Is it possible to use a MC34063 boost converter for a negative voltage? - Page 1
Forum section: electronics / projects

author: Ampere
author_rank: Regular Contributor
timestamp: June 12, 2015, 12:04:43 am
content: I'm currently designing a portable microphone mixer / pre-amp. It's meant to be powered by two 9V batteries.That's fine for the op-amps. I can regulate the batteries down to +6V/-6V with a linear regulator to power them. But any good microphone pre-amp needs to have phantom power to run the microphones which means +48V and at least 100mA.I've tried the Cockroft-Walton multiplier, but it just seems way too inefficient for what I'm doing. I need massive capacitors to make it work which isn't really ideal because they will dwarf all of the other parts in what's meant to be a very small mixer.Then I saw Dave's video on the MC34063, and switchmode converters (which had always seemed really complicated) suddenly seemed to be a reasonable alternative. But would an IC like the MC34063 work for this? Four chips, one on each of the +24V, -24V, +6V, -6V rails? Dave mentioned that the MC34063 can be used to invert a positive to a negative voltage, but he didn't say anything about using it to boost a negative voltage.On the topic of DC-DC ICs, how do the equations shown in the MC34063 apply to other circuits? For all of the other ICs I've looked at, there isn't a list of equations like the MC34063's datasheet has.If it makes any difference, the circuit which needs phantom power is the one from page 7 of the INA217 manual:http://www.ti.com/lit/ds/symlink/ina217.pdf

author: moffy
author_rank: Super Contributor
timestamp: June 12, 2015, 12:20:19 am
content: Putting a switchmode supply so close to all your sensitive mic preamp circuitry would be tricky. Your signal levels are so small that any generated noise, on the supplies or EMI will cause issues. Could you section off the switchmode completely from the rest of the circuitry, by shielding?This post seems to be exactly what you want:http://electronics-lab.com/projects/power/029/index.htmlp.s. Use another 9v battery for your phantom supply.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2015, 12:34:38 am
content: Ugh, definitely not MC34063 for a battery powered application. Ugh, avoid it in general, it's old and it's bad!Nice thing about switchers is, if you're not afraid of leaving the beaten path slightly and treading your own circuit, you can make almost as many voltages as you want. Take close note of which parts of the waveform are regulated, and grab those parts with diodes. Avoid paths where, when the switching transistor turns on, your diodes also turn on: because those tend to be transistor-switching-into-capacitor conditions, i.e., momentary short circuits. Switchers don't like that. Anyway, those paths tend to be the unregulated ones anyway.Hmm, I don't have a schematic handy. I've used a TPS54233 regulator, with a dual winding inductor and a couple diodes, to obtain +/-5V (positive regulated, negative nearly as good) from +12V (actually 7-28V would work). Other devices in the series, and with different inductor setups, would be fine making your voltages, I think.For low noise, some additional filtering is probably a good idea, or a couple LDOs can be used. Note that LDOs rarely do much high-frequency cleanup (they almost all have poor PSRR over 10kHz or so -- you have to check the datasheet to find out), so it may not be worth the bother.Tim

author: Ampere
author_rank: Regular Contributor
timestamp: June 12, 2015, 12:36:42 am
content: Quote from: moffy on June 12, 2015, 12:20:19 amWhat would be the best way to do this? Is it good enough to just use shielded inductors? Sorry for all of the questions. This is my first real project.QuoteThat's what I'm looking for, but I might need to switch out the resistor and inductor because my mixer has four mic inputs, and therefore uses four times as much current.QuoteGood idea. It's better that the phantom power alone go dead than the entire mixer.

author: Ampere
author_rank: Regular Contributor
timestamp: June 12, 2015, 12:49:44 am
content: Quote from: T3sl4co1l on June 12, 2015, 12:34:38 amI'd be willing to try, but I've never made any circuit like this before. Are there any good resources on how to make one of these from scratch? Will it be better than the MC34063 circuit moffy linked to? Power consumption and circuit size are my two main concerns.QuoteIt doesn't necessarily need to be +/-. I can add another 9V battery as suggested by moffy and boost +9V to +48V. The need for a positive and negative supply was due to the limitations of the MC34063 which was the source of my problem.QuoteMy mixer will have an 80Hz high pass and 30kHz low pass filter after the amplification stage, if that's what you mean. Will the inductors cause any interference in that range?

author: moffy
author_rank: Super Contributor
timestamp: June 12, 2015, 12:57:21 am
content: Quote from: Ampere on June 12, 2015, 12:36:42 amI would enclose the switch mode supply completely, a box within a box. You can use blank PCB material single sided to do this.QuotePhantom power requirements are usually very small. Check whatever the specs say

author: Ampere
author_rank: Regular Contributor
timestamp: June 12, 2015, 01:07:48 am
content: Quote from: moffy on June 12, 2015, 12:57:21 amThe phantom power standard allows for up to 10mA per mic. If four 7mA mics are plugged in then that's 28mA, 40% higher than the circuit allows.

author: moffy
author_rank: Super Contributor
timestamp: June 12, 2015, 01:27:30 am
content: Just increase L1 to 100uH and make sure it can handle at least 1A. Go for a closed magnetic core like a toroid, to reduce EMI. That will give about 40ma.Pout = freq*L1*(Ipk^2)/2

author: Ampere
author_rank: Regular Contributor
timestamp: June 12, 2015, 01:53:46 am
content: Quote from: moffy on June 12, 2015, 01:27:30 amThis solved my problem nicely. Thanks for the help.

author: moffy
author_rank: Super Contributor
timestamp: June 14, 2015, 03:55:42 am
content: Glad to help