750mA LDO current regulator for luxeons

[MrAl]

Hi again,

evan:
If you give me the dimensions i can tell you the approximate
inductance, but even a few microhenries or less can cause
oscillation. It's not a matter of how much inductance
is present, it's more about the circuit topology and
the switching speed of the components combined with
the inductance. With a small inductance, the only
difference is the circuit will oscillate at a higher
frequency than with a lower inductance.
Generally you dont want a linear circuit oscillating
if you can help it. Sometimes it doesnt matter but
if you can easily stop it you should.
Sometimes a circuit will oscillate about a given
operating point. In this case, it looks like it's
working normally until you look at the output wave,
which could be banging the LED with high current
pulses in order to achieve some average current level,
when it should be stable and at a nice, safe constant
current.

Newbie:
That's not a bad idea either, i'd try it that way
myself. It's a little easier to handle the wire.

Take care,
Al

 

[evan9162]

Al,


For the 750mA version, the coil is 0.4" long, 0.1" in diameter, with 33 turns.

Looks like I'll have to get myself an oscilliscope soon /ubbthreads/images/graemlins/tongue.gif

 

[MrAl]

Hello again evan,

If i did the estimate correctly you might end up with
as much as 0.7 micro henries with that coil.
Not a lot, but then oscillation doesnt require a lot

If i get time tomorrow i'll plug an inductor in place
of the sense resistor and add 0.180 ohms series resistance
and see what frequency it oscillates at. Hopefully
the cap swamps most of the oscillations. I would guess
something in the 1MHz range wouldnt be surprising.
Perhaps you can hold it next to a radio, then a tv set,
and see if there is any disturbance in the appliances
sound/picture. Because it will most likely be a square
wave (if it is really oscillating) it will be sending
out harmonics way up there, so the radio or tv set should
pick up something, even if it's just a few lines in
the picture.

Newbie's solution sounds pretty good though if your
wire is insulated (like enamel). If it's not
insulated you'll have to use my suggestion and keep the
turns away from each other.

If you're getting the right current level and you dont
see any interference on the tv set then perhaps the
cap is swamping all the oscillations in which case
you wont even have to worry about it
It's good to know it could happen though, and it may
break into oscillation at a given current level,
possibly high, low, or somewhere in between.

Take care,
Al

 

[evan9162]

Al,

Thanks for taking a look at that.

I've been bench-testing a version of this regulator on a breadboard - for the sense resistor, I have 3 0.47 ohm resistors in parallel for 0.156 ohms - giving a set current of 420mA. Removing one bumps the current down to 280mA.

For the RC network, I have a bunch of different capacitors, and a 1K pot. I wasn't able to obtain the same stability as my built circuits, even with the same 0.1uF, and adjusting the pot to obtain maximum current - the current dropped off gradually as the input voltage was increased.

Is it possible that the inductance from the coil is aiding in supressing some of the oscillation? Or could the construction difference (breadboard and wires vs. soldered PCB) account for the behavior difference?

 

[MrAl]

Hello again,

evan, it starting to look like maybe the resistor
and cap would be better placed on the sense pin of
the ic chip. I took a quick look at this tonight,
and probably have more time tomorrow.
Anyway, when i put the cap across the "R" terminal
(on your schematic you posted in this thread)
and the 390 changed to 100 ohms in series with
the sense resistor, i reduced oscillations even
with higher value inductors.

Of course this requires a bench test...

The test would consist of the following...

1. remove the series 0.1uf cap and 390 ohm resistor.
2. place the 0.1uf cap across the "R" terminal
(on your schematic)
3. break the connection between the 'current sense'
resistor and the "R" terminal, and place a 100 ohm
resistor between the "R" terminal and the 'current
sense' resistor.

Test with a load and various voltage inputs.

This might actually work better then having the cap
and resistor where it is now, although it does need
actual bench testing. If you are able to try it
please let us know how well it works.

Take care,
Al

 

[evan9162]

Al,

I'm not sure where to put the resistor - put it between R and GND? or R and Vin?

I did try the 100 ohm resistor between the R pin and the sense resistor - this caused the LED current to drop to a little less than half as before.

The 100 ohm resistor is restricting the base current of the transistor to 0.4mA, so is increasing the voltage drop of the transistor to about 0.9V. Meanwhile, the voltage drop of the sense resistor is reduced to 0.025V, so it's a bit of a wash in the dropout voltage department. Also, current slowly rises as the input voltage rises, since the base current is being reduced, and thus, the voltage drop of the sense resistor must increase to make up the 0.065V required between R and V- of the 334.

I've also found that (at least in my test circuit) replacing the whole RC network with a 2.5uF capacitor makes the circuit stable.

 

[MrAl]

Hello again evan,

Yes, the 100 ohm resistor would go between the top
of the sense resistor
and the terminal marked "R" on your schematic.
You also need the capactor, from the "R" terminal to
ground.

Of course before you can connect the 100 ohm resistor
you need to break the existing connection between the
sense resistor and the "R" terminal. Instead
of a direct connection there you will now
have the 100 ohm resistor.

This is just something to try, as the simulation showed
less oscillation for larger inductance in the leads
of the sense resistor.

Take care,
Al

 

[evan9162]

Al,

The results in my post above yours reflect the setup you've described. The circuit was stable, but behaved considerably different than before.

 

[MrAl]

Hello again,

Come to think of it, i think i had another circuit that
required a small 'low pass filter' like this to be
inserted before the 'R' terminal of the LM334.
I'll have to look up that info next...

Take care,
Al

 

[Doug Owen]

Ya gotta remember that there's current comin' out (or going into depending on what you know to be true...) the R lead. Basically it's the base current from the external PNP pass element. Expect a voltage drop across any series resistor between it and the shunt that will lower the total current.

Doug Owen

 

[MrAl]

Hello again,

Yes, the circuit i was talking about was using the
LM431 regulator...big 'ooops' again on my part.
I keep getting these two regulators mixed up for
some reason...i think because i did a lot of work
with the LM431. What this means of course is that
the filter i proposed earlier in this thread wont
work with the LM334. Im very sorry for the mixup.
Thanks to Doug for bringing this to my attention!

Apparently the LM431 is a little easier to use, but
still requires this small RC network. The input Z is
much higher so it's easier to filter.

Take care,
Al

 

[Doug Owen]

Al,

Ya got me confused, the LM431 I know is a rather mundane fixed voltage 3 terminal regulator (plus 5, 12 and 15). No external componenets called for??

Doug Owen

 

[MrAl]

Hello again,

LMV431 Did i post the wrong number?

I think the circuit was similar to the LM334, but
i could look it up...

Take care,
Al

PS
Ok, looked it up and found that we used a Zetex high
side current monitor chip as well as the LMV431, so
it would turn out to be a bit more complex, with a
better temperature characteristic. Maybe not worth the
extra part.