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Need help finding a 5V LDO with good PSRR at 400kHz

Caleb Begly
Prodigy 30 points

I have a buck converter that is operating at 400kHz with 250mVpp and I'm looking for an LDO (output of 5V) that can smooth out that ripple to below 1mVpp (roughly 50dB attenuation). Since the output is a triangle wave with a fundamental at 400kHz, I need something that has good PSRR at 400kHz (and above due to the higher frequency components in the triangle wave). I've gone through the datasheets for about 30 chips so far, and they all have PSRR at that frequency of no more than about 36dB. Current capacity must be over 1A, and low dropout is preferred to keep the thermal dissipation low.

Alternatively, I may be going about this all wrong, and it would be better to use a PI filter (or ferrite bead PI filter) prior to the LDO. Feel free to comment if I'm taking the wrong approach. I've also heard of people having good success with a capacitance multiplier prior to the LDO, although that will burn more power which isn't ideal for a battery-powered situation.

  • 0
    TI__Mastermind 37065 points

    Hi Caleb,

    What is the output voltage of the buck converter (input voltage to the LDO)?
    I would definitely review options to reduce the buck converter ripple.
    The ripple is a type of conducted EMI noise source, and can cause other EMI noise problems, so it is best to reduce it as much as practical.
    Often this can be accomplished by increasing the value of the buck output stage inductor.
    A secondary stage LC filter is not uncommon for switching converters, so that may be an option, but please remember to dampen the Q of the LC filter.
    LDO's respond very quickly to line transients, and undamped input filters can create noticeable effects on the output of the LDO if the input supply has a large transient with the right frequency content.

    Keep in mind that the PSRR plots at this frequency are dominated by the choice of the output capacitor.
    Typically you will see a sudden (and significant) improvement at high frequency in the datasheets PSRR plots.
    This is the result of the output capacitor of the LDO.
    If you need to lower the frequency of the improvement, you can increase the output capacitor.
    If you need to increase the frequency of the improvement, you can decrease the output capacitor.
    Make sure you review the datasheet requirements on stability and output capacitance before you finalize your design.

    If you need more than 1A of load, your options will begin to diminish unless you wish to parallel LDO's, which can be done using ballast resistors.
    The TPS7A4701 can support up to 1A, and the PSRR is hovering around 55-60dB at 400kHz, so this may be an option.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Mastermind 37065 points

    Caleb,

    One more comment.  If the output of the buck converter looks like a triangle waveform, then the ripple is dominated by the ESR of the output capacitor.
    You may see significant pk-pk ripple improvements by switching to a lower ESR output capacitor, or adding some parallel X7R ceramic capacitors to the output stage.  My favorite resource on this topic is Christophe Basso's "Switch-Mode Power Supplies: SPICE Simulations and Practical Designs" text, if you would like to look into it further.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 30 points

    I can adjust the output voltage to whatever is needed, but this was set for a dropout of 500mV. 

    The ripple is definitely larger than I want but most of the power is within 75 mVpp, so I mostly just needed filtering on the switching artifacts (it's not ) which I'm thinking a ferrite bead should take care of. It's not triangular, and the ESR is pretty low (lots of small capacitors in parallel, each with low ESR). The power supply I'm using at the input of the buck is really noisy, so I'm probably going to add more capacitance to the input which should help.

    The TPS7A4701 looks good, and is much better than the one I'm using right now. I think a combination of a ferrite bead PI filter for the higher frequencies and that LDO for the low frequency and offset should produce better results.