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TPS7A96: Enquiry on cascading LDO

Ernest Yong
Expert 4315 points
Part Number: TPS7A96
Other Parts Discussed in Thread: TPS7A57, TPS7A47, TPS7A3501

Hi Team,

I have a customer who have a question on cascading LDO.

Assuming that we have a cascaded power supply tree 24V-6-5-3.3 where the last 2 stages are LDO, customer want very low noise from the 3.3V rail. Which LDO should be low noise? The one outputting 5V or the one for 3.3V? Or both? Is there a rule of thumb to amplifiers, where typically the upstream amplifiers dominate the output RMS noise.

Please kindly advise.

Best Regards,

Ernest

  • 0
    TI__Mastermind 33965 points

    Hi Ernest,

    An LDO will attenuate external noise according to the PSRR specification.  It will also generate internal noise.  The summation of these two is the total output noise from an LDO.  In modern high performance LDO's, such as the TPS7A96, the PSRR is very high and the noise is very low.  Thus, if you have a low noise device on the 5V rail then it is safe to assume that the LDO's internal noise will be the total output noise.

    You want the TPS7A96 on the 3.3V rail. The 5V rail wants to be low noise but high PSRR.  Options include the TPS7A57, TPS7A96, or TPS7A47. 

    If you need very high PSRR above 1 MHz, and you can change the 6V rail to be 5.5V instead, you may also consider the TPS7A3501.  The TPS7A3501 regulates the voltage differential between Vin and Vout and due to this unique capability it has unusually high PSRR above 1 MHz.  The TPS7A3501 can only offer 1A loading, so if you need more than this then you'll want to use one of the other choices mentioned above.

    Thanks,

    Stephen