WEBENCH® Tools/LM25119: LM25119 Output voltage Low

Steve Chau

Part Number: LM25119

Tool/software: WEBENCH® Design Tools

based on webench design, my Vin=28V, using LM25119 as an interleave/single output, per recommended schematic, Vout is expected to be 24V, Iout=19A (9.5A each channel). However, upon power up Vout=1.2V. With selected Rt, the switching frequency is about 172Khz. I can see this switching waveform on scope.

Can anyone points me to where to start troubling shooting?

over 5 years ago

0 Qian Chen over 5 years ago

TI__Mastermind 24075 points

Hi Steve,

Can you provide schematic in pdf?
Also, can you povide switch node and Vout waveforms?

Thanks
Qian

0 Steve Chau over 5 years ago in reply to Qian Chen

Prodigy 85 points

webench_design_265218_14_562016787.pdf

0 Qian Chen over 5 years ago in reply to Steve Chau

TI__Mastermind 24075 points

Hi Steve,

Thanks for the schematic. Do you have switch node and Vout waveform?

Thanks
qian

0 Steve Chau over 5 years ago in reply to Qian Chen

Prodigy 85 points

Hi Qian,

I found that Ccomp2 on my board is cracked and Rfb2 measured 12.1K. After replacing Ccomp2=100pF and Rfb2=34K (closest values I have), Vout measured 20.8V (not 24V I intended). At least the design is somewhat working.

Attached is a scope output to look at tested at 1A load. CH1=vout, CH2=SW1. The frequency seemed right. I can tweak Rfb2/Rfb1 a little to get Vout near 24V unless you have other suggestions.

Thanks for the quick reply!ScopeVout.TIF

0 Steve Chau over 5 years ago in reply to Steve Chau

Prodigy 85 points

If I want to be able to adjust Vout (24V to 12V), would you recommend adjust Rfb1 or Rfb2? Not sure if this is possible. Thanks

0 Qian Chen over 5 years ago in reply to Steve Chau

TI__Mastermind 24075 points

Hi Steve,

I suggest to change bottom resistor, because the bottom resistor is not related to compensation.

Thanks
Qian

0 Steve Chau over 5 years ago in reply to Qian Chen

Prodigy 85 points

Hi Qian,

Vout is adjusted using the bottom feedback resistor to obtain an output voltage of 20V (no load). As Load is increased to 330-ohm, Vout drops to 17.62V. at 100-ohm load Vout drops to 8.51V. The converter was supposed to support at least 16A (8A each channel). See attached Vout waveforms. The inpuVout_vs_load.pdft power supply is 28V 50A. Any suggestions?

0 Qian Chen over 5 years ago in reply to Steve Chau

TI__Mastermind 24075 points

Hi Steve,

I sent a message with my email address.
Let's continue the discussion by email.

Thanks
Qian

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WEBENCH® Tools/LM25119: LM25119 Output voltage Low