Quick check of my step-up based on lm2577 - Page 1
Forum section: electronics / beginners

author: mrkva
author_rank: Unknown
timestamp: February 03, 2014, 04:41:31 pm
content: Hello everyone,I was hoping some of you experienced designers could look at my circuit and check whether my calculations seem reasonable. It is my first time designing step-up converter, so just to be sure.My goal is to generate 48V from 9V, as clean as possible (audio use), with maximum current 30mA. I know the lm2577 seems a bit of an overkill, but I haven't found any other solution yet.I know the circuit will produce actually 50V, will work on that later - but that is not the critical part. I am particularly not completely sure about L, R(C) and C(C).Thank you very much for any input on the circuit!Jonas

author: BravoV
author_rank: Super Contributor
timestamp: February 03, 2014, 04:46:53 pm
content: Quote from: mrkva on February 03, 2014, 04:41:31 pmI'm afraid you need to define your minimum specification on that.

author: mrkva
author_rank: Unknown
timestamp: February 03, 2014, 04:49:34 pm
content: Quote from: BravoV on February 03, 2014, 04:46:53 pmDon't have one. Not sure what I can expect from this kind of a setup. So just omit that comment if it is troublesome

author: fcb
author_rank: Super Contributor
timestamp: February 03, 2014, 05:13:40 pm
content: Looks like a phantom supply for two mikes from a PP3???Most of the calculations look OK, to cope with a minimum load conditions (the FB network will draw 1.2mA, you may need a little more - experiment), just load the output with a resistor.Be warned though - if you want to run it from a PP3 it won't last long. Your drain will be something like 1.488W/efficiency, which is alot of a PP3 to support - i'd guess at 1-2hrs life.As for noise, your friend will be bulk capacitance and filters (CLC at the very least).Personally, if thats what you are building, i'd do it using 4xAA cells.

author: mrkva
author_rank: Unknown
timestamp: February 03, 2014, 09:11:29 pm
content: Quote from: fcb on February 03, 2014, 05:13:40 pmYes indeed, my goal was to have single 9V battery to generate phantom power for two microphones and one LED to signalize it.I am guessing you are calculating with 10mA per microphone, which seems like an quite and overkill (I know it is spec, but it is maximum spec, not average as far as i recall). According to this article:www.sounddevices.com/notes/general/phantom-powering-basics/it is usually less then 3mA, which makes it a bit more reasonable. And chances are, that only one will be used at the time usually.Anyway, you're probably right about the AAs.Also, what do you think of replacing CLC circuit with LM317HV solution? I never designed a CLC circuit before...

author: mrkva
author_rank: Unknown
timestamp: February 03, 2014, 09:16:35 pm
content: Also, my deal with 9V batteries was to have to and 'easily' generate dual rail supply for the rest of the circuitry (there is preamp too...).Maybe you have some ideas how to get both dual rail supply and 48V with single chip?If I split the 48 to +/-24V, it won't be 48V anymore for the phantom...

author: fcb
author_rank: Super Contributor
timestamp: February 04, 2014, 02:46:27 pm
content: Current draw: You spec'd the 30mA, not me.LM317HV: sure - that'll lower the noise quite alot (the 317 is a GREAT regulator for low noise), but bare-in-mind that your audio is in the microvolt range, so picovolts of noise will be relevant. Probably use filters on the output of the regulator - use bulk capacitance!When I've done this in the past I've built fixed frequency fixed duty cycle boost-converters (basically unregulated) to avoid in-band regulation loop noise creeping in (555 astable, driving MOSFET boost config, driving chain of zeners).As for the rest of your design requirements (other rails etc...), no ideas - you haven't published the specs.

author: Bassman59
author_rank: Super Contributor
timestamp: February 04, 2014, 05:40:53 pm
content: Quote from: mrkva on February 03, 2014, 09:11:29 pmThe spec is indeed 10 mA, and there are some microphones and direct boxes that use all of it.Remember that phantom power is provided to the mic through a pair of 6k81 resistors on pins 2 and 3, so the more current pulled by the mic the lower the voltage available to it.So if you intend to power two mics, 30 mA is a reasonable design spec.-a