LM5576: Power issue when addressing Multiplexer select pins

Ayhan Evren

Part Number: LM5576
Other Parts Discussed in Thread: TM4C123GH6PGE

Hello Everyone

I am working on a PDU Card for a project.

The circuit basically works like this: 40-60V from external power supply feeds to DC DC converters and LM5576. The LM5576 drops this to 5V and feeds the circuit components. Meanwhile, TM4C123GH6PGE selects channels for multiplexers and at reads the values on the Multiplexer pins at that moment.

This is the schematic of how we use LM5576 on the circuit:

This is the schematic of some of the Multiplexers and current senses:

On this card whole components feeds with 2 x LM5576.

If I selected a specific channel on multiplexer with my TM4C123GH6PGE there is no issue. But when I start to switch address pins on the Multiplexer the circuit behaves weirdly. Power goes off and on very fast, and the current which the whole circuit draw is goint to zero. If I disable the LM5576 and feed the circuit with external power supply there is no problem on working. This could be the key part of the problem.

What could be the reason of this issue? And how to avoid it?

Thanks, regards.

over 4 years ago

0 Andy Chen over 4 years ago

TI__Mastermind 33935 points

Hi Ayhan,

Can you upload the image of circuit and layout? not the link

B R

Andy

0 Ayhan Evren over 4 years ago in reply to Andy Chen

Prodigy 50 points

Hello Andy,

Of course

This is the schematic of how we use LM5576 on the circuit:

This is the schematic of some of the Multiplexers and current senses:

Thanks.

0 Andy Chen over 4 years ago in reply to Ayhan Evren

TI__Mastermind 33935 points

Hi Ayhan,

Do you use the LM5576 Quick Start (calculation tool on TI.com) to design the application circuit?

Pls also upload the PCB layout of LM5576 section.

B R

Andy

0 Ayhan Evren over 4 years ago in reply to Andy Chen

Prodigy 50 points

Hello Andy,

No I did not use the quick start, could you please share the link, I can't find caldulation tool in the TI.com

I'll upload the PCB layout when I go to the office in the morning.

Thanks for your interest.

0 Andy Chen over 4 years ago in reply to Ayhan Evren

TI__Mastermind 33935 points

Hi Ayhan

https://www.ti.com/tool/LM2557X-557XQUICK-CALC

B R

Andy

0 Ayhan Evren over 4 years ago in reply to Andy Chen

Prodigy 50 points

Hello Andy,

This is the altium files of main board part of the project. The LM5576 section is on this board.

Wetransfer download link:

https://wetransfer.com/downloads/f548b9b88cf186e3545a919ebd1bb0c320210819081623/4c1bac4f1a00bead927b9684cd53ebaf20210819081637/8cd91f

Thanks

0 Andy Chen over 4 years ago in reply to Ayhan Evren

TI__Mastermind 33935 points

Hi Ayhan,

Seems I can't download it, can you upload screen shot of the PCB layout of LM5576 section?

B R

Andy

0 Ayhan Evren over 4 years ago in reply to Andy Chen

Prodigy 50 points

Hello Andy,

I downloaded the calculation tool and tried some different solutions but the result was same at every time.
I'm observing the output of the LM5576 with the scope. A clicking sound is heard from the circuit when a voltage drop occurs. At this time I took a photos of the signals.

The voltage drop occurs in two different situation;

The first voltage drop occurs a few milliseconds after first feeding the circuit. As far as I have observed, this does not cause a problem in the operation of the circuit. But still, the levels where voltage rises and falls are dangerous.

The second voltage drop occurs in a completely random time when I changing the multiplexer address pins with the MCU. This reduces the current drawn from the power source to zero levels, causing many elements in the circuit to turn off and on.

By the way these are the 2 layers PCB layout LM5576 section of the mainboard:

Thanks, Regards.

0 Andy Chen over 4 years ago in reply to Ayhan Evren

TI__Mastermind 33935 points

Hi Ayhan,

The PCB layout was bad, pls try below modification:

Also remove R42 C83 first.

B R

Andy

0 Ayhan Evren over 4 years ago in reply to Andy Chen

Prodigy 50 points

Hello Andy,

I applied the modifications you mentioned.

I measure the output of the LM5576, after it worked for about 15 minutes without any problems, the signal was rippled again and this caused the processor to turn off and on. Ripple seems to have decreased compared to before, but it is still at a size that will cause the system to restart.

0 Andy Chen over 4 years ago in reply to Ayhan Evren

TI__Mastermind 33935 points

Hi Ayhan

Can you check the IC case top temperature after about 15 minutes before the output becomes unstable?

B R

Andy

0 Ayhan Evren over 4 years ago in reply to Andy Chen

Prodigy 50 points

Hello Andy,

Sorry for the late answer but there is a problem on my MCU board, and I am waiting to new ones to arrive, that is why I can not measure the temperature of the IC case for now. I'll inform you about temperature when I try it with the new boards.

Thanks

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LM5576: Power issue when addressing Multiplexer select pins