LM25119: low voltage with no load , 700mS delay when 20amp load applied.

Steven Wharepapa

Prodigy 50 points

Part Number: LM25119

Hello.

I am having a coulpe of issues with the LM25119.

I am only using 1ch from the chip for now and have disabled CH2.

Running at 50khz Vin18V , Vout 12V. Max output current 25amps.

Issue 1 - I power up the reg with no load, and the Vout is 4volts. And the FET is getting pulses very slowy. Once you connect a load (400mA) the reg come to life and Vout is 12V.

Issue 2 - With reg running a light load, When I connect a 20amp load , Vout drops down to 4volts and after approx 700mS the reg kicks back into life and supply's 12V and 20amps.

Has anyone come across these issues before?

Regards

Steven

over 4 years ago

0 Qian Chen over 4 years ago

TI__Mastermind 24075 points

Hi Steven,

Can you provide schematic in pdf file for review?

Issue1: Can you provide high-side Vgs and low-side Vgs waveform?

Issue2: If you increase load current slowly, will this problem occur?

Thanks
Qian

0 Steven Wharepapa over 4 years ago in reply to Qian Chen

Prodigy 50 points

Hello Qian

My Sch was from the webench power-designer which was for the interleaved supply. I have changed it to be Dual mode
and disabled CH2 , I will get a sch together with values during the week.

While I was trying to debug my board last night I damaged the chip trying to use an external Vcc supply. I have ordered
more chips which will be here in a week so will be able to get scope pics for you then.

I have been using and excel spread sheet called LM(2)5119 QuickStart ICC Modified.xlsx to calculate the component values , but I noticed when I use webench power-designer I get different values for comp/ramp components. not sure which one I should use for
component values now.

Regards

Steven

0 Qian Chen over 4 years ago in reply to Steven Wharepapa

TI__Mastermind 24075 points

Hi Steven,

I will review the shematic first when it is available.
The excel spread sheet can be used as starting point.

Thanks
Qian

0 Steven Wharepapa over 4 years ago in reply to Qian Chen

Prodigy 50 points

Hi Qian

Sch attached.

I left most of CH2 parts off as I only using CH1 for now.

Regards

Steven

LM25119 1CH Sch V001.pdf

0 Qian Chen over 4 years ago in reply to Steven Wharepapa

TI__Mastermind 24075 points

Hi Steven,

Attached is the schematic review and calculation spreadsheet based on your design. Please try below items one by one.

LM(2)5119 QuickStart ICC.xlsx

LM25119 1CH Sch V001_reviewed.pdf

1. Add RCR filter on current sense traces, and place RCR close to controller. The RCR filter is needed for most application to filter noise on sensing resistor.

2. Decrease Ramp value to add more slope compensation.

3. Refer to spreadsheet and tune compensation.

Thanks

Qian

0 Steven Wharepapa over 4 years ago in reply to Qian Chen

Prodigy 50 points

Hello Qian

I have mostly sorted out the issues I was having. The main problem was using the wrong mosfets and not under standing the DEMB mode. I had DEMB off most of the time so I thought I didn't need the lowside fet and I just had a schottky diode. then realised we still needed the Lowside fet to switch the inductor to gnd for a very short time.

I am now doing some tests on the interleaved design and is working good at 18amp load, but switching gets unstable when I increase load to 23amps. I am getting some double pulses and

the switching freq drops down to 20khz instead of 50khz? I will look into this some more later this week.

regards

Steven

0 Qian Chen over 4 years ago in reply to Steven Wharepapa

TI__Mastermind 24075 points

hi Steven,

The issue could be related to loop. Can you use the speadsheet (excel file) I provided to increase slope compensation and tune the compensation?

Also, I suggest to add RCR filter on current sense traces if it does not exist in your design.

Thanks

Qian

0 Qian Chen over 4 years ago in reply to Qian Chen

TI__Mastermind 24075 points

Hi Steven,

I don't hear from you for some time. Hope the issue has been solved.

Please let me know if you need further support.

Thanks

Qian

0 Steven Wharepapa over 4 years ago in reply to Qian Chen

Prodigy 50 points

Hello Qian

All working now. The last test I did I was running power supply from 24V down to 12V and was pulling 27 amps.

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LM25119: low voltage with no load , 700mS delay when 20amp load applied.