**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* - https://datasheet.lcsc.com/lcsc/2201121630_FOJAN-FRC0805F3402TS_C2933420.pdf**Coff:** 470pF 50V X7R - *FOJAN FCC0805B471K500DT* - https://datasheet.lcsc.com/lcsc/2208181800_FOJAN-FCC0805B471K500DT_C5137598.pdf**I-LED:** 4635mA**Rsense:** 0.05Ω 1% 3W - *NCT LRBN25GFER050* - https://datasheet.lcsc.com/lcsc/2208021730_NCT-LRBN25GFER050_C5121147.pdf**ΔiL-PP:** 650mA**L1:** 5.5A 33uH 74Ω - *KOHERelec MDA1050-330M* - https://datasheet.lcsc.com/lcsc/2109092030_KOHERelec-MDA1050-330M_C2847566.pdf**IL-MAX:** 4960mA**ΔVIN-PP:** 1.55V**Recommended Cin:** 22uF**Cin-1:** 22uF 35V X5R - *TDK C2012X5R1V226MT000E* - https://datasheet.lcsc.com/lcsc/1912111437_TDK-C2012X5R1V226MT000E_C338080.pdf**Cin-2:** 22uF 35V X5R - *TDK C2012X5R1V226MT000E* - https://datasheet.lcsc.com/lcsc/1912111437_TDK-C2012X5R1V226MT000E_C338080.pdf**Cf:** 1uF 50V X7R - *FH 0805B105K500NT* - https://datasheet.lcsc.com/lcsc/1811061720_FH--Guangdong-Fenghua-Advanced-Tech-0805B105K500NT_C215803.pdf**Q1:** 60V 14A 110mΩ - *Diodes Incorporated DMP6180SK3Q-13* - https://datasheet.lcsc.com/lcsc/1912111437_Diodes-Incorporated-DMP6180SK3Q-13_C461099.pdf**D1:** 150V 10A Schottky - *Diodes Incorporated MBRD20150CT-13* - https://datasheet.lcsc.com/lcsc/1912111437_Diodes-Incorporated-MBRD20150CT-13_C151357.pdf**V-TURNON:** 20.6666V**V-HYS:** 1.034V**Ruv1:** 3kΩ 1% 125mW - *UNI-ROYAL 0805W8F3001T5E* - https://datasheet.lcsc.com/lcsc/2206010200_UNI-ROYAL-Uniroyal-Elec-0805W8F3001T5E_C17661.pdf**Ruv2:** 47kΩ 1% 125mW - *UNI-ROYAL 00805W8F4702T5E* - https://datasheet.lcsc.com/lcsc/2206010200_UNI-ROYAL-Uniroyal-Elec-0805W8F4702T5E_C17713.pdf**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: