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LM3409: Using IADJ to program maximum current

Part Number: LM3409
Other Parts Discussed in Thread: TPS92641

I have a circuit with LV3409 working well. It is essentially the design from EN PIN PWM Dimming Application for 10 LEDs along with IADJ being controlled by a 12-bit DAC.

I have connected a 24V power supply and can control the current flowing through the LEDs. The question I have is around being able to programmatically compute the right IADJ value (so that a user can specify it as e.g. "300mA" and we can set it appropriately).

I've seen the formula I LED = VADJ / 5 x RSNS - Delta IL(pp)/2 in the spec sheet. However, working through the formulas indicates there's dependency on Vin/Vout as well (or perhaps just Vin), so I'm not sure what I'm trying to do is possible without the application being able to measure at least the Vin.

So basically:

- How to compute I LED for a variety of different input voltages and output LED configurations?

- Would I need to additionally measure input and/or output voltage to compute the curve?

Many thanks! 

  • Hi, Elan, 

    LM3409 is switching power supply detects peak current in inductor. As you can see from datasheet, I LED is related to Delta IL(pp)/2 which is affected by Vin, output voltage, inductor value @ different temperature,  You may refer to Equ.10 in datasheet for calculation. Other facts like VCST-offset, propagation time etc. also can lead to some tiny deflection. 

  • Thank you for your reply!

    Let me present a more concrete scenario. Let's say I want a design to work with LEDs which want 350 mA, and I have a 36v power supply. I can work through the computations, and come up with a design for the LM3409.

    Now I want to add another LED in series (also at 350 mA of course). My understanding was that within reason, the same design would work for that scenario and continue to provide 350 mA (V output would of course ~double).

    Is this roughly correct, or do I need to have a specific design per *exact* configuration?

    My goal is a driver which can be configured for different input voltages and output current requirements with a fixed design.

  • Certainly you can use one design to cover wide application, there will be some trade off when selecting components and performance. 

    If you designed with Vo max, then you can drive the similar current with lower Vout. 

    For detailed design, you may refer to chapter 9 Application and Implementation in datasheet. 

    EVM guideline also includes design procedure as well as four specific design with slight change.

  • Is it possible that given my desire for "programmable" flexibility, a chip like the TPS92641 might work better for me? Its IADJ pin seems to work in a more easily computable way given it doesn't depend on Vin or Vout. Or any other TI chips you can recommend?

    Many thanks!

  • Yes,TPS92641 can be considered for better LED current regulation.