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# LM5180-Q1: LM5180 power problem

Part Number: LM5180-Q1
Other Parts Discussed in Thread: LM5181, LM5180, SN6501

Hi,

I cannot achieve the required output characteristics. Almost all denominations are calculated according to the datasheet. The transformer is selected close in inductance to the recommended one. What could be the problem?

The transformer is selected based on the size requirements. Perhaps you need to choose another transformer from the LPD3015 line.

In this circuit, I need to have at least 100mA. But now, with a load of 100 Ohm, the voltage drops to 0.2V. This means that the current is about 2mA (0.0004mW) - this is much less than it should be.

Additionally, I attach oscillograms, they suddenly suggest an idea. Large enough needles are visible

• Hi Max,

The LPD3105-223 has a sat current spec is only 420mA, so the magnetic is likely saturating here.Also, the winding resistances are quite higher, 1.9 Ohms.

The LM5180 peak primary current limit is 1.5A typical, hence the datasheet recommends a sat current of 2A. The peak current may be high at startup while charging the output cap. Please use the quickstart calculator to assist with component selection. For this 3.3V/100mA spec, consider using the LM5181 with peak primary current limit of 0.75A.

Hope this is clear. Let me know if you have further questions.

Regards

Tim

• Hi,

Do I understand correctly that you need to choose a transformer with a high resistance?
According to the calculator, the transformer is needed like this:

The closest transformer in terms of resistance is LPD3015-152MR. But in this case the inductance differs significantly from the recommended one.
Unfortunately, I am still limited to this particular line of transformers because of the size.

• Hi Max,

No, lower DCR is better as it reduces conduction loss. A 3 x 3mm magnetic is likely not feasible with the LM5180. As mentioned, the main parameters that are necessary are Lmag-min and Isat.

Maybe consider a buck converter followed by a push-pull non regulated solution (SN6501).

Regards,

Tim