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TPS7A47: how to improve LDO PSSR in 200KHz-2MHz interval

Part Number: TPS7A47
Other Parts Discussed in Thread: LMZM23601

QCL-DFB laser needs ultra clean supply. LMZM23601 following tps7a4700 are intended to be used to output the required voltage. But a problem occures: the PSSR of tps7a4700 is very low in  200KHz-2MHz frequnce interval and frequency of PWM switch regulaor is in this interval. So something has to be done to reduce the switch noise before LDO, or the ripple may unacceptable. Could you give me some suggestion to it?

Thanks  a lot in advance.

  • Murata simulation shows PI LC is a good choice. Now is LMZM23601 output cap and LDO input cap a part of the LC cap value?

  • Hi Rongjun,

    Since your frequency of interest is so high it is difficult to improve the PSRR from an LDO perspective. Most techniques to improve LDO PSRR specifically only have a significant effect at lower frequencies, as you can see from this PowerPoint:

    5125.Application Conditions Impact on PSRR and Noise.pdf

    A common method to improve the overall system PSRR is adding a PI filter as you have mentioned. To answer your question, yes, the LMZM23601 output capacitor will contribute to C1, and the TPS7A4700 input capacitor will contribute to C2. Some additional things to keep in mind:

    • Make sure to tie the LMZM23601 feedback network to the node before the inductor (P2 in your simulation schematic)
    • With 1 A output current, there will be a sizable voltage drop across the PI filter inductor due to its series resistance
    • It is important to prototype multi-stage power designs first and ensure they are stable with load transient tests

    This application report goes into some more detail about using PI filters and LDOs to reduce the output ripple of a switching converter:

    www.ti.com/.../slva549.pdf

    Thanks,

    Gerard

  • Hi Gerard:
    Thanks a lot. Filter from Murata will be used to reduce the noise near 1MHz before LDO, it should work according the rules in references.