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LM3409HV: Output current/voltage not matching expected values.

Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409,

Hi all,

For some reason, I am getting a different output current and voltage for my LED than I expect based on my calculations. Here are my LM3409 values and component choices:

VIN: 24V

VO: 17V

η: 95%

Fsw: 201.6KHz

Roff: 34kΩ 1% 125mW - FOJAN FRC0805F3402TS -

Coff: 470pF 50V X7R - FOJAN FCC0805B471K500DT -

I-LED: 4635mA

Rsense: 0.05Ω 1% 3W - NCT LRBN25GFER050 -

ΔiL-PP: 650mA

L1: 5.5A 33uH 74Ω - KOHERelec MDA1050-330M -

IL-MAX: 4960mA

ΔVIN-PP: 1.55V

Recommended Cin: 22uF

Cin-1: 22uF 35V X5R - TDK C2012X5R1V226MT000E -

Cin-2: 22uF 35V X5R - TDK C2012X5R1V226MT000E -

Cf: 1uF 50V X7R - FH 0805B105K500NT -

Q1: 60V 14A 110mΩ - Diodes Incorporated DMP6180SK3Q-13 -

D1: 150V 10A Schottky - Diodes Incorporated MBRD20150CT-13 -

V-TURNON: 20.6666V

V-HYS: 1.034V

Ruv1: 3kΩ 1% 125mW - UNI-ROYAL 0805W8F3001T5E -

Ruv2: 47kΩ 1% 125mW - UNI-ROYAL 00805W8F4702T5E -

Cout: None

Despite calculating for 17V output at 4635mA average current, I'm only getting 15.5V out at 2.85A. I redid the calculations a few times and tried a few of the calculator tools out there, and I'm pretty confident all my values are correct, so I'm not sure what's going on. The EN pin is pulled high to 5v and there is no PWM. I did try moving Cf to between VCC and CSN, even though my PFET has a Qc of < 30nC, but it made no difference.

The other thing I tried was a different sense resistor of 0.033Ω instead of 0.05Ω. This brought the current up to 3.6A, with the same 15.5V

I'd really appreciate some help figuring out what I'm doing wrong. I'm thinking maybe my choice of PFET or diode is incorrect for some reason I'm unaware of. Here's the circuit schematic and footprint, it's a 2 layer board and the entire backside is a ground plane:



  • Hi Mathew,

    How many LEDs do you connect between L+ and L-? How many are in parallel? How many are in series? Could you provide the LED part number?

    Best Regards,


  • Hi Steven, thanks for your help. It's a UV-A LED module with 25 individual LEDs, in a 5S5P configuration, typically used for SLA 3D printer applications. Here's a photo (the small passive in the bottom right is just a thermistor):


    Unfortunately, since the LED came as a kit with heatsink, power supply and Fresnel lens, I do not have an exact model number for it. I've searched quite a bit to find one, but it seems to be custom. The datasheet for the entire kit specifies a 17V 5A supply to the LED.

    The power supply that came with the LED is an LM25116MH-based design. It's much too large in size to fit my requirements, which is why I'm trying to build my own LED driver circuit using the LM3409. I can confirm the power supply they included does put out 17V at 5A when fed from the same 24V/10A bench source I am using to test my design, so I'm confident that's what I need. I'm just not sure how to have the LM3409 match that.

  • Hi Mathew,

    Got it. I will carefully check your parameters and give you a feedback within 24 hours.

    Best Regards,


  • Thank you very much Steven.

  • Hi Mathew,

    Sorry but I am having an urgent biz travel these two days. 

    I do apologize for my delay. I will reply to you once I am back in office. Thanks for your understanding.

    BR, Steven

  • No worries, thanks Steven.

  • Hi Stephen, just checking in since it's been 5 days, would really appreciate your help if possible.

  • Checking in again. Still would appreciate knowing what I'm doing wrong.

  • Hi Mathew,

    Sorry. I apologize for the delay. I am now looking at this issue and are working on it. I will give you the reply by tomorrow.



  • Hi Mathew,

    • I have checked your calculation and I do not find any problems.
    • I see you put too input capacitor Cin-1 and Cin-2 in your calculation details but I do not see them in your schematics and PCB layouts.
    • The PCB layout can be improved a lot. But I am not sure at this time if the layout is the root cause of the "inaccurate current" problem.
    • The below point is suspicious. Please try below items to help me identify:
      • Please try to measure the voltage of IADJ pin.
      • Please try to connect the IADJ pin to an external constant voltage source = 1.24V and see what the output is.
    The other thing I tried was a different sense resistor of 0.033Ω instead of 0.05Ω. This brought the current up to 3.6A, with the same 15.5V

    Best Regards,


  • Hi Stephen, thank you for looking into my issue and verifying my calculations.

    You are correct about the input capacitor, there is only one in my layout and schematic. I did not realize two input capacitors were recommended until after the PCBs were designed and ordered. During my testing, I tried both a single 22uF input capacitor, as well as a stack of two in parallel on the same footprint that I measured at 44.6uF. The extra input capacitance had no measurable effect on the output voltage and current to the LED. Regardless, my next board revision will have an extra footprint for a second input capacitor.

    I measured the voltage of the IADJ pin during operation to be 1.241V. Despite being the correct voltage, I followed your suggestion and connected it to an external constant voltage source of 1.24V. This also had no measurable effect on the output voltage and current to the LED. I still measure 15.5V at 2.85A.

  • Hi Mathew,

    • The second 22uF input capacitor may not be needed as per the theoretical calculation. But you can have that extra capacitor footprint for further decoupling intents if needed.
    • I will test your parameters on our EVM today.
    • Please measure the I_LED current waveform with a current probe and provide me with the oscilloscope capture. (If a current probe is not available, then you can probe the voltage of CSP pin and CSN pin and then do a mathematical calculation on the oscilloscope.)

    Best Regards,


  • Hi Steven,

    Thanks to your recommendation I was able to find the source of the problem. I've discovered my trace to CSP is too long and too thin, creating additional resistance across the sense resistor. To test, I soldered an 18AWG wire in place of the suspected trace to lower the resistance, and I'm now getting the expected output from the LM3409HV.

    So, you were correct to suspect my layout was the problem. I will rectify this in my design. Thanks so much for your help!

  • Hi Mathew,

    Glad to hear that you have found the root cause. I strongly recommend you refer to Section 11 Layout from the datasheet for your layout.

    If you have any further questions, please feel free to contact me again. I am glad to help.

    I am going to close this thread.

    Best Regards,