LMS3635-Q1: Troubleshooting no output from regulator

To be clear, the output isn't 0V but hovers around 1.3V. This was measured before the PTC.

Hi Umair,

Can you send waveforms during this operation? I would like to see VIN, VOUT, and SW?

Is this behavior present on multiple boards? Can you send the entire part number of the device that you are using?

Thanks, Jason

Hi Jason,

I checked 3 boards. All exhibit the same issue. The part we used was LMS36353QRNLTQ1 from DigiKey.

This is VIN:

This is Vout:

This is Vcc pin:

and this is the SW waveform:

That SW waveform is pretty weird.

Hi Umair,

What type of capacitors are you using for CIN, COUT and CBOOT? The minimum required should be Low-ESR ceramic type. Place the ceramic caps close to the device pins. 

Also, can you share your layout?

Thanks, Jason

Hey Jason,

See below for layout. The ground plane is continuous. The power plane is split between 3.3V and 24V underneath the IC.

Caps used:

Cin: https://www.digikey.com/short/nwbjzmvv - Same as WEBENCH recommendation

Cboot: https://www.digikey.com/short/3803zm9d - Used equivalent to WEBENCH recommended https://www.digikey.com/short/74bpv88t 

Cout: https://www.digikey.com/short/wbvhfjjv - Used equivalent to WEBENCH recommended https://www.digikey.com/short/27djwp20 

Thanks for taking a look.

Can you help me understand what the deformed PFM SW signal suggests?

Hi Umair, 

I need to discuss with someone who may know the device better. I will respond tomorrow.

Regards, Jason

Hi Umair, 

Can you measure the resistance between VOUT and ground?

Thanks, Jason

Resistance between Vout and GND is 18 kohm. This is was measured at the Cout with the PTC present, i.e. with a load.

If we remove the PTC, i.e. no load, the resistance is 330 kohm.

Umair, That sounds ok. I have been discussing this with several colleagues.

Can you confirm that the Cboot is making a proper connection? Can you measure the BOOT voltage?

Have you tried powering up without the PTC?

Is there any other load on the device?

Regards, Jason

Hi Jason,

Sorry for the late reply.

In terms of continuity with a multimeter, Cboot is making proper connection.

I removed the PTC for a no-load scenario and the output voltage from the regulator was approximately 1.5V.

The Cboot pin waveform was as below:

Hi Umair, 

So, the output voltage changed slightly? 0.5V?

Have you tried removing R27, (Rbias)? Shouldn't have effect, but would like to isolate VOUT.

Also, check to see if there are any shorts between adjacent pins.

I am also curious about the FB trace running directly below the SW node. Noise can be coupling to the FB pin and disturb the regulation.

Regards, Jason