LM2577 Regulation

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I was hoping there could be some assistance in this group. This is a longer than typical description but please ask questions if I’ve left out details or I’m overlooking something.

I’m using an LM2577 Boost regulator to take a 3.7V supply (which is currently a benchtop power supply but was going to be a LiPo cell later). I have setup the circuit as per the spec sheet and included a potentiometer on the feedback pin instead of a fixed resistor voltage divider to ensure it can get exactly 5.0V output.

When there is no load on the output, it regulates to 5.0V consistently. However when I put a 5 Ohm, 20W resistor as a load (really 2 x 10 Ohm 10W resistors in parallel), the output voltage drops to 3.0V. The maximum output current is 3.0A; well above what should be around a 1A output.

Things I have checked already: The circuit layout is exactly like the spec sheet, component values are correct, the input current is not being capped by the benchtop power supply, the input current is only 0.65A; well below the maximums, the input voltage remains the same, nothing is getting hot, there are no shorts, the “Comp” pin that indicates the duty cycle is 0.5V; below the maximum of 2V, all complementary components have more than sufficient power ratings.

Any ideas what I could be missing here?

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Hi Michael
Try a load of 10Ω (disconnect 1 resistor) and see what happens. I have some doubts about the 1A capability. The figures in the Data sheet mention loads of 300mA, 600mA and 800mA.
Yes the “Switching” current is in the order of 2A but this is not necessarily the output load current.
Worth the experiment anyway.
Cheers Bob
Thought. I don’t suppose the 3.7V you input is a bit close to the published minimum of 3.5V. I never like to operate things that close to borderline. Depending on your bench supply it might sag just that little bit at the currents involved. At least the 1A you are asking for plus the conversion efficiency.

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Thanks Bob. I’ll give that a go. I’m a high school Systems Engineering teacher so it’s been good to have students experience troubleshooting etc. I’ll give the higher resistor a go.

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If you are using the module rather than your own circuit then it looks like it has an input reverse polarity protection diode, which further raises the input voltage minimum for reliable regulation.

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Hi Jeff

Probably puts it right out of reach for a nominal 3.7V cell.
Cheers Bob

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