Single package, dual linear reg... - Page 1
Forum section: electronics / projects

author: crispytato
author_rank: Regular Contributor
timestamp: November 29, 2013, 05:45:46 am
content: Hi there everyone. I've been working on a new design, which is an extremely small, single lipo powered headphone amplifier, which still produces reasonable sound quality. I will be running a boost and an inverting switch mode regulator, and then fed into a linear reg to clean up the rails.I've been taking my time with component selection, and I have rounded up many of the necessary components already, I'm just stuck on the linear regulator at this point.I foundthiscomponent, which fills the role of using discrete +/- regualtors, but in a single package. It's ideal for my design, in every regard, except for the package. I have no problem with tiny smd stuff, msop and similar is fine, but bottom-mounted thermal pads are a real pain, and go against the design I've been aiming towards thus far - nothing too special needed to assemble the board ie. reflow gear. I'm very keen to avoid having an extra IC if possible, as I'm already squeezed for space, and want to keep wasted power as low as practically possible.I was hoping that someone would be able to point me towards an alternative to the LT3032, in a easier to deal with package. A combined positive/negative linear reg seems like a hugely useful component, and something that would be used a fair bit in modern integrated designs, but I've had no luck finding any other similar components. If someone could enlighten me what, and where I should get it, it'd be much appreciated, as I can finally get to making this design into reality. If there's really no other easy option, I'd love to hear some suggestions for inexpensive, available, SMD parts, with low power, low quiescent current, and both positive and negative regulators (in a matched pair, like the 79xx and 78xx). It's been very hard finding a pair like this with the search tools on RS or component manufacturers sites. (my normal supplier is RS, so bonus points if they're available there)Thanks in advance for any help.Chris

author: codeboy2k
author_rank: Super Contributor
timestamp: November 29, 2013, 07:41:45 am
content: I don't have any suggestions for a dual +/- LDO to replace the LT3032, but I just want to ask what is your switch frequency?I expect that you want to be high frequency simply because you want smaller magnetics, but this is harder to filter with linear regulators, as most regulators have poor PSRR at high frequencies. So you probably want to try to operate your switch at as low a frequency as possible, balancing between the size of the inductor and frequency, and placing it into a region where the linear regulator can perform better to clean it up.Just thought I'd put that out there for you to consider if you haven't already been made aware of this.Cheers!EDIT: actually, looking closely at the datasheet for the LT3032 it has only around 30 dB of PSRR at switching frequencies from 100KHz up to 1MHzThe several datasheets I've seen for the 78XX series don't even have charts for ripple rejection.30dB means that 200mV of switching noise at the regulator's input would be attenuated by 30dB and end up at the linear regulator's output at around 6.3 mV or so. You'll want to know if that 6.3mV of switching spikes will affect the amplifier or not.You can also chose your opamps to have a good PSRR at the switching frequency to reduce the switching noise even more.