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Old 10th October 2005, 01:10 AM   #1
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Default Regulated PS with shutdown

Hi all

I'm designing the power supply section of some audio project. It's based on a very standard LM317/337 pair to provide +/- 12V.

As an added bonus, I wanted to be able to "shut down" the audio section, from the microcontroller that I use to control everything.

I came up with this: (and likewise for the 337)

Click the image to open in full size.

It's quite simple really, practically textbook. It doesn't shut down entirely - there is a ~1.25V output when in the "shut down" state, but the audio section should draw very minimal power at this voltage, and not function at all, so it seems good enough to me.

What do you guys think? Any ideas, suggestions? Would a relay be a simpler/better solution? (But I like this solution because it's cheap, has no mechanical part and is silent.)

Thanks!
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Old 10th October 2005, 01:55 AM   #2
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Try to skip C3. Eigher mosfet or cap or both won't like immediate shorting the energy stored in a cap.
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Old 10th October 2005, 01:59 AM   #3
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Well, C3 helps stabilizing the circuit a lot, especially when the FET is switching...
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Old 11th October 2005, 11:35 PM   #4
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Ok, I came up with something that I think is much better. The drawback is the need for an additional power MOSFET (with possibly heatsink, depending on power draw).

C4 "smooths out" transitions to reduce current spikes. (Don't mind the MOSFET references here, I chose the closest that was available in LTspice... I will probably use more common ones, like an IRF530 for the power mosfet and a BS170 for the switching MOSFET).


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Old 24th October 2005, 06:58 AM   #5
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Ok, I haven't gotten around to building this yet. Does anyone see anything that could prove problematic before I go on and build it?

Just to make things clearer, the switching part will be used to power a "hot-plugging" device. A microcontroller will detect when the device is plugged in/plugged out and switch the power supply to it accordingly. Again, anything I should be careful about?
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Old 24th October 2005, 07:57 AM   #6
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Neat idea! The original circuit will certainly work. Since you have dual supplies, you can make a -/+ 1.2V supply with two times diodes in series and take the adjust terminal of the pos supply to -1.2V to get a total shutdown to zero. Likewise, take the neg supply adjust pin to +1.2V.

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Old 24th October 2005, 10:34 AM   #7
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Thanks Jan,

actually I'd rather use the second version. The device in question will be connected via a cable that can be a few feet to a few tens of feet long. To get the best regulation, I intend to put the regulators in the device itself. The cable will transport about +/- 20V to the device, then it will be regulated on-board to +/- 12V. +/- 20V should be enough to get a good overhead to make up for losses along the cable. Since the regulators will be on-board, the capacitive noise coming from the cable will be basically a non-problem.

So the 'base unit' will provide +/- 20V, switched by MOSFETs. With a 40V max difference, we're still in the safety zone, I guess (regarding shock hazard).

By the way, a bit off-topic (or not) but if anyone happens to know a good source where to buy Neutrik Minicon (12 conductors) connectors for a decent price, please let me know. The only reseller I have found so far sells them for about 20 to 30 euros a piece - so to make a cable and get two plugs for each end, that's about 80 euros (not to count the cable itself! A bit pricey.
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Old 24th October 2005, 12:55 PM   #8
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why not just use a regulator with a shutdown pin? The LT1963 is lower noise, low dropout, thermally protected -- a much better device than the LM317 and it has shutdown. It's a $2 part if you have the patience to work with SOIC-8. The TO-220-5 part is about $5.

The smt LT1763 and LT1964 are much quieter than the LM317, but will only do 500 ma --
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Old 24th October 2005, 03:32 PM   #9
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Thanks for the pointers.

Ripple rejection is still better on the LM317 (80 dB vs. 63 dB, IIRC), though, although the other parameters are indeed better on the LT parts... which is more important, that is the question... any guru out there who can say something about ripple rejection?

Anyway, as I last said, it turns out I don't really want or need to directly shut down the regulators. It makes more sense that I be able to shut down the power supply from the "base unit", since I want my device to be "hot-pluggable" (if that is a word...), so that's the power lines before the regulators... hence the MOSFET solution, which seems simple enough and robust.

Local shutdown of the regulators could be a plus, but I don't think that's really needed here. What I want is that no power run through the connecting cable until the device is fully plugged in... (and as a bonus, I want to be able to put my device to "stand-by" on demand while plugged in, but again it's better in that situation if no current runs through the cable...)
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Old 31st October 2005, 12:23 AM   #10
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A bit late, I think you should use FKV550 MFet's.

They have a lower Rds(on) .01 and they only require 1-2V Vth.

just a suggestion
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