LM70840-Q1: Issues with noisy power line and odd behavior when the IC is touched by finger.

Hi Nico,

Have you filled out a quickstart calculator? If so, can you share that with us?

Are there any scope shots you can share with us? Schematic?

Ben

Hi Ben,

Here is some supplemental information

Quickstart:

LM70XX0_quickstart_calculator_A0.xlsm

WBench design:

TI_18_3A.pdf

Altium Schematic:

Altium PCB Layout:

Top:

Bottom:

Scope captures/pictures:

1. Start of experiment light load, power line scope is OK:

2. 30 seconds elapsed, system is warming up during light load and some collapsing of the 18V line can be seen:

3. 1 min elapsed and 18V line is loaded and becomes very unstable:

4. 18V line the moment before I touch the IC:

5. 18V line the moment after I touch the IC:

Hi Nico,

What is the max current of your application? Looking at the provided quickstart calculator, the loop doesn't appear to be stable (less than 45 degrees of phase margin). See attached updated quickstart calculator with changes that I have added. Is there a particular bandwidth your design is requiring? In the updated quickstart calculator, I have the x-over frequency at 60kHz.

LM70XX0_quickstart_calculator_A0_updated.xlsm

Ben 

Hello Ben,

The max current is 3A.

I implemented the design in the WBench design I provided above. I've had pretty good luck so far with WBench, is this design fundamentally flawed?

Why is it that the issue disappears when I touch the IC? 

Best,

Nico