LM25116: Performance review and troubleshooting

Noel Binny Abraham

Part Number: LM25116

Hi
I have a few doubts regarding the LM25116 buck regulator which we have configured for 20V/5A output.

Vin:21V to 29.4V
Vout:20V/5A
22uH inductor and fsw of 82kHz

1.Currently the regulation is being well maintained at 20V, but the issue we are running into , is that my board in not able to ramp up quickly to 5A of load. It has an immense lag when i give a load step of 0 to 5A and most of the times the load stays at 20V/0A. When i increase the load gradually from 4A to 5A it does not have any issue, but during the load step it does not ramp up. I have selected the Cramp as per the calculation given in the datasheet and I've also kept the Rsense such that my continuous current is set at 9A. Also, I've added a soft start of 15ms and this did not help too. Can you suggest any way so that this issue is resolved?

2.The inductor heats up to about 70 deg.C (40 deg above ambient) in my case when i run the load at 5A for 2-3mins. Is this expected? Can this be due to the core losses, and should i go for another inductor? Currently the inductor saturation current is 9A and has a DCR of 37mOhms (Inductor part number: (TMPC 1265HPV-220MG-D)

Please look into this and suggest possible solutions. I can attach the schematic and PCB files on private chat if needed.

over 2 years ago

0 EricLee over 2 years ago

TI__Guru 60941 points

Hello Noel

1. I think the PWM operation is limited by the maximum duty cycle limit during the light-load to heavy-load transition because VIN is close to VOUT. Can you check ?

Also, please use the Quick Start calculator and check your component selections especially Rramp and Cramp value.

Please measure the peak voltage at RAMP.

2. DCR loss is ~ 1W now this is enough to heat up your inductor.

-EL

0 Noel Binny Abraham over 2 years ago in reply to EricLee

Intellectual 535 points

1.Ya will check and get back to you

Ya i used the values from the quickstart. Another observation is that when i use the Rramp, my current limit hits at 2.5A every time. So i ended up leaving the Rramp pad open. I couldn’t figure out why this was happening.

So currently only the Cramp is populated on the board.

2. Using a lower DCR inductor is my only option then i guess?

+1 EricLee over 2 years ago in reply to Noel Binny Abraham

TI__Guru 60941 points

Hello Noel

I use inductor manufacturer's tool to find the best inductor I can use. Please refer https://www.coilcraft.com/en-us/tools/power-inductor-finder/#/search

-EL

0 Noel Binny Abraham over 2 years ago in reply to EricLee

Intellectual 535 points

Noel Binny Abraham said:
a i used the values from the quickstart. Another observation is that when i use the Rramp, my current limit hits at 2.5A every time. So i ended up leaving the Rramp pad open. I couldn’t figure out why this was happening.

So currently only the Cramp is populated on the board.

Can you please take a note of this and check if theres anything i can do?