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WEBENCH® Tools/LM5175: Buck boost converter not working

Priteem Behera
Prodigy 30 points
Part Number: LM5175

Tool/software: WEBENCH® Design Tools

Hello,

I have designed a power supply using LM5175 with the help of TI-webench.

Design:

Input: 6-8 V, Output: 8V, Current: upto 3A.

When I fabricated the board and powered it up, it didn’t work.

The output voltage was some 3.8 V. When viewed under oscilloscope it was like a ripple. I checked the inductor legs, it was in some 100mV and ripple voltage. Doesn’t seem to be switching.

Any ideas on where might be the trouble.

(Attached Eagle board, schematic and image of actual PCB)

Thank you

schematic_drawing.pdf

PCB assemble.pdf

  • 0
    TI__Guru* 78270 points
    Thank you for considering the LM5175 in your design. You should have copied the recommended layout by Webench. Basically your layout needs great improvement. For instance, the traces from the low MOSFET Sources to the current sense resistor cannot just be a narrow traces. The connection should short with wide copper traces, because these traces are where the switched power currents flow along. Also, the current sense requires differential traces: traces from both end of the current sense resistor, routed closely side by side along the entire distance to the IC. Routing differently will introduce noise to the circuit and affect the normal application. Please follow the layout guidelines presented in the datasheet, and follow the layout examples in the datasheet, or presented by Webench.
  • 0 in reply to Youhao Xi
    Prodigy 30 points

    Thanks,  for the insight. 

    This is my first ever design on eagle. I never knew layouts could make or break the design.

    Can you please tell me what are the most important guidelines which must be followed?

  • 0 in reply to Priteem Behera
    TI__Guru* 78270 points

    The layout for switching mode power supply is critical for normal operation.  The datasheet layout guidelines are all important and you should follow.   I would recommend you to copy the Webench recommend layout if you want to create your own board.

  • 0 in reply to Youhao Xi
    Prodigy 30 points
    Thanks a ton for your advice. I will try to copy the Webench layout.