WEBENCH® Tools/LMR23615: DC/DC not working

Dirk Handzic

Part Number: LMR23615

Tool/software: WEBENCH® Design Tools

I have designed a dc/dc based on LMR23615 using the webbench tool 23-26V in 5V/1.5A out. See schematic below. I can measure 4.1V at VCC, I can see some switching at 1.6 MHz at pin 1+2 (see picture below). With RT not connected I have expected 400 kHz. Output voltage is only about 0.82V. I have tested to power the board with 5V after the regulator to check power consumption which is only 100 mA at this point. Any ideas are appreciated.

over 7 years ago

0 Brani Dubocanin over 7 years ago

TI__Expert 4100 points

Hello,
Your schematic and screenshot are not visible.
Can you please re-post?
Regards
Brani

0 Dirk Handzic over 7 years ago in reply to Brani Dubocanin

Prodigy 205 points

Here are the schematic and waveform at pin 1 as pdf. Hopefully it works this time.0675.dc_dc.pdf

0 Brani Dubocanin over 7 years ago in reply to Dirk Handzic

TI__Expert 4100 points

Hello,
After reviewing your schematic i would suggest changing C32=100nF.
You are currently using much larger value of 680nF .
Otherwise schematic is OK.
Please try using smaller boot capacitor and let me know if this helped?
Regards
Brani

0 Dirk Handzic over 7 years ago in reply to Brani Dubocanin

Prodigy 205 points

Hi Brani,

the 680 nF is the calculated value from Webbench. I changed it to 100 nF but it did not change the behaviour. I noticed that the BOOT pin will swing between +1V and - 1V since Vsw is about 2V is this expected?

Best regards

Dirk

0 Dirk Handzic over 7 years ago in reply to Dirk Handzic

Prodigy 205 points

I also tried to add 33pF across R24 but that did not help either.

Regards
Dirk

0 Brani Dubocanin over 7 years ago in reply to Dirk Handzic

TI__Expert 4100 points

Hello Dirk,
It is not clear what is going on with your design.
Can you please provide your layout screenshots?
From schematic review , there is no obvious reason for observed behavior.
Regards
Brani

0 Dirk Handzic over 7 years ago in reply to Brani Dubocanin

Prodigy 205 points

Hello Brani,

please find attached the layout it is a 4-layer board with layer 2 being a GND plane and layer 3 planes for +5V and +3.3V.. I have only two prototype boards right now and one has a short from +5V to GND somewhere. The other is working apart from the 5V dc/dc. I will ask our manufacturer to change the LMR23615 maybe it is broken.

Best regards

Dirk

srms_dcdc.pdf

0 Brani Dubocanin over 7 years ago in reply to Dirk Handzic

TI__Expert 4100 points

Hello Dirk,
Your layout looks OK.
I don't see anything obviously wrong that would prevent device from operating.

At this point i would suggest isolating specific IC's (IC2, IC6 and IC16) to determine where is the issue coming from.
As first step i would remove R38 and if possible lift pin 3 of IC6, and test IC16 functionality.
This should show if some downstream load for 5V converter is misbehaving and causing observed behavior.

Unfortunately i don't see other option than troubleshoot it piece by piece to get to the root cause.
Please let me know your findings as well as if replacing LMR23615 has helped.
Regards
Brani

0 Dirk Handzic over 7 years ago in reply to Brani Dubocanin

Prodigy 205 points

Hi Brani,

I will ask our assembly house to replace the LMR23615 for me. When I insert 5V from a lab power supply across C56 the rest of the board is working fine with +3.3V and -5V regulators working as expected and no excessive load (only 50 -100mA).

Regards

Dirk

0 Brani Dubocanin over 7 years ago in reply to Dirk Handzic

TI__Expert 4100 points

Hi Dirk,
I'm glad to hear that you are making progress with debug.
I will go ahead and close this thread, and if you have any questions please feel free to re-open it.
Best regards
Brani

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WEBENCH® Tools/LMR23615: DC/DC not working