LDO regulator isolation - Page 1
Forum section: electronics / projects

author: electosleepy
author_rank: Contributor
timestamp: February 27, 2016, 09:53:09 am
content: Are LDO's able to isolate power supplies? For example if I use a DC/DC converter to convert a battery voltage from 7.4V to 1.5V and then have two loads which are 1.2V and 1.0V that are supplied by LDO regulators for noise sensitive analog circuitry as shown in the attached diagram. Will the LDO's be able to filter the switching noise generated from the DC/DC converter?

author: Ian.M
author_rank: Super Contributor
timestamp: February 27, 2016, 10:07:24 am
content: It depends - generally the HFPSRRof a LDO is quite poor. You will need data for the specific LDO used, the DC-DC converter's switching frequency, output filtering, bulk decoupling after the LDOs and the PSRR of the sensitive analog circuitry. Even if it all looks good, you can be caught out by radiated EMI from the DC-DC converter.

author: electosleepy
author_rank: Contributor
timestamp: February 27, 2016, 10:33:37 am
content: So I just read the following article from TI . Is there anyway to predict what frequency components I will need to filter or do I need to measure the noise? Also would placing a ferrite bead between the DC/DC converter output and LDO input help?http://www.ti.com/lit/an/slaa414/slaa414.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: February 27, 2016, 11:01:54 am
content: The problem with ferrite beads is saturation. If there is enough DC current flowing the bead will start to saturate on the peaks of the waveform it is rejecting. Once its fully saturated, its as much use as a piece of wire!Assuming your DC-DC converter is operating in continuous mode, you can usually assume that the noise will be at odd harmonics of the switching frequency. Unfortunately, if the converter isn't very carefully designed for minimum RF noise, there can be significant energy in the higher harmonics. e.g. it is possible for the rectifier or switching device body diode to act as astep recovery diode, which can generate significant energy in the HF and VHF bands even with a switching frequency below 30KHz.

author: max666
author_rank: Frequent Contributor
timestamp: February 27, 2016, 11:45:45 am
content: fmaimonposted in theUltra Precision Reference LTZ1000thread a reference to this document:AN159 Measuring 2nV/?Hz Noise and 120dB Supply Rejection on Linear RegulatorsMaybe there is something in there that's useful to you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2016, 11:51:15 am
content: Well, the PSRR is shown in the datasheet. Is that enough filtering for your application? How noisy is the switcher, and how quiet do you need it?Don't guess, when you can measure and calculate!Ferrite beads don't do much as they saturate over maybe 20mA. Use inductors!Whatever attenuation that you need, either simply choose a better LDO (there are some types for RF applications which have good PSRR across the range), or pick up the remainder using inductors and capacitors. Filters have characteristic impedance and frequency; use an impedance similar to your application's maximum supply impedance, and terminate the filter with an R+C to dampen it.Tim

author: electosleepy
author_rank: Contributor
timestamp: February 28, 2016, 11:49:57 am
content: Thank you all for the comments so far. How can I determine the noise generated by the switch mode power supply if I do not have a physical prototype? Are there simulation the only methods available? Or any general rule of thumbs like if the switching frequency is 'x' amount this range should be covered?