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LMR14203 does not start

Stefan Helwig
Prodigy 20 points
Other Parts Discussed in Thread: LMR14203

Hello,

I have tried to use two LMR14203 do generate +5V and +18V from a 24V to 36V source. The design was made with Webbench. By reading through related posts in this forum I understand that the layout is not optimal and trace width to the diode and inductor should be increased. However, I don't understand why the circuit does not work at all. On top of that  the two subcircuits react differently. The schematics and top layer of the PCB are shown in the attached file. The design is a two layer PCB with a ground plane below the switching circuit.

The 5V circuit does not show any output voltage when used with 25V input. Also no feedback voltage can be measured which is kind of obvious as there is no voltage at the diode or inductor.

The 18V circuit shows different voltages depending on the input voltage. Typically the output voltage is 2 or 3 V below the input voltage. There is however no sign of any switching at all. When measured with the scope on the diode or the inductor only a constant voltage can be observed.

I should mention that there is a 110uF capacitor in parallel with the input voltage. The measurements and the tests have been made without load and in the circuit the load will be rather small.

Any hints are greatly appreciated.

Thanks

StefanSimpleStepper_design_pcb.pdf

  • 0
    TI__Genius 16025 points

    Hi Stefan,

    This is most intriguing. Your layout and schematic don't appear to have any issues except the perhaps smaller traces. What is your input supply and is it current limited in any way? Also, decrease the amount of inductance to 100uH.

    What tool did you employ to design the board? Cadence, allegro? Can you send over your pcb design so I can inspect it further? Also try applying a very small load to the output to see if it works.

    Finally, replace the IC with a new unit to see if there is any change.

    I am curious to get to the bottom of this.

    Thanks,

    Anston 

  • 0 in reply to Anston Lobo
    Prodigy 20 points

    Hello Anston,

    Thank you very much for your reply. The schematics have been designed using the Webbench tool. Therefore I was not expecting any issues on that side. The circuit will eventually be powered by 3 car batteries but for initial testing I used a laboratory power supply. To be fully exact I used two power supplies in series.

    Based on your reply I repeated the test with two 12V led gel batteries. The input voltage is 26V. The results are the same. On the output of the 18V reel I measure 23V on the output of the 5V circuit I measure 0V. Adding a 20kOhm resitor to the 18V rail does not help. Still the output is 23V. 

    I have populated the 5V circuitry on a second PCB were I left all other parts empty and I get the expected 5V. I will now populate the 18V rail to check if they are influencing each other. 

    Stefan

  • 0 in reply to Stefan Helwig
    TI__Genius 16025 points
    Hi Stefan,

    I cannot comment on your layout, since the image you attached doesn't show the whole circuit. Can you provide everything you have on the layout of the part and I can be of more assistance.

    Judging by your test of the 5V circuitry, it does seem that there is an intended short between the two designs.

    Let me know how this turns out!
    Happy to help,
    Anston