# Linear Regulators & Ripple Rejection

I'm trying to understand the effect of ripple rejection in linear regulators since I'm using 2 LDO's in designing a linear power supply, however I'm unsure about something. A lot of datasheets have values that range from 50dB to 90dB, however they're dependent on the ripple frequency, load current, and other variables.

For example the LT3090 http://cds.linear.com/docs/en/datasheet/3090fa.pdf has the following Ripple Rejection characteristics:

So going by this, my understanding is that the Ripple Rejection capability of an LDO decreases with an increasing ripple frequency, and increasing load current?

Does anyone know how much of a significant impact a 50dB ripple rejection would have on the operation of an LDO? Are there 'common' or 'typical' ripple rejection values that are acceptable? My linear power supply should be able to deliver maximum of ~700mA, so I need to consider the ripple. It's the first time I'm dealing with this so I'm unsure.

For the record, the power supply I'm designing will have a 21.6V Li-Ion battery as input, which will then power a dual rail supply, so I can get anywhere between +/-8V - +/-10V. I'm currently going with the LM337 and LM317 regulators, which both have ripple rejection values of 75/77dB, but they're also rated at 120Hz.

• 700 mA sounds like a lot for a linear regulator, and apparently out of spec for the one in your link. What is your input voltage, and what are you going to drive with it? Feb 28, 2017 at 0:27
• It's just the maximum figure I'm working with, I don't expect it to reach 700mA, but it's the limit for now. I've updated my original post, but the input voltage will be from a 21.6V Li-Ion battery, which will drive a dual rail power supply into a PCB with a 32bit MCU, some filters, ADC, bluetooth, and ultimately a coil antenna.
– deki
Feb 28, 2017 at 0:37