LMR36006: Output rails to max value

Hi Michael,

What's your operating conditions you are running at? (load current)? 

The schematic as I see it looks fine. From what you've described, it looks like the part is entering dropout mode, which is strange as the feedback resistors seem appropiate.

Can you provide a waveform of EN, VCC, BOOT-SW, and FB in steady-state?

Can you also provide a waveform of VIN, VOUT, VCC during startup? 

Finally, can you provide the layout? 

Regards,

Richard

Apologies for the delay Richard - other tasks took priority over the last 3 weeks.

Load current varies from ~0 --> 50 mA at the output - the system is low power.

These measurements don't have ideal ground references, so please excuse the excess noise.

Steady state EN (tied to VIN):

Steady state VCC:

Steady state BOOT-SW:

Steady state FB:

Steady state Vout:

Startup VIN:

Startup VOUT:

Startup VCC:

Layout (quasi-3D view):

Layout (2D view):

Hi Michael,

Sorry for the delay in response as I've been out of office. From what I understand, it does look like the IC thinks it is in dropout behavior. It may have something to do with the layout? Looks like there's a violation in the 2D view. I added the engineer in charge of the E2E forums this month to help with the issue.

Regards,

Richard

Thank you Richard - for context, the 2D view layout violation is just a standard violation for <10 mil spacing between regions without solder mask.

Hello

Be sure that you have a ground plane connecting the input caps and IC ground.

Also, be sure that the two VIN pins are tied together.

You might want to double check that the correct values of resistors were stuffed for the FB divider.

Thanks

Hi Frank,

There are indeed several ground planes connecting input capacitor returns to IC GND, on several layers.

The FB divider was already confirmed given it produces ~2.1V given an output of 12V, which would nominally trigger the LMR36006 to let the output voltage relax to its target value.

Visually both Vin pins are soldered properly, and they are tied together in the design.

Hello

I don't see anything wrong in the schematic or layout.

You might want to try a different slew rate on the input; either faster or slower to see if that has any effect.

Thanks

Hello

I noticed that your inductor is rather large and has a rather large resistance.

For this design you would use an inductance in the 10uH range with a much lower resistance.

You might want to try another inductor.

Thanks

Hi Frank,

I can try adjusting the inductor, but do you know of a reason why an inductor that is oversized would cause the LMR36006 to not regulate properly? I can't see any such reason.

Hello

It may be the large resistance.

Thanks

That still doesn't make sense to me (why would a large series resistance pose a problem? If anything it would just burn more power, but you'd still expect the regulator to regulate effectively the voltage found at the output of the inductor), but regardless, an attempt to replace the inductor with a 1 uH replacement (95 mohm DC resistance, DFE201210U-1ROM=P2) did not change the outcome. The feedback voltage still reads as ~1.95V, and the output is still effectively 12V, albeit with more ripple now than before.

Hello

Can we see a better image of the output voltage waveform.

The devices your tried may have been damaged by mishandling or ESD.

You may want to try to get the EVM for this device and make sure it is working with your input

and load and then put your components on the EVM to see if the issue shows up on the EVM.

Thanks