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LM25180: I can't get it to function properly

Floris Vernieuwe
Prodigy 20 points
Part Number: LM25180
Other Parts Discussed in Thread: LM5181

There is something wrong with the circuit but I cannot figure out what.

  • The expected output voltage is 15V but the output i have is unregulated and changes based on the input voltage. (When the input voltage is 12V the output voltage is 10V)
  • The switching frequency is stuck at 10-12 kHz. (It causes an audible high pitch)
  • When i add a load of 220 ohm the output stops working .

 I have the same behaviour with all 3 of the modules I made.

Can somebody help me?

Below is my schematic:

Below is the wave measured on the SW pin.

  • 0
    TI__Mastermind 34805 points

    Hi Floris,

    Did you design the circuit based on LM25180 Calculator:

    http://www.ti.com/tool/LM25180DESIGN-CALC

    Did you design the PCB layout by referring to LM25180EVM? 

    I suggest you to apply one EVM and modify the circuit for your application.

    B R

    Andy

  • 0 in reply to Andy Chen
    Prodigy 20 points

    Yes I designed the circuit based on the calculator

    I did not follow the evaluation module for the evaulaution module. But i did try to make de high-frequency signal paths short and keep de capacitors close.

    I worked with a bottom grondplane, maybe that is the problem?

  • 0 in reply to Floris Vernieuwe
    TI__Mastermind 34805 points

    Hi Floris,

    The layout is not bad.

    Do you use MBR0520 on D6? this diode will block Vin-max plus Vout if XFMR has 1:1 ratio, so it should be 50V diode at Vin=24V, pls try it with 0.5A 100V diode.

    B R

    Andy 

  • 0 in reply to Andy Chen
    TI__Guru 73822 points

    Hi Floris,

    As Andy mentioned, the flyback diode should be higher voltage rating, either 60V or 100V (also depending on the leakage ringing). Also, the LM25180 has a peak current limit of 1.5A typical, hence why we recommend using a transformer with 2A sat current. In addition, 1uF is quite low for the input cap, try 10uF/50V.

    The output power is quite low here, 1.5W, so the LM5181 is generally more suitable (the peak primary current is 50% or 0.75A). In fact, the LM5181 is more aligned with your existing transformer with high mag inductance of 60uH and sat current of 1A. Take a look at the datasheet and quickstart calculator for that part - it is pin compatible with the LM25180.

    Regards,

    Tim