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LM3492HC-Q1: Clarification on usage of this part

Eric Steimle
Expert 2230 points
Part Number: LM3492HC-Q1
Other Parts Discussed in Thread: LM3410-Q1

I have six LEDs I want to power. They have a 1.1Vmin to 1.9V max Vfw with a 1.65V typical Vfw, and I'm going to drive them at 70mA. So I feel like this is a decent part and I can just put them in series, three per channel. My input voltage will be 5V. Have I picked a good part for this or do you have a better suggestion?

Thank you

  • 0
    TI__Mastermind 18715 points

    Hello Eric Steimle,

    This is a boost converter so you want the output to be greater than the input voltage.  If you separate the 6 LEDs into two  sets of 3 series LEDs then the output voltage would be 3.3V to 5.7V.  You may need to do a sepic, buck-boost or boost into batter configuration if you want to do two separate strings of LEDs.  

    I think we need to know more about the application.  What is the Vin range?  Do you need PWM dimming or just analog dimming?  What is the CC tolerance you need?  I also recommend getting performing the datasheet calculations and then getting an EVM to validate your design.  What is the application?  What are the volume?  What is the start of production?  Once I have more information I can do a better job of recommending the solution.  

    -Francis Houde

  • 0 in reply to fhoude
    Expert 2230 points

    Sure. We're tracking these LEDs on an object using cameras. I don't have to be able to dim them from the controller, but I would like to be able to turn them off with a GPIO. I'd like to be able to adjust the current on the board, a resistor would be fine. Input voltage from the battery pack ranges from 4.5V to 9V.  I don't know enough to spec CC tolerance (these are 850nm LEDs). 

    Volume is 2-3k per year, production start is the end of this year.

    Thanks!

  • 0 in reply to Eric Steimle
    TI__Mastermind 18715 points

    Hello Eric,

    I have a couple more questions. 

    • You stated that Vfw ranged from 1.1Vmin 1.65Vtyp. and 1.9Vmax.  It this for a specific current?  That seems like a wide range for a specific current setpoint.  
    • The battery range is pretty wide too.  Does the 4.5V to 9V range include fault conditions?  Would there be a range where you would operate inside that range?    
    • My main concern with this part is if you have a wide Vf then you will have to drive the boost higher than normal and the part will have to dissipate considerable heat to facilitate the large voltage range.  

    -Francis Houde

  • 0 in reply to fhoude
    Expert 2230 points

    Sorry, yes I just gave the range from the datasheet. According to the datasheet at 70mA (our target) it would be 1.6V, but it could go lower as temperature rises.

    So the battery range is that wide, no fault conditions included. If I have to I could put a little sepic or something in front of it.  I was trying to keep it simple though.

    Okay, yeah I don't want to dissipate a ton of heat :)  We're battery powered so I'm looking for an efficient way to do this, more efficient than just a resistor and LED at least.

    Thank you.

  • +1 in reply to Eric Steimle
    TI__Mastermind 18715 points

    If I were you I would look at the LM3410-Q1 https://www.ti.com/product/LM3410 .  This is a boost/sepic converter that generates a constant current and is more efficient then the boost followed with a linear driver.   It is only one channel but you could just duplicate the design for each string.    There is a sepic EVM that you could try out for your string and battery voltage.  https://www.ti.com/tool/LM3410XSDSEPEV 

    I think it would be worth prototyping it with the EVMs.  Let me know if you have other questions.  

    -fhoude