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WEBENCH® Tools/LMZ12003: Unexpected/unexplained Output Voltage Ripple in Real Circuit

Arkadi Trestman
Prodigy 50 points
Part Number: LMZ12003

Tool/software: WEBENCH® Design Tools

Dear All,

I have a problem with a real circuit based on Webench simulation result:

This is  12V-5V DC-DC converter (load current changed from 0 to 1 Amp) - see the attached pictures.

The point is the real circuit shows unexpected and unexplained output voltage ripple (about of 130 mVpp) at light load ( about of 100mA). From second side the ripple at IL=1A seems to me reasonable (about of 20mV pp). No simulation shows this difference.

The electrical circuit has ceramic capacitors only. The bulk capacitance is about of 90uF (not shown on the electrical circuit). The layout done in accordance with your recommendations.

Could you advise please?

Thanks in advance,

Arkadi.

  • 0
    TI__Mastermind 47060 points

    Hi Arkadi,

    Can you also provide the PCB layout for review? I want to make sure that there is a single point ground to help handle erratic output voltage ripple behavior. 

    How are you probing the output? It is best to use the "pig-tail" or "probe and barrel" method to get a more accurate representation of the output ripple. Having large ground loop probes will result in more noise pickup that could couple onto the probe and result in more exaggerated voltage measurements. Here is a good blog which outlines different probing techniques and the resulting measured voltage. 

    Also note that when running at lighter loads (100mA and lower) the device is operating in DCM which will have larger output ripple than in CCM. It may be worth looking into experimenting with the feedforward capacitor (C67) to in your real circuit and tune the capacitor accordingly at light load.

    Regards,
    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 50 points

    Hi Jimmy,

    1. I am probing the output using "probe and barrel" only.
    2. The PCB Layout is attached. I have to make some explanation here- this is not classic "single point grounding" - there is ground plane in next low layer. But. When I elevated ground sides of R51, R52 and C70 and connected them by wire to U15/4 - the result remained the same.
    3. Another interesting point. In the same circuit I have similar DC-DC converter 12V-3.3V, which properly works with the same light load and the same layout (the circuit is attached). May be you are right and C67 correction would help me.
    4. Last question. Actually, this ripple at light load of the converter does not impact on its customers in the circuit. Do you see some problem to keep the circuit as is?

  • 0 in reply to Arkadi Trestman
    TI__Mastermind 47060 points

    Hi Arkadi,

    The PCB layout looks okay. The input and output capacitors are close to the device pins. I would recommend tuning the C67 feedforward capacitor depending on loading and output voltage if that is desired. 

    As you have seen the output ripple voltage is only an issue at light load/DCM condition. Output ripple voltage becomes an issue if your customer is using this circuit in noise sensitive applications such as a high resolution ADC. Large output ripple voltage may appear as noise spurs on the ADC. If the customer is fine with the noise performance of the circuit, there is no issue with keep the design as is. 

    In the case where the customer does desire a clean output voltage, please refer to this app note. 

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 50 points

    This resolved my issue