Linear LDO pill!

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evan9162

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While a regulator pill isn't very exciting, this is a bit different than the normal pill. This has an LM334 based LDO linear regulator inside (can you tell I'm obsessed with this circuit?). This is meant for AA lights (specifically, the Brinkmann 3AA with a 1W luxeon), so it's 14.5mm in diameter. It's set to 540mA of current, so will be quite a bit more gentle on the 1W luxeon with fresh batteries - and the batteries will probably last a bit longer too.


A look at the sandwich fixins:

lpill1.jpg


My first time soldering SMD caps/resistors. Not too bad, just takes a steady hand and some planning ahead. At first, the sense resistor wasn't soldered on one side, which left me scratching my head as to why nothing was lighting up - I couldn't see the missed solder, and had to do some resistance measurements to find out the culprit.


After putting it all together, and potting in epoxy:
lpill2.jpg
 
Hello there evan,

Hey that's pretty neat evan.
Nice and small too :-)

Take care,
Al
 
Wow

an LM334 based LDO that can actually drive a lux without too much dropping the voltage? nice /ubbthreads/images/graemlins/thumbsup.gif
maybe using this to drive lux from 1AA li-ion from a MMM is a good idea !? /ubbthreads/images/graemlins/grin.gif
 
[ QUOTE ]
shiftd said:
Wow

an LM334 based LDO that can actually drive a lux without too much dropping the voltage? nice /ubbthreads/images/graemlins/thumbsup.gif
maybe using this to drive lux from 1AA li-ion from a MMM is a good idea !? /ubbthreads/images/graemlins/grin.gif

[/ QUOTE ]

Yep, about 0.1V drop out right at regulation. This would be a good solution for 1W driven from a single Li-Ion or 3 Alk/NiCD/NiMH cells.
 
[ QUOTE ]
MrAl said:
Hello there evan,

Hey that's pretty neat evan.
Nice and small too :-)

Take care,
Al

[/ QUOTE ]

Thanks,

I made the PCB circles by hand. I found that the hole of a CD is the PERFECT size to trace a PCB circle that will fit in a AA-sized light.
 
Could you send me a parts list and a pcb layout or are you going to make somefor sale?
this looks like something I want to make
Chris /ubbthreads/images/graemlins/grin.gif
 
Don't really plan on selling these - however, I can post that information for you when I get home tonight.
 
Parts list (and DigiKey part number):

Zetex FZT968 (FZT968CT-ND)
150 ohm axial resistor, 1/8W or something tiny (P150CACT-ND)
2.2uF 0805 smd ceramic capacitor (490-1704-1-ND)
National LM334M, SO-8 package (LM334M-ND) - Note - pin layout is important on this part
0805 SMD sense resistor (0.12 ohm is RL12S.12FCT-ND)
R=0.065/I where I is the target current. I used an 0.12 ohm resistor for about 540mA.


The pin layout of the LM334 is important. You want the '334 with the following pin layout:

lpill3.jpg


You'll want to break off (yes, break off), the the two V- pins between the R and V+ pins (pins 2 and 3), and break off the NC pins (pins 5 and 8).

Also, on the FZT968, which is in SOT223 format, you will break off the center pin(not the big tab), which is the collector pin, I'm using the tab as the collector connection.

Here's the schematic:

lpill4.jpg



Here's the PCB boards. Use the center hole of a CD to trace the circle, or just trace a 14mm circle. There are two boards. The "inside" layers of each board are the ones which you will cut the copper into 3 planes. The "outside" copper layers. PCB #1 is the bottom PCB. Its backside will be the + contact (to the batteries). PCB #2 is the top PCB. Its backside will be the - contact and will contact the top of the battery tube. There are two "vias", I just drilled a hole and used a short piece of wire to connect the two planes together. The LED + contact goes through a large hole in the top PCB. The LED - contact also goes through a large hole in the top PCB.

lpill5.jpg



Here's the circuit layout, complete:

lpill6.jpg
 
Darin, I'm pretty sure that those NC pins have no internal connection so there is no need to break them off. It is easy enough to check. Use diode function on DMM with (+) on the V- pin and probe each of the NC pins with the (-) lead.
BTW, the STM version of this part in the 234 temp range will install on your layout without removing *any* pins. It is cheaper too; less than a buck from Mouser.
 
Thanks. That looks like an excellent match.

Digikey (unfortunately) doesn't carry the ST versions. So no real good one-stop shop for these parts.
 
evan9162,

This is a very neat circuit and I thank you for posting.

If a PWM signal were to be used to control the brightness, which point of the circuit can it be applied to control either way:-

1) pulse to enable or disable

or

2) convert to variable voltage using RC filter

-- dj
 
Could a boost circuit be used to power this regulator? I'm thinking 1.5 volts boosted to 4.5 volts so it could be a constant current supply running off one AA battery.
 
[ QUOTE ]
djpark said:
evan9162,

This is a very neat circuit and I thank you for posting.

If a PWM signal were to be used to control the brightness, which point of the circuit can it be applied to control either way:-

1) pulse to enable or disable

or

2) convert to variable voltage using RC filter

-- dj

[/ QUOTE ]


Hmm...I'm not sure about that. I don't know of a good way to PWM this circuit into variable brightness. You might try switching on/off the base current of the transistor rapidly - which you could do with a transistor.
 
[ QUOTE ]
rdshores said:
Could a boost circuit be used to power this regulator? I'm thinking 1.5 volts boosted to 4.5 volts so it could be a constant current supply running off one AA battery.

[/ QUOTE ]

It could, though you might be better off (efficiency and parts-count wise) to use a boost circuit that can be configured for constant current. You would be best to match the voltage output of a boost circuit to be just above the Vf of the LED, then the LDO would regulate the current through the LED.
 
I can't seem to find an IC that will run in constant current mode and run off a single cell down to about 1 volt. Does anyone know of a good one? Thanks.
 
depends on the power level.. less than 400mA it's not too difficult.. TI and microchip have very nice boost circuits... look also for 'white led driver'
 
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