LM2678: LM2678 Problem

LUIS FELIPE CARDOSO

Part Number: LM2678

Hi there!

I'm designing a buck converter that converts the voltage around 20v-40v to 5.5v at 3.5A.
I followed the WEBench guides, but I don't know exactly what's wrong with my design, below are pictures of the circuit.
I believe that the main problem is in the pin on/off, but I believe that it is not just this error.

Webench

My desing

over 4 years ago

0 Frank De Stasi over 4 years ago

TI__Guru 72145 points

Hello

What is the issue you are seeing? Is it not switching? Or not regulating?

The ON/OFF pin can not be connected directly to the VIN. This may cause damage

to the device. The ON/OFF pin should be left floating or pulled-up to VIN through a 100K

resistor. Please provide waveforms of the voltage on the SWITCH_OUTPUT node, and any

other relevant voltages in the circuit. Also, please provide a PDF of your PCB layout.

Thanks

0 LUIS FELIPE CARDOSO over 4 years ago in reply to Frank De Stasi

Prodigy 10 points

First, thank you for replying, second, i may have damage it, because at first i did not know about and i turn the circuit on with the on/off pin connected to Vin and than it did not respond to anything i have done since, attached bellow are the pdf of the pcb desing.

4578.WBDesign5.pdf PCB_PCB_monitor revisado_2021-06-29.pdf

0 Frank De Stasi over 4 years ago in reply to LUIS FELIPE CARDOSO

TI__Guru 72145 points

Hello

In general your PCB layout could use some improvement as well.

It is best to use a ground plane on both sides to connect all of the grounds

for the device and the diode and input/output capacitors.

Try to follow the PCB layout example in the data sheet.

Thanks

0 Frank De Stasi over 4 years ago in reply to Frank De Stasi

TI__Guru 72145 points

Hello

I will close this post due to inactivity.

You can re-open if you need more assistance.

Thanks

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LM2678: LM2678 Problem