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LM26484SQ - output voltage and inductor

Dear TI,

We have a design with LM26484SQ and using only the switched outputs. For 1.3V and 1.0V it works fine. Using FB resistors according to DS and 0.5uHy output inductor (LPS4414–501ML) also following the DS recommendation.

We also use LM26484SQ for 1.8V and for 3.3V we are also using 0.5uHy output inductor (LPS4414–501ML) and FB resistors according to DS. With no load, the outputs are 1.73V and 3.188V while connecting loads (~40mA for 1.8V and ~330mA for 3.3V) the voltage drops to 1.6V and 3V and I hear monotonic noise form the 3.3V output inductor. Input, Output and feedback capacitors are according to the datasheet as well.

Tomorrow, I'll try to replace the 3.3V inductor to 2.2uHy, as recommended in the datasheet, but still not clear why the voltage on the 1.8V is so low with and without load.

It would be great to debug it with TI support engineer.

Thanks,

IK

  • Hi,

    I'm not the original apps engineer for this device as he is out of office this week, but generally speaking, you may want to check several things.

    1. Isat > I peak : Inductor selection should follow this rule and it seems you already have big enough one.

    2. Vout ripple : when Vout ripple is huge, you can hear audible noise. As described in the datasheet, capacitance can degrade very fast by bias voltages. If voltage rating of the cap is very close to output voltage, it may happen. Use big size of capacitor if possible

    3. Layout SW node : SW node should be very close to the device to minimize any artifacts of noise. If SW node is far from the device, boost operation will be unstable and may cause audible noise on inductor/capacitor not meeting target output voltage.

  • Dear Sung Ho Yoon,

    Thanks for your reply.

    For 3.3V output, I've replaced the 0.5uHy inductor with MLCC 2.2uHy inductor we had in stock (different vendor, current capability is 1A - good enough for the 0.33A load) and the output voltage of both 3.3V and 1.8V became almost same as the nominal values. The voltage ripple on the 3.3V is too high (~100mV). No audible noise.

    I've just received samples of the suggested LPS4012–222ML (Coilcraft) inductors and will try them too.

    In the LM26484SQ datasheet it is mentioned that the 0.5uHy is the nominal inductor and recommended inductor value, but practically it is not so accurate. Please make it clear in the DS.

    BR,

    Igal

  • Hi Igal,

    Thanks for the feedback and your try.

    I will make sure your findings when the original apps engineer comes back.