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lmz12002 and lmr10520

Guang-lei Zhang
Prodigy 20 points
Other Parts Discussed in Thread: LMZ12002, LMR10520

8831.6332.5224.accessory.rarHello!

I am working on a power which is lmz12002 and lmr10520. Input 12V, lmz12002 provide 5V power, lmr10520 provide 3.3V power. when i used not analog 3.3V, 5V power have 400mv of burr and 3.3V power have 2v of burr. When i add analog 3.3V, 5V power have 2v of burr and 3.3V power will not work properly. Here attch my circuit and the output waveform. device packaging and pcb layout is according to the docimentation.Ask what's the problem.

Thank you!

  • 0
    TI__Genius 15810 points

    Hello Guang-Lei,

    Thank you for the schematic and waveforms. Where are you measuring the voltages? You seem to have very high switching noise at your outputs. That can be caused by an unoptimized layout. Please send us your layout and we can take a look at that.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 20 points

    Hello Akshay

        Thanks for reply.When PGND to DND I got 5V_not.png of LMZ12002 Vout and 3.3V_not.png of LMR10520 D3.3V.When DC_5v to 5V,POWER_GND to PGND, VDD3.3 to D3.3V,DGND to DND I got 5V_AVCC.png of lmz12002 Vout and 3.3V_AVCC.png of LMR10520 D3.3V. Here attch my circuit and layout.8461.accessory2.rar

  • 0 in reply to Guang-lei Zhang
    TI__Genius 15810 points

    Hello Guang-lei,

    I looked at the layout of the design and I see many issues with it. This is the reason why you see such large voltage spikes at the output and have interference between the two designs. Here are a few pointers:

    1) On U1 the DAP of the IC should be sitting on a good amount of copper pour. This helps with heatsinking.
    2) ON U2, the ground of C5 and anode of D1 should be on the same plane and very close to each other. On your layout they are not even the same net. All of power should have the same single point ground.
    3) You have thin traces going everywhere. The power traces cannot be so small, thin, and long. The SW node on U2 should be  fat and thick copper pour. 
    4) The Vout sense line should be away from any switching component. On U2 it is going right below the switching diode. 

    Please go through the layout guidelines section in the datasheets of both the parts to get a better idea and improve the layout. You could even refer to the eval board layouts in the application notes.

    I hope this helps.
    Regards,
    Akshay