Li-ion power source for OP-Amps - Page 1
Forum section: electronics / projects

author: FHR
author_rank: Contributor
timestamp: October 31, 2016, 08:55:45 pm
content: Hi,I'm making a portable headphone amplifier and I need some help with power supply design.I need some way to step up the 3 - 4.2V of Li-ion battery to around 20V, which is split by capacitor divider to provide dual power supply/virtual ground for opamps.What I have:* Boost converterhttp://www.ebay.com/itm/2A-booster-board-DC-DC-step-up-input-2-24V-to-5-9-12-28V-Replace-XL6009-M92-/301724758278?hash=item4640327106. I get noise in the subwoofer spectrum (around 30Hz) + a little bit of switching noise. I have set the output to 20V, input is 5V; switching frequency is AFAIK 1.6MHz.* Adding a ~47uH coil + 220uF (LC filter) to converter's output was an improvement. No switching noise, but I still get that low-frequency noise.Here's the noise, measured AFTER LC filter.What I've tried:* Adding a linear regulator 7815 between boost converter and opamps. Seems like it's better, but a) it does not get rid of all the noise b) it's bulkyIt looks like I'm out of ideas now...Could anyone help me how to get rid of the low-frequency noise, please?Thanks,Filip

author: edavid
author_rank: Super Contributor
timestamp: October 31, 2016, 09:02:28 pm
content: The SB6284 has "Automatic Pulse Frequency Modulation Mode at Light Loads". If you apply a reasonable load, the LF modulation should go away. If not, you would have to use a DC-DC converter based on a different IC.

author: FHR
author_rank: Contributor
timestamp: October 31, 2016, 09:26:15 pm
content: Thank you, I didn't know about this "feature".I have messed around with resistors a little bit, and it seems that I need ~540R load in order to have a clean output.At 20V that means 0.74W of wasted heat and energy - that would be a problem, as I want as much battery time as possible and wasting energy as heat is not one of the ways to achieve itThat means I will have to use another regulator. Do you have any recommendation? I need around 200mA @20V output.I can't really do SMD (I use veroboard), so either through-hole package or module like the one I already have would be nice.

author: kxenos
author_rank: Frequent Contributor
timestamp: October 31, 2016, 09:40:22 pm
content: Have a look here:https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=14F3A86B85747199Use their Webench software. It's perfect for quick designs like this.Here's the spice model for TPS55340 to simulate it on your own.http://www.ti.com/lit/zip/slvm589edit: Oops, just read the not smd sentence...

author: edavid
author_rank: Super Contributor
timestamp: October 31, 2016, 09:43:27 pm
content: A regular "2577" or XL6009 module should be fine. It would be nice to find a module that already has bipolar output, but I don't know of one.

author: FHR
author_rank: Contributor
timestamp: October 31, 2016, 10:48:13 pm
content: Thanks for all responses,I've looked at XL6009 and the datasheet states minimal Vin is 4V. I need it to go down ideally to 3V. LM2577 has min Vin 3.5V so it's better, but not ideal.When browsing eBay I came across this MC34063 chip and it looks like it might fit the bill. It's an older chip, but min Vin is 3V and it's available in DIP8 package.Do you think it will be better than what I have now noise-wise?

author: Buriedcode
author_rank: Super Contributor
timestamp: November 01, 2016, 12:52:58 am
content: The MC34063 is old, and ubiquitous. It generally runs at a lower frequency than most modern SMPS devices, and as such, requires larger value inductor and output cap. I would see that as a step backwards. It's efficiency is pretty poor - although can be rather good for boost converters, but it'll have larger ripple on the output than your current setup.It will have to be a compromise between physical size (including inductor and caps), cost, efficiency and noise. As you found out, adding a post regulator can hinder efficiency whilst reducing noise and increasing size. Many boost converters achieve 90%+ efficiency at an incredible range of output currents by pulse skipping or PFM. The price we pay for this is that the switching frequency can vary and its output spectrum spread - making it harder to filter out.If it is specifically for opamp circuits I suggest creating a true 'bipolar' power supply. This way things are referenced to a real ground, so any noise on the power rails will be subject to the opamps CMRR which is often at the least 60dB. It also removes the need for a capacitive voltage divider. It complicates matters, difficult for moderate power (5W+) but many use a boost converter and a charge pump for the negative rail. Alternatively use a coupled inductor or a centre tapped transformer with an appropriate SMPS device to create both rails from the lower voltage. Whilst charge pumps can be very efficient, generally efficiency takes a hit when creating both positive and negative supplies, with 'typical' figures of 60-75%.As I said a compromiseI would use a boost converter than can easily cope with your maximum current (derate for higher efficiency) and a charge pump. Very few charge pumps can go to 20V but I'm sure they exist. You can then add LC filters on each rail to further reduce ripple, and let the opamp do the rest of the attenuation.

author: FHR
author_rank: Contributor
timestamp: November 01, 2016, 07:05:56 pm
content: Thank you all for your great support!I decided I'm going with MC34063. I calculated I need a ~68uH inductor, so not that big.If this does not work for some reason (ie too noisy), I will probably buy one of the newer converters in SOT-23 package (LM27313) and use SOT-23 => DIP adapter board.@BuriedcodeI looked at charge pumps, but I can't find any that would suit my purpose.Building an isolated supply seems like an overkill to me. Sure, I would love a "true" bipolar power supply - but I'm not sure it's really that needed. The solution I have now works pretty good, apart from the PFM noise.Thanks again,Filip