Problems with LM25088 Load current

Cormac O'Sullivan

Other Parts Discussed in Thread: LM25088

I am using an LM25088 voltage converter with an input voltage of 28V (regulated) and an Load voltage of 18.75 V. I am using a current sense resistor of 0.01ohms and expected an available load current of up to 10 amps. The system works well at Load currents under 0.7 amps, but at Load currents over 0.7 amps the Load voltage drops. When the load current reaches 1 amp the Load voltage has dropped to 9.0 volts.

I would appreciate any help resolving this issue

over 11 years ago

0 Cormac O'Sullivan over 11 years ago

Prodigy 60 points

Just to supply a few more details of the power supply I am posting some schematic details and a view of the layout.

0 Cormac O'Sullivan over 11 years ago

Prodigy 60 points

Just to supply a few more details of the power supply I am posting some schematic details and a view of the layout.

0 EricLee over 11 years ago in reply to Cormac O'Sullivan

TI__Guru 58481 points

Here are something you can try

1) CS-CSG connection should be kelvin-connected to your sense resistor. Just for a test, please cut the CS-CSG traces and use external twisted pair wires to connect CS-CSG to your Rsense.

2) Try R-C snubber from SW to PGND to minize switching noise.

3) Try R-C filter at CS-CSG pins

Regards,

0 Cormac O'Sullivan over 11 years ago in reply to EricLee

Prodigy 60 points

Hello

I tried the following with no success

Cut the traces from CS-CSG and used 28 awg twisted pair to connect R Sense.
Placed a 1000 pf capacitor accrossR Sense.
Placed an RC snubber to Pgnd; R = 6.8 ohms, C=1000pf
Placed a short between the CS-CSG traces next to the board.

The result is always the same, the output voltage drops to about 9 volts when the load current > 0.7 amps. The fourth test would seem to indicate that the trouble is not with R Sense as the short should make the load current appear 0 amps to the chip.

Regards,

0 Cormac O'Sullivan over 11 years ago in reply to Cormac O'Sullivan

Prodigy 60 points

Looking at the waveform on PIn 14 HG, I see a large amount of ringing. This is the gate drive and I expected to get a square wave with a small amount of ringing. Maybe this is the problem. Don't know how to fix it though.

Any help appreciated;

Thanks

0 EricLee over 11 years ago in reply to Cormac O'Sullivan

TI__Guru 58481 points

Would you please post the waveforms for me ?

Regards,

0 Cormac O'Sullivan over 11 years ago in reply to EricLee

Prodigy 60 points

Posting images of the waveforms. Sorry for the poor quality but my 'scope does not export screen images so I just took photos with my cell phone. First image is with the correct output voltage (18.75V), Second image is with the current > 0.7A and the out voltage = 9V.

0 EricLee over 11 years ago in reply to Cormac O'Sullivan

TI__Guru 58481 points

Please check your RT resistor value. It might be damaged or wrong part is populated now.

Your switching frequency does not match with RT resistor value.

Regards

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Problems with LM25088 Load current