LM5013: Output is not 12v, goes from 2v down to 1v, then back up to 2v

Hi John, 

I believe you are referring LM5013 EVM schematic mentioned on the LM5013-Q1 EVM user guide. If you have any other schematic/layout that you are referring, please share it. 

Also, Please capture Vin, Vout, SW and FB node waveform for further debug.

Thanks,

Nitya

Yes I'm referring to LM5013 EVM. Please see my schematic from Eagle.  I've double checked it, it should be identical to the schematic on the evaluation board.  I've added the optional caps and resistor, all the way on the right on the schematic.  See photo

Vin Pin & Vin Scope48v in

SW Pin & Scope

FB Pin & Scope

Vout looks just like FB

Hi John,

The scope plots can you re-attach again?. The plots origin point is not clear in the attached image. Also, can you confirm this is happening across the load condition and output voltages?

Thanks,

Nitya 

This is to power a microcontroller, I also used your development board also and that works fine.  Can you please double check my schematic.

I added a couple of extra capacitors and a resistor that were optional components, do you think the larger capacitance on the output side is creating some sort of fault?

Yes the outputs all the same, raises to 2v, drops to 1v in 2.5 seconds, then back up to 2v.

Does PGood need to be connected?

Hi John,

PGOOD is used to indicate if the output voltage is within the regulation level. It can be left floating if unused and it should not cause any issue if left floating. 

In schematic, the values are not mentioned and the part numbers also not clearly visible. I request you to attach BOM component values or more clear schematic for review.  If you can't provide, please help to fill below excel sheet with your operating conditions and BOM values used. 

https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-CWXsXlF9WP/01.00.00.0A/snvr514a.zip 

I would like you to couple of experiments.  Remove the microcontroller from device output and observe the output voltage under no load and with resistive load conditions. Let me know if you still see output voltage down to 1V/2V.

Thanks,

Nitya

Hello Nitya,

I input the values in the excel ss and some components are different from the ones I use.  I copied the schematic exactly from the LM5013-Q1EVM evaluation board datasheet.  I use 1mohm Ruv1 and 330k Ruv2,  also Ra of 453Kohm.   The Excel SS is saying to use 178k for Ruv2 and 226k for Ra... this the datasheet wrong? Do you think thats the problem?

Also, I removed the MCU and any other component that I thought might be a potential short but no change, same issue.  Also connected PGOOD with a resistor and still nothing.

.  

Hi John,

Regarding the RUV2, the value can be different depending upon the UVLO level that is set and Ra value would be different considering the ripple amplitude for COT mode control. 

Having said that, i would like to double check the BST capacitor value and measure the BST-SW node waveform through differential probe for further debug.

Also, can you confirm in how many devices this issue has been seen?. Does the issue goes away after replacing the faulty device with good LM5013 device? Please confirm.

Thanks,

Nitya