LM5177: LM5177 schematic review

Hi Kerim,

can you check the voltage level the VCC pin.

You are working with quite high voltage levels on input and output but within your schematic I do not see any gate resistors or snubber. So you might have large under or overshoot on the SW node (SWx) which can violate the abs max ratings and lead to a damage of the device.

Best regards,

 Stefan

Hi Stefan,

thank you for your answer, I checked the VCC it is 5V. Should I add gate resistor or snubber? Which one will be the best choice? I have checked the evoluation board there is 3 ohm resistor and 10nf capacitor. Do you mean like this resitor and capacitor? Will 3ohm and 10nf be enough?

Best Regards,

Kerim

HI Kerim,

if VCC is 5V then this OK.

So lets keep the gate resistor and snubber in the backlog for an design update. Also add filter for CSA/CSB here

The issue needs to be somewhere else, so please check:

Probe:

- EN/UVLO, SS, VIN, VOUT

- SW1 and SW2

Disable the current limit to reduce the options for issues:

- put IMONOUT to VCC

Best regards,

 Stefan

Hi Stefan,

I want to be sure, Do you mean I need to connect IMONOUT directly to VCC , with no resistor or RC filter required? Then I will measure the pins you want me to check.

Best Regards,

Kerim

Hi Kerim,

yes, this will just disable the average current limit.

Best regards,

 Stefan

Hi Stefan,

I connected IMONUT directly to VCC, then I measured these pins and the results were;

EN/UVLO = 1.5V

SS = 0,47V

VIN = 11,40V

VOUT = 46V

SW1 = 0,165V

SW2 = 0,165V

Best Regards,

Kerim

Hi Kerim,

sorry for not being clear enough.

Can you probe them with a scope?

From above it looks like the device is continuously restarting.

You can also try to increase the SS cap to e.g. 100nF.

Best regards,

 Stefan

Hi Stefan,

Sorry for the late reply, I wasn't at the office.

I shared pictures, the names of pictures are same as the name of tests. I tested them with 100nF SS cap.

Best Regards

Kerim

Hi Stefan,

as you can see from the SS plot the device is continuously restarting shortly before reaching the 1V.,

Can you try to increase the SS cap to e.g. 150nF as a first step.

The probe VIN, SW1 and SS in one plot.

Best regards,

 Stefan

Hi Stefan,

my scope has 2 probe, sorry for that.  I shared the measurment result, with 150nF SS cap.

Best Regards,

Kerim

Hi Kerim,

sorry for the delay, accidentally i set the wrong tracking flag for the thread.

Checking the scope plot showing VIN, it looks like the supply drops the moment the controller starts to operate and the VIN voltage drops below the UVLO level set with the resistor divider on EN/UVLO.

Check if the supply could be more stable and does not drop. Check current limit, add buffer caps,...

Try to set the UVLO trigger to a lower voltage level.

Best regards,

 Stefan

Hello Stefan,

No problem, thank you for your help.

I obtained the value shown in the images when no load was connected, and I was able to achieve the desired voltage level in the absence of a load.

However, when I connect a load, the output voltage drops, and depending on the load, it can decrease by as much as 3 volts.

Best Regards,

Kerim

Hi Kerim,

so without load you get the required output voltage and this also looks stable?

If you add load the output voltage drops - right?

How does the load correlate to the load. Is this linear? Does it drop even with very small load? .....

From the schematic it does look like that you do not have the filter at CSA/CSB  can you try to add that?

Would be also good to see the layout - esp. the connection of CSA/CSB to the controller (once above questions are clarified)

Best regards,

 Stefan

Hi Stefan,

Thank you for your response.

Yes without load voltage looks stable.

Yes, when I add load to the output voltage drops

It is a smart load. Yes it is linear. Yes I just arrange the load to pull 40mA again the output voltage droped

Maybe it will be difficult to add filter at CSA/CSB. I think maybe my problem is deal with the layout. it looks bad.

Best Regards,

Kerim.

Hi Stefan,

Thank you so much for your help.

I will redesign my board based on the guidelines you shared. Prototyping will take time. 

When I get the new board, I will test it and then inform you.

Best regards,

Kerim