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I'm fairly new to circuit design, so excuse me for asking maybe a noob question; I did some research, but because of my lack of understanding I'm not quite sure if my conclusion is the right one.

I have a simple circuit with a high voltage input (24 V), a step down converter with components (XL1509 3.3 V), and an ESP32.

When I read the datasheet of the XL1509, I read the minimum current is 0.2 A. Running the ESP32, the current necessary is below this 0.2 A.

My question:

How can I increase the current to at least 0.2 A, so the step-down converter will work properly? I think I need like a 16.5 Ω resistor (3.3/0.2). Is this a correct assumption?

My second question regarding this: I'm not married to the XL1509, an other step-down converter will also do, but all step-down converters that can go from 24 V to 3.3 V have a minimum load of 0.2 A. Am I missing something?

And, is, for the above circuit (efficiency wise) a linear regulator maybe a better idea? The ESP32 is the only component (besides the regulator and its assisting components) in my circuit.

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  • \$\begingroup\$ "XL1509 I read the minimum current is 0.2A" Poor design. Get another converter? \$\endgroup\$
    – winny
    Commented Feb 1, 2022 at 12:21
  • \$\begingroup\$ You should supply a link to the datasheet where you found this information regarding minimum load. I have never seen a step down converter that specifies a minimum load to work. \$\endgroup\$
    – Klas-Kenny
    Commented Feb 1, 2022 at 12:23
  • \$\begingroup\$ 24-3.3= 20.7 * 0.2= 4.14 power dissipation. I don't think you should use linear regulator. \$\endgroup\$
    – Tryingtogetsome
    Commented Feb 1, 2022 at 12:24

2 Answers 2

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The 0.2 A mentioned in the datasheet on page 6 (the only mention I could find) is one of the test parameters of the testing circuit they used for measurements.

Lower currents may give other testing outcomes, but the 0.2 A is not a specified minimum load, and not a specified limitation of the XL1509.

DC/DC converters that need a minimum load to function properly do exist, but as far as I read it, the datasheet doesn't say the XL1509 is one of them.

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  • \$\begingroup\$ Thanks! I misunderstood this part. \$\endgroup\$
    – Vincentvw
    Commented Feb 1, 2022 at 12:43
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    \$\begingroup\$ The implication is that it may regulate less well at lower currents; at the very least, you will want to test this. \$\endgroup\$
    – user16324
    Commented Feb 1, 2022 at 13:29
  • \$\begingroup\$ There is a reason why the specs are worse at light loads as I mentioned on deaf ears in my answer \$\endgroup\$
    – D.A.S.
    Commented Feb 2, 2022 at 1:03
  • \$\begingroup\$ The stability margin is not determined by the IC or switch alone but in the linear case is a function of Q =X(f)/(RdsOn+DCR+ESR) when there is no load to dampen it. \$\endgroup\$
    – D.A.S.
    Commented Feb 2, 2022 at 18:54
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Yes, your assumption is correct.

I don't have the datasheet but simple converters often had a 10% output preload for improved stability specs, as this load provides some damping to the high series RLC switch.

Stability is a spec. parameter for overshoot, when there are step load changes in current.

This can be precluded with Differential error compensation gain (think PID) perhaps at the compromise for ripple or some other method of feedback or forward conversion.

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