3.3v and 5v from two AA cells - Page 1
Forum section: electronics / projects

author: andyturk
author_rank: Frequent Contributor
timestamp: October 13, 2012, 10:01:57 am
content: I need to come up with a power supply strategy for a hand-held diagnostic tool that will be powered by two AA cells. The digital side requires 3.3v, but there's some analog stuff too (sensors and such) and unfortunately those require 5v. The mcu and display logic will need about 40mA @3.3v and the analog side should be under 50mA @5v. The LCD backlight isn't included in the logic figure because it's less sensitive to voltage and doesn't generate much noise. Figure another 30mA for the backlight.My current thinking is to use a dc-dc boost converter to increase the battery voltage (2 to 3 volts) to 5.5v so that an LDO regulator at 5v can scrape off most of the boost converter's ripple (~ 30mV). With filter caps and a not-too-horrible PCB layout, that will form the analog supply. The digital side will get its own LDO at 3.3v, also fed by the 5.5v boost output. I'm hoping to keep the two supplies as separate as possible because the digital side is quite noisy (especially when driving the SPI LCD). The backlight can run directly off the 5.5v so as to not burden either of the LDO regulated rails.Just to throw some part numbers around, the MCP1640 boost converter can generate 5.5v (on paper) and an MCP1824 LDO regulator can take that down to 5v with only 200 mV of headroom. Another MCP1824 will be set up to generate the 3.3v rail. I haven't built anything with these parts yet, I'm just going off the spec sheets for now. One assumption I'm making is that the 5v LDO will put a significant dent in ripple occurring at 5.5v. If that's unrealistic, then I could save some board space and just set the boost converter to generate 5v directly.Can an LDO regulator be used to remove voltage ripple on the input? Are there other ways to generate these two supply rails?

author: Rerouter
author_rank: Super Contributor
timestamp: October 13, 2012, 10:27:32 am
content: you could well get away with a switchmode due to such a small current requirement, you simply need to set it up like an RC filter, as the frequency is high, you can get away with some nice and low resistances, its not impractical to get the noise into the sub-mV range using a 200Khz or higher one using this,

author: Baliszoft
author_rank: Frequent Contributor
timestamp: October 13, 2012, 10:30:41 am
content: Exactly! I would not waste that much energy (dissipating at ldos), when it is going to be powered from batteries.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 13, 2012, 10:44:30 am
content: Quote from: Rerouter on October 13, 2012, 10:27:32 amYou're talking about SMPS, right? I don't know much about them. Have you got a link for some basic info?

author: digsys
author_rank: Unknown
timestamp: October 13, 2012, 01:28:30 pm
content: For low current requirements, have a look at Linear Technology's switched capacitor converters - eg LTC1515Most use only 2-3 caps (no inductors at all) and are up to 97+% efficient !! I've used the LTC1044 on many occasions.Go to LT's site and search through the options. Also, VERY low quiescent currents.

author: ptricks
author_rank: Frequent Contributor
timestamp: October 13, 2012, 01:53:34 pm
content: Quote from: andyturk on October 13, 2012, 10:01:57 amNo. A regulator will simply reduce the output voltage, ripple will still exist . You need to remove the ripple with filtering before the regulator.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 13, 2012, 04:50:15 pm
content: Quote from: ptricks on October 13, 2012, 01:53:34 pmUgh. Boost converters seem pretty sloppy:NCP1402, Vin = 1.4, Vout = 3.3NCP1402, Vin = 2.8, Vout = 3.3Coming from this:PS. I built a breakout board for the NCP1402 boost converter back when I thought the whole system could run at 3.3v. Since then, I found that the 3.3v sensors wouldn't work and had to change to the 5v ones. That necessitated a big redesign on the analog side and it's why I've been looking at other boost converters like the MCP1640.

author: Rerouter
author_rank: Super Contributor
timestamp: October 13, 2012, 10:24:00 pm
content: It strikes me as odd is that in pwm its only working at about 7.5Khz, I was talking about the fixed frequency PWM models working at 100Khz+but lets just start with what you have, a 7.5Khz pulsed waveform with a ripple of about +-100mV,now on your breakout board i cannot actually see any significant capacitance, i assume its the stock standard 100nF,so now onto an RC filter, while on the higher frequencies you can get away with significantly smaller values, we will start with what you have,for this lets say we use a 2 pole filter to get you close to +-1mV @ 30mA load,a resistance of 4.7 ohms and a capacitance of 22uF per pole. should work nicely for you, (see image)

