LM5117: Control issues with VCCDIS function

Saurabh Tewari

Part Number: LM5117
Other Parts Discussed in Thread: TINA-TI

Hello everyone,

I am designing a 48V to 14V converter using LM5117 (100 Watts output). I used the reference design snvm619a.tsc as a start with good results for the most part, but I would like to power the controller LM5117 from the output voltage. Since the output voltage is 14V, I intend on adding two diodes to drop some of the output voltage before it reaches the Vcc pin.

Based on the datasheet I should be able to just leave the VCCDIS pin open although I have tried both -- using a resistor divider from output to VCCDIS, and also leaving VCCDIS open. My output voltage does not stabilize when I attempt to power LM5117 from the output. Whereas if I just use the internal Vcc voltage of ~7.5 Volts, the simulation works just fine.

I would like to have a higher Vcc to reduce RDS_ON and also to reduce the controller power dissipation. I am attaching the TINA-TI file. Any insights will be helpful!Buck_CL_upload.TSC

Thanks,

Saurabh

over 7 years ago

0 Samuel Jaffe over 7 years ago

TI__Mastermind 25590 points

Saurabh,

I've been running some sims from your provided .TSC file and I'm seeing the same thing you're seeing. I've tried just attaching dedicated voltage sources and I'm still not seeing proper operation.

Let me get in touch with the sim team and ask around. We'll get back to you.

-Sam

0 Arash Loloee over 7 years ago in reply to Samuel Jaffe

TI__Mastermind 30451 points

Hi, we will take a look at model and see what might be the cause and the possible solution.

Kind Regards,

Arash

0 Saurabh Tewari over 7 years ago in reply to Arash Loloee

Intellectual 670 points

Thanks guys!

Assuming that it is a simulation artifact, do you see anything wrong with doing it the way I am doing it (i.e. using diodes to drop a couple of Volts and setting VCCDIS to disable internal LDO at ~7.5 Volts output)?

0 Samuel Jaffe over 7 years ago in reply to Saurabh Tewari

TI__Mastermind 25590 points

Saurabh,

Your circuit looks good to me!

-Sam

0 Saurabh Tewari over 7 years ago in reply to Samuel Jaffe

Intellectual 670 points

Thanks Sam!

0 Saurabh Tewari over 7 years ago

Intellectual 670 points

Hello everyone,

Update: I built the prototype and tested it at low current. I can see the Vcc voltage transition from internal ~7.5V to ~12V as the output builds up. The operation is flawless so far. So I can confirm that this is a simulation issue.

Regards,

0 Samuel Jaffe over 7 years ago in reply to Saurabh Tewari

TI__Mastermind 25590 points

Thanks for the followup! I'm glad the circuit is working.

-Sam

0 Saurabh Tewari over 7 years ago

Intellectual 670 points

I marked the issue as still unresolved because I don't know what's going on with the simulation file. That being said, the Vcc voltage seamlessly transitions from internal to external level derived from the output. Ch1 (Yellow) is the output, Ch3 (Purple) is the Vcc.

Thanks!

0 Samuel Jaffe over 7 years ago in reply to Saurabh Tewari

TI__Mastermind 25590 points

Saurabh,

That looks great! Thanks for the updates!

Arash,

Please keep us posted on progress.

-Sam

0 Imran Dharwad over 7 years ago in reply to Saurabh Tewari

TI__Intellectual 1440 points

Hi Saurabh,

I was able to solve the issue by just cut-pasting the same model in the same spot. Please try it your from your end and let me know.

This looks like a simulator bug/issue, we will check with the team and let them know.

Thanks,

Imran

0 Saurabh Tewari over 7 years ago in reply to Imran Dharwad

Intellectual 670 points

Interesting fix but it works beautifully, thanks!

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LM5117: Control issues with VCCDIS function