LM3478: Low frequency ripple on the DCDC output

Niklas Hagman

Part Number: LM3478
Other Parts Discussed in Thread: LM5155

Hi,

I have a prototype circuit board with a boost converter utilizing the LM3478 switching regulator.

The output voltage on the DCDC has a low frequency ripple on it when no load is connected on the DCDC's output (2V, 5Hz). When the load connected to the DCDC's output is increased the frequency for the ripple increases until the ripple disappears. I have measured the voltage across the MOSFET (VDS) and it seems like the regulator is only driving the MOSFET for a short period of time (Burst-mode?, over voltage protection triggered?).

What causes the output voltage ripple at low loads? Is this a feature like pulse-skipping our is some internal protection triggered at low loads?

*I have attached the schematic below.

Schematic:

Regards,

Niklas

over 6 years ago

0 Garrett Roecker over 6 years ago

TI__Mastermind 21720 points

Hi Niklas,

Thanks for reaching out with your question.

Can you please try reducing C41? Please try 100pF.
Also what is the input voltage range for this design? I'm surprised to see an inductance value of 330uH.

Thanks,

Garrett

0 Niklas Hagman over 6 years ago in reply to Garrett Roecker

Prodigy 110 points

Hi Garrett.

Thank you for your reply.

I will check if changing C41 from 10nF to 100pF removes the ripple on the converters output.

The input voltage is 8-40V with a 45V output voltage. The boost-converter is from a design we are currently updating/re-designing.

What is your concern with the inductance value? I think the design i generated through WEBENCH.

Regards,

Niklas

0 Niklas Hagman over 6 years ago in reply to Niklas Hagman

Prodigy 110 points

Changing C41 from 10nF to 100pF did not remove the low frequency ripple from the converters output.

0 Garrett Roecker over 6 years ago in reply to Niklas Hagman

TI__Mastermind 21720 points

Hi Niklas,

At light load / no load condition the LM378 will enter pulse skipping mode. This can be overcome by adding a dummy load to the output.

If this is not desired please look at the LM5155 to replace the LM3478.

Thanks,

Garrett

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LM3478: Low frequency ripple on the DCDC output