LM5117: Unable to output much current

Keshav Raj

Part Number: LM5117
Other Parts Discussed in Thread: CSD19534Q5A, CSD18563Q5A

Hello,

I have an issue with a 35-50V input 12, 18, 24 V selectable output buck converter designed using the LM5117 and the TI Webench for 10A output.

The issue is that when I apply even a small load e.g. 40ohm at 18V, the output voltage goes into a sawtooth. I think what is occurring is that the LM5117 is going into current limiting of some kind as I see a ramp at the same frequency on the SS pin. I have tried tying the RES pin to VCC, the performance gets better but the output voltage is still in a sawtooth waveform.

Below is my schematic and PCB design. I have a ideal diode circuit using the LM5050 on the output however I don't believe this is what's causing the issue.

I have replaced the 1uF capacitor on the CSG, CS pins with a 560pF and have a 10nF capacitor on the SS pin rather than a 47nF. FETs are IPB042N10N3GATMA1. Currently the external voltage regulator U2 and the divider R10,R11 are unpopulated (as I was concerned with the LM5117 overheating however in practice this was not an issue).

Below are scope pictures of the output voltage, voltage at SS, VCC and waveform at the drain of Q2 along with a close up of the switching part of that waveform.

OUTPUT

VCC

DRAIN OF Q2 (SW)

Any help would be highly appreciated.

Thanks in advance!

over 3 years ago

0 Samuel Jaffe over 3 years ago

TI__Mastermind 25590 points

Keshav,

It looks like the schematic and the last image did not attach properly. Please attach again and I'll take a look.

Thanks!

-Sam

0 Keshav Raj over 3 years ago in reply to Samuel Jaffe

Prodigy 170 points

Hi Sam,

Sorry about that, I directly copy pasted screenshots in and I guess it didn't like that.

Here's the schematic and the last image which was the PCB layout.

Cheers!

Keshav

0 Timothy Hegarty over 3 years ago in reply to Keshav Raj

TI__Guru 57152 points

The FETs are too capacitive. Check if the VCC regulator is hitting current limit and the voltage dropping.

Try CSD19534Q5A 100V NexFETs. Also, I recommned completing the LM5117 quickstart calculator to check your component values. There should be very little filtering on CS (certainly not 1uF).

Regards,

Tim.

0 Keshav Raj over 3 years ago in reply to Timothy Hegarty

Prodigy 170 points

Hi Tim,

Thanks for the response.

Would this issue also be the cause of the LM5117 using significant amounts of current (~10-40mA) when sitting idle?

Also could I supply a more powerful external VCC as a temporary fix or would this potentially damage the LM5117 depending on the amount of current required to activate the FETs.

Cheers,

Keshav

0 Keshav Raj over 3 years ago in reply to Timothy Hegarty

Prodigy 170 points

I have the scope plot of the VCC pin of the LM5117. It appears that the internal regulator is switching off.

0 Timothy Hegarty over 3 years ago in reply to Keshav Raj

TI__Guru 57152 points

Hi Keshav,

As mentioned, I recommend changing to SON 56 FETs, for example the 60V NexFETs from TI such as the CSD18563Q5A. The D2PAK FETs are really inefficient and the layout is too big (too much parasitic inductance). See app note snva803 for layout detail.

Regards,

Tim

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LM5117: Unable to output much current