented Apr 10 at 22:48
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    \$\begingroup\$ @chami That's true even in the practical case if the switch resistance and the inductor DCR is much lower than the load resistance, which often isn't hard to achieve. However, in your case the switch resistance was on the order of the load resistance so you saw the output voltage drop accordingly. One downside of not having a control loop is that if you have a load transient the output voltage can ring at the LC resonant frequency. \$\endgroup\$
    – John D
    Commented Apr 10 at 22:51

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