Okay! So I bought a new chip and soldered it to some stripboard the way the schematic suggests. I am using 4 x AA batteries (1.2v 2450mAh each) into the LM2577.
I am now getting approx 7.2v out! What have I done wrong now!?
EDIT: It seems as if the longer the batteries are connected to it, the more the voltage drops. Its now at 6.99v.
If it still doesn't work with the cap added, then the inductor is probably saturating or otherwise unsuitable. Is it or the chip getting warm? What load do you have on the output?
Having no load shouldn't cause the output voltage to be low. My guess is that the inductor is unsuitable. If you can tell me the dimensions of the toroidal core and the approx number of turns, I can probably work out whether it is saturating. Also useful would be its dc resistance and the current you want to draw from the +12v.
Ps is the diode the same type as shown in the regulator datasheet? Is it or the output capacitor at all warm? If the problem is iron losses in the inductor, it will be heating from the core and it may take a while for the windings on the outside to get warm.
Okay so the inductor has about 32 turns on a
Outside dia: 23mm
Inside dia:12mm
Width: 7mm
Yep the diode is the same. No the capacitor isnt warm.
Interesting though - when powered from the 5v of my arduino at ~4.9v the output is about the same, but when powered from batteries at 4.1v, it gives 7.2(ish) volts out.
Just then when I was testing it, I managed to bridge +12 and gnd, and it gave a small spark if that helps too.
idiotjohn:
Interesting though - when powered from the 5v of my arduino at ~4.9v the output is about the same, but when powered from batteries at 4.1v, it gives 7.2(ish) volts out.
I suspect that the peak current that the regulator is drawing may be greater than the current that your Arduino can supply - that is why it works better from a battery, and why I suggested the input capacitor (have you tried it yet?).
Using 42uH instead of 100uH would definitely be a problem. What load current are you intending to draw? I suspect that you might be better using an inductor greater than 100uH.
I would expect the input capacitor to help - especially when you are driving the circuit from a battery - but probably not as much as you need. I see that the test circuit on the data sheet has a 220uF input capacitor.
Looking at Fig 7 on the data sheet, for 12v@250mA output from 5v input, you should be using an inductor with code L220 or L330 i.e. 220uH or 330uH.
I suggest you add a load resistor to draw at least 100mA from the output at 12v, because the regulator may behave very differently when unloaded.
The resistor attached to the comp pin (and possibly also the capacitor) needs to be adjusted to suit the conditions. For 250mA load I calculate 1K maximum, for 100mA I calculate 430 ohms. See page 18 of the application note.
If the output drops with time, then something is probably heating up - probably the chip or the inductor, if you are not running from a battery.
I see - I didnt realise that the inductor depended on what output current you needed - I assumed that the 100uH would also accomodate for current less than 800mA.
I will try a 220uH with the new resistor and such.
I'm still surprised that you are not getting a higher output voltage. Unless the input capacitor is more important than I thought, it may be that the inductors you have to choose from just aren't good enough. I think a lower value inductor than the optimum should still work, however it will make the regulator less efficient and the input capacitor will be more important. It should be easier to get the circuit working with a higher value inductor.
Will I try with the 220uH inductor and a 1k resistor, or do i need to get someone to work out if I need a different output capacitor also?
John
I would work out the minimum and maximum current that you need to draw. Then work out what inductor you need based on the maximum current, and choose an inductor. Then work out the minimum value of the output capacitor, and choose a capacitor. Then work out the minimum value of the compensation capacitor based on the output capacitor you intend to use. It's all described on page 18 of the datasheet. You may find that your existing output and compensation capacitors are OK.
It's possible that your regulator is unstable and that's why you aren't getting 12v output.