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LM3671: LM3671MF-3.3NOPB too low output and failure under load

Tomasz Bialas
Prodigy 10 points
Part Number: LM3671

Hi,

I am a second year student and I was using the LM3671MF-3.3NOPB device to output 3.3V for a system with typical load 70mA with bursts up to 400mA. However, the device we're using is outputting a significantly lower voltage under load, ie 2.5V instead of 3.3V with a 100mA resistive load applied to it. There is audible coil whine. The device seems to fail after a short while, developing a short between the output and the ground and pulling a lot of current. Without any load, the output is a perfect 3.33V.

Since I am a student and beginner PCB designer, I was hoping I could get some pointers on why my design is not behaving properly. Here's the PCB layout as well as the component IDs of the passives used:

The passives used were GRM188R60J475KE19D 4.7uF input capacitor, GRM188R60J106ME47D 10uF output capacitor, DFE201610E-2R2M=P2 2.2uH power inductor.

I have tried putting an additional ground connection closer from the chip to the capacitors with a wire (where the missing GND trace is on the PCB) as well as adding more capacitance on the output, however this has not helped.

Any feedback and advice would be greatly appreciated.

Thank you very much in advance,

Sincerely,

Tomasz Bialas

  • 0
    TI__Guru**** 150710 points
    Hi Tomasz,

    Thanks for posting your layout and BOM. The schematic did not come through.

    Yes, you definitely need to connect the GND of the caps to the GND pin of the IC directly. As well, that output cap does not have the minimum specified effective output capacitance listed in the D/S at a 3.3V bias.

    If you have already tried both of these, and for any general debugging case, you need to look at the various waveforms with a scope. I would start with your Vin and Vout--these should be DC.

    Make sure you measure the output voltage across C4 and with the additional GND connection implemented. This voltage should be constant, as there are no other IR drops across the board which would lower it.