LM5155: Boost converting not boosting

Hi William, 

Thanks for reaching out. 

The Webench schematic looks ok. Can you also send the actual schematic you built as well as the output current spec?

Can you please also share some scopeshots of Vin, Vout , switchnode and CS pin in no-load and loaded condition?

Thanks and best regards

Moritz

Hi,
Yes I rated the system for 1A in normal operation I expect to pull around 500mA peak.

VCS Unloaded:

VCS Loaded:

Vin Unloaded:


Vin Loaded:

Vout Unloaded:

Vout Loaded:

Vsw Unloaded:

Vsw Loaded:

Schematic:

0285.pdf

I don't know if the quality of the image is good enough but I included it in this pdf as well.

Its a first time for me working with boost converters, but my left toe presumes Vsw isn't supposed to look like that.

Kind regards,

William

I see something went wrong with the formatting this is the order of the images

VCS Unloaded

VCS Loaded

Vin Unloaded

Vin Loaded

Vout Unloaded

Vout Loaded

Vsw Unloaded

Vsw Loaded

Schematic

Hi William,

You are right, the switchnode should not look like that. 

You can see that the switch is only turned on for a very short time in each cycle. 

The reason for this could be that the OCP is triggered. Can you measure again on the CS pin please? But use a probe with a very short GND connection ( tip barell) and zoom in, so that cycle by cycle is visible. The expected signal is a triangular waveform with a peak in the range of 50-100mV.

What is the saturation current of the inductor? Is it suitable for this application? The peak current will be 7-8A.

Can you please also measure the VCC voltage? 

Best regards

Moritz

Hi, 

this coil, I selected another one first but it wasn't in stock.

R = 36mohms
Isat = 5.6A
https://www.lcsc.com/product-detail/C3151182.html?s_z=n_C3151182

CS pin:

SW pin:

VCC pin

Best Regards,

William

P.S.
These measurements were made under a load of 1K ohm, so expected to draw 30mA.

Hi William, 

So the inductor is the wrong choice. It is not suitable for this application. The saturation current should be hiigher than the maximum current you expect. 

The current sense waveform is really strange, why is it constant at 500mV? Can i see a picture of your setup please?

Also the FET is not really turning on. The VCC voltage should be at 6.8V, not at 5V. Can you please try to remove the circuit at PGood for the next try?

 And how does the Gate signal look like? Does it also have only short spikes? 

Best regards

Moritz

Hi,

Thank you very much I'll see for a replacement for that coil.

Yeah sure thing:

I read that as well in the datasheet, that VCC should be 6.8V but I didn't find a exact cause for this phenomenon.

I removed all the PGOOD components.
Gate Voltage:

Best Regards,

William

Hi William, 

Is there anything else connected to VCC that can pull it down? There is something wrong if the voltage is not 6.8V.

The gate signal shows a shorter duty cycle than expected but it looks clean. Is it measured at the gate pin of the IC or the FET? 

Did you check if the MOSFET you use is suitable? Can you share the datasheet of it? 

Best regards

Moritz

Hi, 

Nothing is connected to the VCC pin except the 1uF Capacitor.
I remeasured it and it sits on 5.1V.

This is measured on the FET gate pin.

I used the CSD18543Q3A
https://www.ti.com/lit/ds/symlink/csd18543q3a.pdf?ts=1759817752385&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FCSD18543Q3A
I believe I took this over from the TI Power designer.

Kind regards,

William

Sorry, i missed that the input is only 5V, then its normal that VCC is not higher.

Anyway the FET should be able to switch, but the SW signal looks like it does not really turn on. 

I will have a look on your layout and come back to you tomorrow.

Best regards

Moritz

Could you please share all layers of the layout? 

Thanks!

Hi, 

Sure, thank you!

Kind regards,

William

seperate.pdfsingel Layers.pdf

Hi, 

Oh I have indeed missed that in the design fase.

I tried removing the capacitor and patching a think wire from Rs to ground but it didn't make a difference.

My coil is also still the wrong kind.

I'll make the changes for sample V2.

Thank you very much for your time!

Best regards,

William