Low Voltage Constant Current Source

[thetrueman]

Hi All,

I need a constant current source which can be operatable as low as 3.5V. I used 2 transistors current source which works fine but not below 4.5V.

Actually I want to drive a white LED with small 6V battey and want it to glow uniformly from 3.5V to 6.5V.
Is there any solution? Please Help!

Shahzad

[8K51]

Hi
please could you be more precise to allow better answers ?
what's your Led voltage and current requirements ?
do you need good efficiency ?
regards

[thetrueman]

Hi,

LED voltage are 3.5V and current is 20mA.

Shahzad

[8K51]

If you want to drive a 3.5V LED from a 'small battery' even when battery delivers 3.5V there is a solution with a small ready made buck-boost regulator with serial mounting white led with inductor. (constant current)
I have no precise chip reference but Maxim, Linear, NS... and maybe Microchip have such buck-boost switching regulators. http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2590&param=en528261

Your current need is low so another solution could be 'charge pump constant current'

Regards

[bogdantk]

Hello
You should use a FET for lower dropout
Might be of interest
http://ww1.microchip.com/downloads/en/AppNotes/01138A.pdf
http://ww1.microchip.com/downloads/en/Market_Communication/mcp1252jan03.pdf
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2590&param=en528236

[Parmin]

Look at LM78XX series datasheet there will be a simple constant current circuit that use one capacitor, one LM78XX and one resistor.

[pama]

hi,

i used webench for special led applications selections, it's good tool for national semi. parts, and you can get easily samples:

hxxp://www.national.com/appinfo/webench/led/pled.html;

but for low powered applications i tried some parts from prema:

hxxp://www.prema.com/Application/whiteleddriver.html;

but it's depend on your requirements, it can be solved also with discret components.

best wish
pama

[belinao]

if dimensions are not your problem you can use an op amp. i can send you the circuit if you want

[thetrueman]

Yes please. I'll be thankful.

Once again I want to clear my requirement that when the battery is charged fully the LED should be glow at 20mA and when the battery discharges gradually till 3.5V (because the LED voltage is 3.5V) it should provide constantly 20mA to LED throughout.

In this way I want to get most from small battery for long time at same brightness of LED. Thank you.

Shahzad

[pama]

hi,

also something interesting, but this is for a 1,5 V. It's again a working circut, tested by me.

hxxp://www.emanator.demon.co.uk/bigclive/joule.htm

good luck

[8K51]

Hi

You want to get 'the most' from a small battery, it's an interesting challenge !!
I googled with 'low drop white led constant current'.
Among many good answers I choose this one :

http://radiolocation.tripod.com/LEDdimmer/LEDlampDimmer.html

-only 0.2V lost (voltage drop) vs 0.6V with 2 transistor solution,
-well explain so you can adapt it to your exact requirements, (single led, 20mA)
-easy removable dimmer components,
-no special, hard to find,  components ..... no inductor,
-does some temperature compensation usefull for white led safe use,
-article links towards others solution like that ones :
http://electronicdesign.com/Articles/Index.cfm?ArticleID=4703

Regards

[igq]

How many LED you want to drive?

[twonuts]

Look at LM78XX series datasheet there will be a simple constant current circuit that use one capacitor, one LM78XX and one resistor.

I'd go with this solution. Almost any fixed voltage 3 leg regulator can be used this way,
I use this method very often.

[piclover]

Look for LM317, You need only 1.25 volts voltage difference input-output. For low current u can use LM317L (TO92 package).

[thetrueman]

8K51 showed enhanced example of simple two transistor with one resistor circuit version. I'll give it a try.

78xx and LM317 are good but voltage drop between input output is minimum 1.25V and when we add 3.5V of LED voltage drop then voltage drop becomes 4.75 volts which is not acceptable at low voltage.

Any body have any idea with PWM. I tried 555 with some success but output is something non-linear. I directly drived LED through a transistor without any current limit resistor. But yet feedback is not controlled to Pin 5 of 555 control voltage. Can anybody have any idea such above circuit i.e. with op-amps etc. like SMPS...

[twonuts]

There should always be the same  volts drop across the diode, ~3.5V depending on type, no matter what system you use, assuming you are driving the LED at its normal current rating..
This means there will always be ~2.5 volt drop across your regulator, when supplied from a 6V battery.

Multiply that by the current required to drive the LED, ~25mA, and you should get the same power loss no matter what constant current system you use.

[vayupak_01]

For low voltage try to you this (Linear topology)

LDO
= Low voltage drop out

many company that have this product

I think LDO is very simple to design because that design same another linear power supply
ummmm

[surachai]

This is simple circuit constant current source for led.

[anemon06]

http://pdfserv.maxim-ic.com/en/an/AN3464.pdf