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Author Topic: Trouble with a boost converter  (Read 2193 times)
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I'm trying to increase the voltage of my system from 5 volts to 25 volts using a boost converter LT1303 (the adjustable one), found here: http://cds.linear.com/docs/Datasheet/lt1303.pdf  My circuit is set up as in the first figure on page 9.  I'm using a 47 microhenry inductor, a 1N5819 diode, two 100 microfarad capacitors, R2=1000 ohms, R1=51 ohms, yielding about 25 volts according to the equation there. LBO, LBI and shutdown are grounded.

Instead I'm reading 1.8 volts.  Is there anything obviously wrong with this setup?

Thanks for your help
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LBO is an output, you don't need to ground that.
Shutdown Pin Response, bottom middle of page 4, seems to an  output being generated when Shutdown is high.
Try letting it float, or pull it up to 5V.
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Hmm, I've never paid much attention to those graphs, figuring them for applications more sensitive than my tooling around.  I'll try that, but first...

Anyhow, more complication.  The voltage (rails and Vout) aren't constant.  Instead they start at a high value (around 2.5 volts) and lower over a few seconds.  Removing the power for a few seconds reverses this.  As a result, my rail is 0.43 volts right now (so I can't trigger the shutdown/stop the shutdown) with Vout at 0.18... Argh.  Disconnecting the boost converter and measuring, the rail is the correct 5 volts.

*Sigh*  I guess I'll take another look in the morning with fresh eyes.
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It sounds like your circuit is not oscillating.

Making a boost converter is quite a tricky thing, it is not something that you can just throw together and expect to work. What sort of current are you expecting to use? The saturation current of the inductor is vital for this.
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LBI should be connected to Vin.  Shutdown should be connected to ground.  LBO should not be connected.

At the moment you are crippling the device with pins connected to ground that shouldn't be.
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Grumpy_Mike: I'm using an off-the-shelf IC.  I'm looking for about 10 mA and I'm using an inductor that saturates at 0.5 amps so unless it works differently than I'm thinking I think it should be fine.

Alright, attached LBI to Vin...  Didn't think it mattered.

Though I'm still having the issue of the device pulling my rails down way too low.  I guess it's drawing too much power?

How does the load figure into the setup?
« Last Edit: July 17, 2012, 01:44:54 pm by aoeud » Logged

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Here's the circuit as it's set up.  Is it correct?


* Circuit.png (15.52 KB, 810x422 - viewed 34 times.)
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Do you have another chip? you may already have damaged it
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I do.  What would have damaged it?
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Quote
I'm using an off-the-shelf IC
I know but this sort of circuit isn't something you can just throw together on a bread board. Layout is crucial.   
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Perhaps having something connected improperly, shorting something out or similiar, especially with it drawing alot without a load,
Also where is the 5v supply from?
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I've been as careful as I could be in hooking it up, but I tend to make a lot of mistakes converting from schematics to layouts...

I'm using the 5 volt pin in the "power" section of the Arduino Uno.  It's functioning correctly with other loads as far as I can tell.
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Boost converters need a substantial amount of current to start, as they have to charge the output capacitor
before the converter starts regulating.
You really need to run them off batteries or a supply that can deliver a few amps , or they wont start.
Whats the current capacity of your 5 V supply.
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I've been as careful as I could be in hooking it up, but I tend to make a lot of mistakes converting from schematics to layouts...
What are you using for a ground plane?
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...that would explain it quite well.  I guess this is what Mike was getting at. So if I used a smaller capacitor (said as "possible" in the data sheet) this would be more likely to work.  It's 40 mA/channel though I was expecting to ultimately be using several channels.

Alas, no time now!

I'm using a solderless breadboard and grounding to one of the rails.
« Last Edit: July 18, 2012, 06:29:55 pm by aoeud » Logged

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