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LMR36006: Capacitors Datasheet vs Webench

Nic Gough
Prodigy 385 points
Part Number: LMR36006

Tool/software:

Hi Power Team,

We are looking at an LMR36006B design.

The datasheet claims that you need a pair of 220nF (0.22uF) capacitors for HF decoupling at the two power input pins.

The Webench simuation, however, places 22nF (0.022uF) capacitors instead. 

When I investigate the design further, it allows for alternate selection in the range 0.022uF to 0.047uF, 100V rated.

Please can you advise whether the data sheet or the Webench decoupling capacitor recommendations are correct.

The datasheet also calls for a 100V rated 4.7uF capacitor.  The Webench simulation matches this, with a 4.7uF C3225X7S2A475M200AB part from TDK (NRND).  This part has a minimum capacitance at 50V bias of 1.507uF, so its capacitance drops by 68%.  I’m assuming that this is acceptable, and that I can replace this with a pair of 1210, 2.2uF parts that will lose 57% of their capacitance at 50V, leaving me with 1.9uF capacitance and a lower ESR.  Can you confirm this is acceptable? 

Thank you for your help.

Best regards,
Nic Gough

  • 0
    TI__Mastermind 24620 points

    Hi Nic,

    I would recommend following the datasheet recommendation for the HF decoupling.

    For the input capacitance, I would recommend a more conservative approach of having a 4.7uF capacitance including derating. The example shown in the datasheet recommends a 100-V 4.7uF capacitor for a nominal 12V-24V input with transients up to 60V. If you are operating normally at higher input voltages, I would add another pair of the 1210, 2.2uF parts just to make sure that you will have enough. 

    Thanks,

    Richard