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LMR62014 output ripple

Gabriele Pellegrinelli
Prodigy 50 points
Other Parts Discussed in Thread: LMR62014

Hello.

Sorry for my bad english, but i'm not native language.

I try to realize boost converter with this product, particularly Vin=5V and Vout=12V, but it isn't work correctly. I realize circuit show at page 11 of LMR62014 datasheet. C1 and C2 are tantalum capacitor (2.2uF and 4.7uF respectively), D1 is a shottky diode, L1 is 10uH inductance Isat>4A, R3=47k, R1=100k, R2=12k and Cf=270pF disk ceramic capacitor. All components (except LMR62014) are a through hole components, because i realize this test circuit on bread-board.

Vout mean is about 12V, but ripple is very high, about 1,5Vpp. I try to insert 47uF tantalum capacitor at Vout pin and entity of ripple was little decrease (and period was increase). I did another test adding electrolict capacitor with 470uF capacitance value, but entity of ripple little decrease (1Vpp) and period was increase (frequency of ripple, in this case, was about 7Hz. With oscilloscope i measure Vin and i see DC component with low noise. When i connect DCDC converter, the 5V voltage present a square wave add with DC component.

In all test, Rload=390ohm, so Iout=31mA.

Where am i wrong? Can you help me?! Thank you. Gabriele.

  • 0
    Prodigy 50 points

    Maybe i found a solution.

    I try to increase load current from 31mA to 100mA and in this case circuit work fine; ripple was really small.

    Sometimes Vout change value from 12V to 13V for 0.5-1seconds, but after return at nominal value of 12V.

    Maybe this converter work fine when a significative current was pulled into the load?

    Do you think this is a good solution or not fine?

  • 0 in reply to Gabriele Pellegrinelli
    TI__Genius 16025 points

    Hi Gabriele,

    Can you send us your schematic and waveforms of the problem? It's hard to pinpoint the problem and find an appropriate solution without it.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 50 points

    This is the schematic i used. R2=12k, R1=100k, Cf=270pF. At Vin and Vout i connect 470uF electrolitic capacitor added to the schematic.

    Now i can't provide you any image of waveforms because i'm not at work. However you can think an output voltage as a classical charge-discarge capacitor waveform centered around 12Vcc. The period was correlated with output capacitor; if i don't connect that period was short and if i connect it period was long.

    This behavior this is when load corrent is about 30mA (RL=400ohm). When load corrent was increas around 100mA (RL=120ohm) ripple turn off and his amplitude was reduce really hard.

  • 0 in reply to Gabriele Pellegrinelli
    TI__Genius 16025 points

    Hi Gabriele,

    On closer inspection, it appears your design may be unstable. We would recommend playing around with your capacitor selection. Both type (Tantalum vs Ceramic) and value (follow datasheet guidelines for correct capacitor value selection).

    I would suggest replacing your Tantalum capacitors. Can you replace these to ceramics X7R and give it a try and let us know.

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    TI__Genius 15810 points

    Gabriele,

    I haven't seen your layout, but if you are seeing large ripple voltages at your output and if your output is not stable for no apparent BOM reason, then it could be the layout. Firstly, breadboarding an SMPS with thin traces/wires is not a good idea. You could instead obtan the EVM from the TI website and use that for prototyping. Please adhere to the layout guidelines on page 10 of the datasheet. I have also attached two presentation. One of them talks about good layout techniques to mitigate EMI. The other talks about ways to reduce the output high frequency ripple by appropriately choosing the right ceramic output caps (http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/858/8015.Capacitors-Selection-for-High-Frequency-Noise-Mitigation.pptm).

    Regards,
    Akshay

    Switching Power Supply Design_ EMI.ppt
  • 0 in reply to Akshay Mehta
    Prodigy 50 points

    Hello!

    Thanks for your answer.

    I realize pcb with LMR datasheet instruction and results are there:

    - When RLOAD = 120ohm, Vout=12.4V and ILOAD about 100mA. In this case voltage output is stable and there isn't ripple.

    -When RLOAD = 240ohm, Vout=14.8V mean and ILOAD about 60mA. In this case voltage output has an high ripple (about 1.5Vpp).

    - When RLOAD = 180ohm, Vout=13.8 mean and ILOAD about 77mA, In this case voltage output has an high ripple (about 1Vpp).

    In the end, i think this DC-DC converter must have ILOAD >100mA for work properly. Isn't it?

  • 0 in reply to Gabriele Pellegrinelli
    TI__Genius 15810 points

    Gabriele,

    Would you share your layout with us? A PDF printout of the layers would be very helpful. Also if you can share scope shots of your circuit, that'd be nice. The part should not have a minimum Iload requirement.

    Regards,
    Akshay