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 13, 2012, 10:33:47 pm
content: MCP1460 is very cheap - why not just use two of them, one for each rail.With a 5V plus LDO for 3.3, every milliamp you draw at 3.3v will pull about 2mA from the battery.To minimse ripple, use a higher inductor value and if necessary a second LC filter on the output will reduce ripple substantially.

author: TerminalJack505
author_rank: Super Contributor
timestamp: October 13, 2012, 10:43:53 pm
content: If you go the switching-with-LDO-post-regulator route then here's a video by the late, great Jim Williams that explains how to reduce ripple and spikes when using a switching/LDO combination. Check out the app note referenced in the description as well.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 13, 2012, 11:12:27 pm
content: Quote from: ptricks on October 13, 2012, 01:53:34 pmIs this empirical evidence to the contrary?I hacked up another NCP1402, this time boosting up to 5v. It's output is shown below as the blue trace. The yellow trace is the output of a follow-on LM1117 set up for 3.3v. It's not exactly apples-to-apples compared to the the traces I posted before since there's a 100uF bypass cap on the output of the NCP1402 and another on the output of the LM1117. Even so, the noise coming out of the LDO is significantly less than what it sees on the input.It seems like the LM1117 does a pretty good job at taking care of everything except the spikes, at least when it has enough headroom. I don't know whether that's a general result or not, but I wanted to try it and see.Here's a graph from the NCP1402 datasheet showing expected ripple for a setup with roughly similar voltage and current. The ripple in my circuit has a lot more amplitude (even with those bypass caps!), but the shape of the noise is about the same. If anyone can explain why my implementation is so much more noisy, I'm all ears.My circuit/scope at the same time/sensitivity:

author: andyturk
author_rank: Frequent Contributor
timestamp: October 13, 2012, 11:21:06 pm
content: Quote from: TerminalJack505 on October 13, 2012, 10:43:53 pmGreat video! Thanks!Maybe I'll bang up another LM1117 with some ferrite beads as Jim suggested.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 13, 2012, 11:31:40 pm
content: Quote from: Rerouter on October 13, 2012, 10:24:00 pmThe battery/input capacitor is 10uF (C1). Here's the schematic. Note that I actually built these with 100uF tantalums on the output (C2), not the specified 47uF.Thanks for the 2-pole filter idea. I'll read up on those and give it a whirl.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 14, 2012, 12:42:00 am
content: Hooray for Jim Williams and ferrite beads!Even my rinky-dink little 0603 beads cut the post LDO spikes in half:The bead on the LDO input is responsible for most of the improvement. The one on the LDO output didn't seem to matter much. My measurement setup wasn't exactly "coaxial" though.

author: TerminalJack505
author_rank: Super Contributor
timestamp: October 14, 2012, 12:59:00 am
content: Nice. The secret sauce worked.You might try some small (10nF to 100nF) ceramic bypass caps to see if you can get any of the remaining high frequency stuff out of there.

author: ptricks
author_rank: Frequent Contributor
timestamp: October 15, 2012, 05:30:23 pm
content: Quote from: andyturk on October 13, 2012, 11:12:27 pmWhen you have something like noisy 5V input to a 3.3V output the noise MAY be reduced. The problem with doing this is that the regulator will regulate based on the noise and you could get unsteady output voltages anytime the noise causes the input to peak or sag. If your base voltage is 5V put you have ripple that goes between 5.2V and 4.8V that is the voltage the regulator will see, depending on the voltage the regulator needs over the output it can be a bad situation. Creating a clean input is always desirable and easily done in most situations.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 15, 2012, 10:30:30 pm
content: Quote from: ptricks on October 15, 2012, 05:30:23 pmGotcha. What I'm still struggling with is whether I can use a 5v boost "regulator" to actually put out a clean five volts. The examples that I've built so far have had spiky noise far greater than what the datasheet suggests. I didn't use their layout exactly, but I thought it was pretty close and I have followed their guidelines for the other components. Maybe the NCP1402 is particularly sensitive to whatever it is that I've done differently.With the filtering ideas from this thread (and Mike's suggestion of using a separate boost for each rail), I'll try again and see what happens. Hopefully, the MCP1640 regulator will be better behaved than the NCP1402.