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Original question:

WEBENCH® Tools/LM5180: WEBENCH don't work for LM5180

LM5180: LM5180 fails to come up using identical circuit to datasheet and Webench

Brent Bolton
Prodigy 115 points
Part Number: LM5180

I'm trying to get a simple isolated supply to work based on the LM5180. I'm using a circuit nearly identical to the one in the datasheet and have also run the design through Webench to yield the same circuit. Some component values are different, primarily the feedback resistor and the primary side clamping zener as I want to get 12V out with 11-15V in at only about 50mA. When I power up the design with a load resistance of about 800 ohms (to meet minimum output current spec), I only get 3.9V at the output.(the rest of my circuit is disconnected so the supply is operating in isolation)

As a sanity check I changed the feedback resistor to 158k, at which point, except for the zener and slightly different input and output capacitances, it was identical to the datasheet circuit. It should have the output 5V, but was still at 3.9V. I then changed the load resistor to 100 Ohms, thinking it might be a minimum current issue. The chip then drew so much current at startup it overwhelmed my current limited bench supply. I attached a higher current supply and the chip promptly let out the blue smoke as it tried to start up into my 50mA or so load. I'm using the Coilcraft YA8779-BL transformer, same as TI's 5V reference design.

Any ideas why it doesn't work and why the part is failing to current limit to 1.5A?

I don't see a place in this form to attach a schematic, but can forward via email.

Best Regards,

Brent

  • 0
    TI__Mastermind 22301 points

    Brent,

    That is weird, especially since changing your FB did not change the outcome can you start by forwarding me your schematic?

    o-murray@ti.com

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 115 points

    Hi Orlando,

    I sent a pdf of my schematic to your ti.com address so hopefully you have that in hand. I've since done more tests, checking waveforms at various nodes and trying different component values. One odd thing is that the part does not seem to be regulating at all. The switch node waveform looks normal, but at 13V input I get 3.9V out. As I increase the input voltage, the output tracks right along staying at about 1/3 of the input up to 7V out at 21V, at which point I stopped the test. I then changed out the YA8779 transformer for a YA8916 and used the 1:1 coil pair. After setting the feedback resistor to get 12V out, I actually got 1V out and the LM5180 only drew 1mA from my bench supply at 13V in and a 360 Ohm load on the secondary. I'm beginning to think that the part simply can't regulate, or perhaps even start, at light loads, even though the datasheet claims a 0.5% minimum. Also for some reason, perhaps related to high pulse currents from my current limited bench supply, I can't operate at high currents either (though I did try one test using a battery that could supply 25A at 13V and the LM5180 simply blew up.)

    Any ideas?

    Brent