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: Mixed-signal and digital signal processing ICs | Analog Devices 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?
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.
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.
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.
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?
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?
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.
...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.
a solderless breadboarrd is your first mistake... try to "Dead Bug" the ic on a small piece of PCB material ESPECIALLY the components connected to the Catch diode, inductor and output filter cap... This is the Most Critical area next to the Vcc connection make your connections as short as possible AND BYPASS THE Vcc Right AT THE IC use the copper as the ground. You Shouldn't need more than a 2 - 3 cm bit of PCB material. If you can, another inductor of the size used for the switcher inductor can be placed in series with the output and another LOW ESR cap to ground on the output of the coil (About 100uF) should bring your ripple down to 20 - 40 mV or less depending on the actual total load current. IMO
I don't have any PCB. Could I just solder parts directly? Nor copper for that matter, though I can scavenge aluminum.
So the idea is a slab of aluminum under the whole thing that serves as a universal ground, with plastic holding the non-grounded parts from touching it. The bypass capacitor on the input voltage should have the cylinder holding the good stuff as close as possible to the lead and the grounding wire should be very close to the lead as well. Then I add another inductor in line with the load, and connect the load input to ground via the 100 muF capacitor.
Right all but something you can solder to, The Ground Plane. Seriously the single reason that I mentioned copper (PCB Material) is it's ease of soldering. Aluminum is Most difficult to solder to. A Flat piece of .2mm tin plated steel would be an acceptable substitute. Not because it is steel quite the contrary. It's just easier to solder to. Doesn't need to be big as I said a 25 to 35 mm square should be big enough for all the parts, certainly no more than 50 mm on a side (2 inches) the smaller the better as it (the size) forces you to make SHORT DIRECT connections which are the KEYS to correct operation.
