Leaving linear regulators on another PCB - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: December 30, 2017, 06:46:01 pm
content: So I need to use some linear regulators (LT3080) in an application that does audio-near (40KHz) and precision DC.I need a bunch of different rails. I would like to design a separate PCB that provides the power and has all the power OK indicators on it (like window comparators that run off the rail to indicate power output is within 1 volt or so).What consequences will leaving them off the signal PCB have? I was just going to stick some big caps and ferrite on the signal pcb. I also don't want to heat it from the voltage regulators. Having all those heat sinks and whatnot on the main PCB will be a problem. If I do it on another PCB I could arrange things to make sense from a thermal prospective.I'm thinkin1) noise pickup due to lead wires2) increased inductance (I will use heavier wires to connect the two PCBs)There is a 3.3V digital rail for a DDS on the main pcb, but I thought I would run that off the 5V rail and leave that particular regulator next to the IC.I think I decided to do it the proper way, with regulators near parts to form islands where possible. I won't let the mechanical engineer in me win.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 30, 2017, 07:41:13 pm
content: That doesn't sound to be a big problem. A long PCB trace isn't very different from the same length of cable. Usually if the environment isn't very noisy you don't need any special handling. But this depends on the environment and also on the sensitivity of the load.You should keep some things in mind: I still would place a cap close to the output of the regulator. Big caps don't help for higher frequencies. Use a set of caps e.g. 100nF, 1uF and maybe 10uF to cover a wider range of filtered frequencies. For the digital rail probably completely unnecessary. The digital rail most likely will be fine with the normal 2.2uF required by the regulator plus the decoupling caps close to the supply pins. As for the inductor: keep in mind that you create a LC resonator. With some bad luck you might see unwanted oscillations. So probably place a 0-Ohm jumper instead of the inductor and in case you need additional filtering you can try to place an inductor there.Eventually you can use twisted pairs (if possible) for the supply to reduce the coupling in of noise. You should spend some time to think about any ground-loops you might create. Especially with mixed signal circuits.If you fear thermal issues of the caps. Why don't use dry electrolytic caps? And/or if possible place them on the back-side? Actually what is the power the regulators have do burn? And at what temps you think the regulators will operate?But why do the hassle? Depending on the current requirement I would use ceramic caps** and don't think about the thermal issue at all. And for the analogue supply I would use several in different sizes. Usually the digital isn't very demanding (don't forget the decoupling caps close to the supply pins).** e.g. EMK105ABJ225KV-F 2.2uF/16V which has enough headroom for the voltage deratingEdit: Added values as the type number is pointing to the series and not to a specific device. Added some clarification to the set of caps.

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2017, 08:46:04 pm
content: There is nothing wrong with doing this and it is preferable for many reasons including being able to place the heat sinks in a better location. Some designs also include remote sense so that the DC voltages are monitored at the load improving the load regulation without using excessively heavy wires. If necessary, floating outputs can be used with a single point ground at the load to eliminate ground loops.