buck converter help

mctrivia

Any idea how I could efficiently take a 9v batteries power and drop it down to 3.8v(input of LDS3985M33R)? Don't want to use resistive to drop all the way because i would like the battery to last as long as possible.

grasshopper

You could use a voltage regulator or am I missing something? Even 3 diodes and a resistor makes a great voltage regulator.

mctrivia

linear voltage regulators if I am not mistaken just waste the excess power. I am using several LDS3985M33R to give a clean output for the prop and other electronic components but I would like something efficient to drop the power from the 12v power supply or 9v battery. If I am mistaken that diodes could be used as efficiently as a buck converter please correct me.

Dr_Acula

Have a look at the "simple switcher" range LM2575. There are fixed and variable ones. You just need a diode and an inductor (though these parts can be a little tricky to source - particular low resistance inductors ideally under 1 ohm. I picked some up recently for $2 from a surplus store. Some suppliers only quote you the inductance, not the DC resistance. Those little inductors that look like fat resistors have too high a resistance.) Switching regs will give you the longest battery life - at least twice as long as a simple dropping resistor or linear regulator. Simple switchers are the easiest ones I've ever used and a lot more efficient than various homebrew circuits.
http://www.national.com/mpf/LM/LM2575.html
Futurlec - $1.55 for the adjustable LM2575

Post Edited (Dr_Acula (James Moxham)) : 2/9/2009 5:29:35 AM GMT

grasshopper

How much current will you be needing?

mctrivia

i will be needing less then 500mA for sure but more likely only 100mA. Need to run the prop(50mA normal), and an led screen(20mA), sd card reader(?),g force meter(?)

mctrivia

I will be using all surface mount components as small as posible as space is very limited also.

Leon

Why are you using a 9V battery?

Leon

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mctrivia

because i need to also run a speaker. no need to drop voltage for that though circuit input is 6 to 18v. also not enough room to pack 4 AA bateries

mctrivia

i should mention i do have a MC34063ADR in my stock pile not sure how to hook up though for 3.8-4v output.

Ale

There are circuits in the datasheet for the 34063 that you can use. Be sure to calculate the resistive net to get 1.2 V for the error amplifier. That IC tends to waste loads of power if this voltage is too out of range. This is though a poor choice because its efficiency is in the 65% band. There are better, smaller and more efficient ones from Fairchild and more expensive ones from TI.

Cluso99

If you are thinking of the regular 9V battery, it will not supply 500mA. Have you thought about using lower battery voltage and inverting it up to 9V for your amp section (or is this where the main power is being used?).

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mctrivia

well it is only a $1 chip i do not need to use it. what would you recommend as the best?

mctrivia

I am using a 12V wall power supply, a 9V battery back up and a LTC4412ES6#TRMPBF,FDN306P combination to allow for automatic switch over. this method works well with very little loss in power but requires the wall power to be slightly higher then battery power and the buck converter would half to go after this to prevent the bateries from being drained by the converter so using 2x 1.5v and bringing it up to the 6v needed for the speaker does not work(unless the logic output of the LTC4412ES6#TRMPBF could turn on off the converter).

a 9v can't provide 500mA? that is a problem speaker draw could be as high as 1.1W

mctrivia

here is my current thinking.

dMajo

Have a look at this:

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5224

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1998

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/5656

You get 3.3 directly from one regulator with a wide input range: less regulators in series·more efficiency

mctrivia

The reason I had switching before linear regulator is I was under the impression that the output from switching regulators was not very clean. in all circuits i have worked on there was linear regulators to drop the last little bit. the other reason is the regulators i am using are witched so the prop can turn on and off devices that are necisary only for short times when in battery back up like the screen.

mctrivia

well i figured i can get away with up to 6 AA batteries if i custom make the holder to fit. My screen is almost exactly the length of 3 batteries and 2 high. will use 4 AA gives me the 6V i need for audio. Will use the max 1684 as dMajo suggested though it is very expensive but since this is a 1 off and i want best battery life it is worth it.

Now to make my shopping list.

dMajo

mctrivia said...
The reason I had switching before linear regulator is I was under the impression that the output from switching regulators was not very clean. in all circuits i have worked on there was linear regulators to drop the last little bit. the other reason is the regulators i am using are witched so the prop can turn on and off devices that are necisary only for short times when in battery back up like the screen.

No need for this in my opinion. You can take the output to supply all digital stuff. If you have analog then it's enough a Pi (C-L-C) filter to supply the analog part, if it is very sensible than double stage (C-L-C-L-C). What's important is that the gnd routes on the pcb (the analog gnd and the digital gnd) are kept separate till the power supply capacitors.

Maybe I am not skilled enough but I never had troubles with switching noise even making audio phono-preamps.