Parallel Vs Serial Power supplys
Basil
Posts: 380
Hi All,
The device I am designing will need both 3.3v and 5v power supplies.
I will be using the LM2937IMP 3.3V and 5V versions.
Question, is it best to connect these regulators in Serial (I.E 9V->5V->3.3V) as per the schematics for the prop demo board, or would parallel (I.e. 9V->5V & 9V->3.3V) give a better supply?
I need the supply to have as little noise as possible and efficient.
I was told that having them in serial as per the demo board produces more electrical noise, but I thought i'd ask the pro's because i'm sure you wouldn't design a board with a 'bad' power supply!
EDIT: I should note that every so often, there will be a time when up to 10A or so will be drawn from the 9v battery for < 1/2 second...I'm trying to avoid brownouts, a big cap may be needed...
Thanks,
Alec
Post Edited (Basil) : 4/23/2007 1:20:15 AM GMT
The device I am designing will need both 3.3v and 5v power supplies.
I will be using the LM2937IMP 3.3V and 5V versions.
Question, is it best to connect these regulators in Serial (I.E 9V->5V->3.3V) as per the schematics for the prop demo board, or would parallel (I.e. 9V->5V & 9V->3.3V) give a better supply?
I need the supply to have as little noise as possible and efficient.
I was told that having them in serial as per the demo board produces more electrical noise, but I thought i'd ask the pro's because i'm sure you wouldn't design a board with a 'bad' power supply!
EDIT: I should note that every so often, there will be a time when up to 10A or so will be drawn from the 9v battery for < 1/2 second...I'm trying to avoid brownouts, a big cap may be needed...
Thanks,
Alec
Post Edited (Basil) : 4/23/2007 1:20:15 AM GMT
Comments
Short answer? There is no short answer. Like so many things, you just have to balance the tradeoffs.
-Phil
Thanks for that, I thought it was something like that
FTDI chip will be powered by the USB bus so no problem with that.
The 3.3V will be supplying the prop, boot memory and dataflash memory. (Prop = varys, boot memory = 100na standby, dataflash = 17ma max)
5V will be supplying 12-bit ADC, ADXL78 accelerometer and MPX4115A barometer. (ADC = 400ua, ADXL = 1.3ma, MPX = 10ma)
9V directly powers 3 x Op-Amps (Rail to rail OP777) and also 3 pyro outputs, which will be up to 15A for less than 1/2 second. (OPamp = 300ua each)
As you can see, none of these are particulary high (If I read the datahseets right!)
Does this change any of your thoughts?
Clearly, with the small currents involved at 5V and 3.3V, you could do it either way. With the pyro currents so high, I would carefully filter the supply to the Op-Amps, probably run the regulators in series, and put as large a capacitor as you can afford (space-wise) across the 5V regulated output so that the 1/2 second when the pyros are activated won't drop your logic supply enough to force a reset or invalid data and won't mess up whatever is driven by the Op-Amps. You might use a diode in series with the 9V power supply with a large capacitor across the output to hold the Op-Amp supply while the pyros are essentially shorting it.
Hi Mike,
Thanks for that.
The op-amps aren't a critical part of the design, they are not constantly in use.
Every 0.5 seconds or so, they will be used to detect voltage drop across the ignitors (which is where the high current is going to). This will be used as a continuity test.
When the FET's fire the ignitors and draw all that current, I will disable the op-amps. Once the FET's are 'off' again, I will perform the continuty test with the op-amps to detect if the electric match fired correctly.
I have attached a basic diagram of the setup for clarification.
Would I still need the diode and cap?
EDIT: You can ignore the resistor values, they have been changed since this was done Hi Mike,
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Paul Baker
Propeller Applications Engineer
Parallax, Inc.
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Paul Baker
Propeller Applications Engineer
Parallax, Inc.
You need caps **AND** a diode in front of them. Or the current drawn from your e-gnitors will discharge them.
You could also calculate the expected voltage drop if you know the impedance of your battery and the resistance of your e-gnitors.
Nick
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Never use force, just go for a bigger hammer!
The DIY Digital-Readout for mills, lathes etc.:
YADRO
I will run 2x 9V in parallel for testing to be safe.
I will design the big cap into the design which should cover everything. Ive been looking around and most other similar devices use a cap and work fine. There are even some that dont use a cap at all, but they seem to only fire 1.25A
I will put my 3.3V and 5V regulators in parallel to help reduce noise on the power rails. The extra power lost when reducing from 9V to the 3.3V supply over 5V to 3.3V is not as important as clean power in this case.
Thanks guys.