MOSFET-based LM334 regulator

OddOne

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I keep sheets of Kapton MT film on hand for just such a purpose. /ubbthreads/images/graemlins/grin.gif

oO
 

OddOne

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Yep, it's DuPont's answer to mica insulation. I've gotten good results with it.

Also, found a P-channel TrenchFET hat might be good for this... SI4427DY. SO-8 package, 30VDC Vds, 11.5A constant Id, 0.0105 ohm at 10VDC Vgs, derating to 0.0195 ohm at 2V Vgs. $2.53 per from Mouser.

In through-hole, there's the SUP75P03-07, with a 30VDC Vds, 75A constant Id, and 0.007 ohm resistance at 10V Vgs. $5.75 per from Mouser.

oO
 

evan9162

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You'll have to watch the power dissipation on the SO-8 part - it can only take brief hits of 3W.
 

NewBie

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[ QUOTE ]
evan9162 said:
You'll have to watch the power dissipation on the SO-8 part - it can only take brief hits of 3W.

[/ QUOTE ]

Note the 85 C/W junction to ambient thermal resistance with a 1"x1" copper pad area on FR-4.

Try out this Thermally Enhanced SO-8 Package with the backside drain thermal pad:
http://www.fairchildsemi.com/ds/FD%2FFDS7079ZN3.pdf

It has under half the thermal resistance at 40 C/W junction to ambient when mounted on 1"x1" pad.

Unfortunately, it's turn-on voltage is between 1-3V, instead of 0.6V.

Of course, if you need a 1"x1" thermal pad area of SO-8 packages, you could always move up to the D-PAK SMT package, which would put you in a package that is alot more thermally efficient.
 

evan9162

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Here's the second version of the regulators (next to a quarter):

ldoregs2.jpg




Here's the new low power version (right) vs. the old (left) (next to a dime):

lpldo2.jpg


The low power version is now on a single PCB. It is also smaller in every dimension, at 0.55" x 0.35" x 0.27"




Here's the new high power version (right) vs. the old high power (left) (next to a quarter):

hpldo2.jpg


I thought of a neat idea for the new version - instead of using a plate for heat sinking, I got some aluminium U channell. The whole regulator sits inside the channel, so it's well protected - plus, it acts like a larger heat sink than the aluminum plate - and can still be attached to a larger heat sink for better power dissipation. The only downside is that it's a bit taller/wider, but a bit shorter, at 0.65" x 0.52" x 0.5".



For those interested, what do you think of the new packaging? Is the larger size okay on the high power version? I'm not envisioning the high power version being used for small applications, so size isn't as big of an issue for it.
 

evan9162

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Thanks,

Look like a good solution for the high power version?

I might start building a few soon.
 

evan9162

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Update:

I was testing two of these regulators, and noticed that one had totally flat output across a wide input voltage range, while with the other, the current decreased slightly as input voltage was increased.

I noticed that the flatter output version was using a National Semiconductor '334, while the decreasing one used an ST Micro '334.

I re-tested the scenario on my test board. Driving a Lux3 at 920mA, the National '334 kept the output steady to an input voltage of 6.7V (the upper limit of my voltage regulator).

The ST '334 started at 920mA right at regulation, but dropped down to 850mA at 6.7V in. The current dropped steadily with input voltage. I suspect the circuit is oscillating slightly with the ST part in place.

So it looks like the National Semiconductor part is the better one for this circuit, as it keeps output flat with rising input voltage.
 

OddOne

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Might wanna try clamping it a bit with a small cap across its power/ground terminals to suppress any ringing.

oO
 

evan9162

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I tried that - it made no difference. The National part is perfectly stable, the ST part, not. Must be a difference in the two parts. I'll definitely be using the NS parts.
 

gpk

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Hello evan and thanks for sharing your work with us. I was looking into your older 334 circuit after you gave me heads up in my older thread.
Q: Is the current output set with the same formula ? R=0.065/I
Keep up the good work!
 

Crux

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The LM334 is by design a temperature sensor, its output current increases with an increase in temperature. The current regulator circuit dissipates more power (heats up) with more voltage applied than with lower voltage.
It should follow (ideally) that both circuits current should increase with higher applied voltage!?!? (Only slightly, +.33%/C, but increase never the less)

But I'm easily confused...

Crux
 

evan9162

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The regulator will only increase in current if the '334 chip its self warms up. With good heatsinking, the pass transistor could stay within 10-20C of ambient, and the '334 may be even cooler. So how much current changes with input voltage depends on how well the regulator is heatsinked.

The current change with temperature change is a known side effect of using this chip as a regulator controller. However, with the regulator at 60C (+35C change), current only increases by 10%. If you heatsink the regulator to the same heatsink as the LED, and things stay cool enough, the current won't change too much.

There's another reason to attach the regulator to the same heat sink as the LED. With a 35C change, a white Luxeon's output will decrease by about 12%, so this regulator will roughly compensate for that and keep brightness relatively flat.
 

Crux

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Evan
I agree, the small heat induced increase in current is low and actually welcome in some cases.
Thats the crux of my confusion... Assuming that either the NSC or ST chip is getting equally warmer because of heat from the MOSFET, why would the ST current go down?

Crux
 

evan9162

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Crux,

The best guess is that the ST part is oscillating. It wasn't visible to the eye, but it's usually a result of insufficient feedback/filtering, and I see it manifest its self as a reduction in current as voltage increases - it usually hits when Vin is just above the point where the circuit is in regulation.

Last night, with the ST part still on my test board, I cranked the input voltage up to where the output dropped by about 70mA. Then I put an additional 4.7uF in parallel with the existing 2.2uF between the gate and source. The current jumped back up to 920mA where it should be (and where it was with the NS part) - so I'm pretty sure that the ST part is oscillating - it looks like the ST part needs a larger cap to keep things stable.
 

Crux

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Evan,

Good work! I have not yet made a high current version of this circuit, its been months since I've had enough free time, but I plan to get back to it soon. I (all of us) appreciate the work and results that we read here, keep it up!

Crux
 

goodyear99

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Hi there everyone,

I just started getting into led lights and flashlights, this forum has been very helpful. Thanks!

I recently bought a few lamina bl-3000's and bl-2000's to experiment with. Right now, im using a 24V 3A adjustable power supply to power the bl-3000 with current limit of 2.3A (max according to datasheet), resulting in about 10.8 - 11.2V, depending on how hot the array is.

I'd like to do two things:

- Power the array(s) from a 12V computer power supply
- Power the array(s) from a few NiMH C cells for a portable and very bright camping light.

Both problems require limiting the current to 2.3A max. If i use 12V power, i guess i could use a simple current regulator like the LM 150, but since it requires an extra 1.25V input to output, it wouldnt be able to power the array at 11.2V when needed. Also, this solution doesnt help me when using a lower voltage from C cells.

Ive read this thread plus one other that debates pwm vs current dimming, but after finding this switching regulator chip - MIC2171, im wondering why everyone isnt just using it instead? Specs: 3V - 40V input, 2.5A peak current, 65V max output. I assume I could design a circuit that would operate on a huge voltage range, so solves 12V and battery operated problem.

Am I missing something? Is using this chip more complicated that I think? Sadly there arent many circuit examples on the datasheet, and im not that good with electronics.

Please help!

Thanks
 
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