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Smallest Possible Linear LED Driver

Discussion in 'LEDs and Optoelectronics' started by modern_messiah, Jan 18, 2013.

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  1. modern_messiah

    modern_messiah

    29
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    Dec 1, 2010
    HI all,

    I have been racking my brain the last two days trying to design a simple linear/buck constant current regulator for driving LEDs. I've looked at everything I can think of - discrete ICs from all the major players (TI, Diodes, Linear Tech etc) to building my own using MOSFETs and so on. I feel my knowledge is borderline hopeless, so I've come here to see if anyone can help out.

    Now before I go on, yes I have read the sticky in this very forum on driving LEDs. I think I should list my requirements to show you why what's listed in the sticky won't work, or at least show me that I don't know how to make them work for me.

    - Single Lithium Ion rechargeable power source (4.2V fresh, 3.7V nominal, 2.7V flat)
    - Single HB LED (XPG, XPE, Nichia 219)
    - Maxium current of 1000mA (over 800mA is desired)
    - Maximum Vf or 3.3V will be needed
    - PWM control. I will be pairing the driver with an uC to control brightness.

    So as you can see all I really need is a linear LED driver. I stumbled across a circuit that meets all my requirements except for the current output (this circuit only manages 400mA). All the circuit contains is a 400mA low drop out linear regulator (PJFQ), an output capacitor and a PIC (Microchip 12F629). Photo of the circuit is below:

    [​IMG]

    How does this circuit function? The confusing part for me is that 99% of the LDOs I find only have an enable pin (which is not appropriate for PWM control) and secondly LDOs have a fixed output voltage.

    It looks like the LDO output pin is connected to the +ve terminal of the LED, but then the LED -ve terminal is connected to ground...LDOs output a constant voltage so how can this be working? I can't see how the uC is connected up as the signal traces are too tight to differentiate :(

    Any help would be highly appreciated as I don't think I can look at anymore datasheets!

    Thanks,

    - Matt
     
    Last edited: Jan 18, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    The ones using a switchmode regulator are not much larger and will result in significantly longer battery life (and far less heat)
     
  3. modern_messiah

    modern_messiah

    29
    0
    Dec 1, 2010
    OK so after consulting with a friend I think I know how it works. I also understand why you are suggesting a switch mode regulator...if you have one in mind or can point me in the right direction I'd be very grateful - I've seriously spent the last 2 days solid looking and just can't find one!!!!
     
  4. rob_croxford

    rob_croxford

    262
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    Aug 3, 2010
    http://uk.farnell.com/diodes-inc/zxld13625ta/ic-led-driver-buck-1a-tsot23/dp/1843876

    Here is one i am currently using. Its a switching high side current regulator. The current level is sensed on the high side and the output current is either set by the sense resistor or can be directly controlled using either PWM or an external DC voltage. Read the data sheet for full calculations.

    I would also suggest having a secondary control loop for voltage as this must be fed with a constant voltage.

    Rob
     
  5. modern_messiah

    modern_messiah

    29
    0
    Dec 1, 2010
    Thanks Rob. The input voltage won't work for me but looking at Farnell I seem to be finding a few that might be appropriate so I'll check them out. The only issue I see is that the switching regulators require a lot of external components which doesn't help me too much. I also have a good understanding of how the driver in the image in my original post works so if need be I can probably fall back on that.
     
  6. rob_croxford

    rob_croxford

    262
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    Aug 3, 2010
    yes that was the 60V version i believe they do an entire range. To be honest most semiconductor manufactures do some kind of range of current regulators. LT and Zetex being two that i have used.

    Using a chip similar to this and a Uc you should be able to develop a fairly tight control loop that doesnt requre too many external components. The best way i have found to do it is to set the current using a variable DC output from a Micro with a built in DAC. This removes the need for PWM.

    As you are running from a battery you should not need to much line filtering and there will be no need for a seperate switcher to convert voltage levels.

    Also from my calculations runnign a pic on 3.3V (DAC output 0-3.3V you will need an 8 bit resolution for decent dimming) and setting the current sense resistor to 0.2 ohms you should be able to vary the output from 350mA up to 1A.

    Unfortunatly the external inductor and didoe are essential to operation however only a very small inductor will be neeed. And you could use a planar inductor making the design even smaller.

    Rob
     
  7. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    Also if you only want a single output current you can set this using either a voltage regulator or just a divider strait from the battery. That way you can erradicate the uC from the design.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    One of the issues is that the battery voltage can vary both above and below the LED Vf. This necessitates a rather more complex design based on buck/boost architecture.

    There are some remarkably small boards available on eBay, but I'm not sure if their input voltage range goes low enough to suit your purpose.

    Life gets easier if you can have 2 batteries (smaller ones may be OK)

    Here is a possible solution.

    Here is someone selling something they claim is good for this, but it appears to be a simple linear current regulator (so the LED will get dim as the battery voltage falls below 3.3V (ish)

    Here is another option which *seems* to fit your needs.

    Beware that some of these solutions will discharge your battery way past the point of no return if you let them.
     
  9. modern_messiah

    modern_messiah

    29
    0
    Dec 1, 2010
    Thanks Steve(?),

    That circuit looks good except for the component count being so high. One thing I don't get is it states it's a boost converter but there are times when the forward voltage of the LED (3.5V) is actually lower than the battery voltage (typically 4.2V fresh off the charger). The driver makes no mention of buck, but there clearly needs to be a step-down stage incorporated otherwise it just won't work with fresh lithium ion cells...

    As for the drivers you have linked I am MORE than aware of the first set - these are all I;ve been using for the past 2 years :eek: The regulators on these boards are called AMC7135 linear constant current regulators. They can be paralleled to add their output. each chip is good for between 350 and 380mA. Paralleling 8 yields the 2.8A output these boards provide. The brightness can be controlled by connecting a PWM input to the Vdd pin of each regulator.

    I've since learnt the circuit I initially asked about works in effectively the same way. A friend figured it out (he knows a bit more than I do that's for sure):

    They basically function in exactly the same manner. The only difference is the regulator is a bit more powerful than the AMC7135 chips. I'm working on a simple circuit now that works the same way, but with a different regulator (good for 1A) and an ATMEL uC.

    Linear regulators suck, but the space this driver requires is minimal (looks like I can fit it on an 8mm PCB) so I'll stick with it for the time being :)

    - Matt
     
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