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searching for buck LED driver

Michael56045
Prodigy 60 points
Other Parts Discussed in Thread: TPS61086, TPS54040, TL4242, TPS61165, TPS54260, TPS54160

Hello,

I'm looking for a LED driver from TI for following specicfications:

7 strings of 5 LED's each (total 35). Current through each needs to be 350mA.
Input DC voltage is more or less equal up to 24 Volts. As a result I need
a LED driver with 18V output voltage (forward voltage 3,6V each LED)
and a total current load of 2,45A.  

Searching for driver I found the "TPS61086". Is that the right driver for my application ?

Regards,

Michael

  • 0
    TI__Guru**** 150870 points

    No, the TPS61086 is not the right fit for this application, as it does not support your input voltage range and is too low of power.

    Since your input voltage is greater than your LED's forward voltage, you need a buck solution, not a boost as most LED drivers are.  The simplest solution is using one TPS54040 for each of the 7 strings, according to this app note: http://focus.ti.com/lit/an/slva374/slva374.pdf 

    We also have an EVM available to evaluate this solution: http://focus.ti.com/lit/ug/slvu345/slvu345.pdf 

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    Thank you for your fast answer.

    In fact the input voltage of my application is not always about the accumulated forward voltages of my LED's (5x3,6V = 18V). To simplify the application I assume a input voltage of 10 or 12 V. So I  need a boost LED driver for my LED-Cluster, don't I ?

    The TPS61086 just has a input voltage range of max. 6V, so it's not the right fit. But is it really too low of power ?

    The IC drives 2,6 Ampere, and I just have 2,45 A (7x0,35A) in use.

    And another question regarding the circuits in the datasheets in general: The capacities are just indicated with a certain value, for example 4,7uF. How can I know, if it's an electroylt capacitor or an unpolar capacitor, or is it not important ?

    Thank you for your help forwards !

    regards 

    Michael  

     

  • 0 in reply to Michael56045
    TI__Mastermind 36110 points

    The TPS61086 datasheet has good information that will help you understand the available output current at different input voltages and currents.  Please refer to figure 8 and go through the datasheet design procedure.  Output current is much less than the peak switch current.  You need to define the input source and its operating voltage range before you can proceed with an IC selection.  For your application, I suggest trying to keep the input voltage higher than the forward voltage of your LED strings, even if you need to make more strings with less LEDs in each string.  This will allow a buck or linear design which will be easier for you.  Also consider the TL4242.  It is and LDO based LED driver which makes it very easy to design with.

     

    For component selection, carefully read the datasheet.  It states that these capacitors are ceramic and the list of capacitors gives part numbers which are ceramics.  You can also review the EVM design bill of materials to find part numbers.

  • 0 in reply to Matthew Day
    Prodigy 60 points

    O.K.,

    unfortunately the layout is predetermined (7 strings of 5 LED's each).
    There don't exist any driver IC, which can deliver about 2,45A at 18 V permanent ? So I do have to assemble one driver per string, to have enough current for every single LED ? In this case, the TPS61165 would be the right choice, isn't it ?  

    The TL242 has a very broad supply voltage, but it also just can drive 500mA, so it's also to less for my application. And are'nt there any problems with lost heat with a linear design ? I guess 44,1 Watt  is really a lot of power I want to drive...

    thank you for your help

    Michael

  • 0 in reply to Michael56045
    TI__Mastermind 36110 points

    Again, you need to specifiy your input voltage first. You can make an LED driver that powers 7 strings of 5 LEDs at 350mA each with a single power supply.  However, you are getting into the area of detailed power supply design that is usually reserved for power supply experts.  For this application, you will need an input voltage that is higher than your LED voltages.  Vin should be greater than the number of LEDs in series times the maximum forward voltage drop of each LED plus the voltage across the current sense resistor.  You need to add a little more to account for voltage drops through the power supply.  You need to try to keep Vin above 5*3.6V+1V = 19V.  Add another volt for tolerances and voltage drops.  24V is standard input voltage.  Your output power is 5*7*.35A*3.6V = 44.1W.  Total current is 2.45W.  This supply can be designed.  I suggest a circuit like the one shown in http://focus.ti.com/lit/an/slva374/slva374.pdf .  You need a higher power IC though.  The TPS54260 might be a good choice.  Can I ask you what your application and end product are?

  • 0 in reply to Matthew Day
    Prodigy 60 points

    Thank you for your detailed answer !

    We are a PCB manufacturer, and we want to exhibit a demo LED lightning system for a traid.

    best regards

  • 0 in reply to Michael56045
    Prodigy 60 points

    I have another question regarding the rating of the components. 
    I did calculate the values like it's specified in the equations in the Step-Down LED Driver Design Guide for the TPS54160.
    Can I assume the values also for the TPS54260 ?
    The SwitcherPro Design Tool did calculate quite different values for some components, as I calculate with the equations from the Design Guide. 

      

  • 0 in reply to Michael56045
    TI__Guru**** 150870 points

    I don't believe the LED driver circuit is available in switcher pro.  So, you cannot use switcher pro to do this design for you.

    The design methodology is the same for the TPS54160 and TPS54260.  However, some of the constants will be different--these could include the equation for setting switching frequency, current limit value, error amplifier transconductance, etc.  Consult the datasheet for each of these parts to determine the differences.