0
\$\begingroup\$

I have designed a PCB (four layer) having SIM7600I LTE Module. The Stackup is:

  1. Signal & Component Layer with Ground Pour
  2. Internal Ground Power Plane
  3. Internal Power Power Plane (3V3, 4V and 1V8)
  4. Signal & Component Bottom Layer with Ground Pour

The datasheet says:

enter image description here

I have followed the datasheet guide and included approx. 700uF capacitors (considering the load transient response of AP6503 as it can handle the transient with 200mV voltage drop):

enter image description here

I have been using AP6503 Buck Converter to step down from 5V to 4V. The schematics of power design specially for LTE module is given below:

enter image description here

And I have placed 700uF capacitors near to Module's Power Pin.

enter image description here

The Power traces from Buck Converter to LTE Module is on Internal Power Plane (Layer 3).

enter image description here

After assembling the PCB and testing it, I found out it is working perfectly but after few days, I put probe of my oscilloscope to the power pin of LTE module and saw weird undershoot and overshoots. I am not sure the culprit behind it. The spikes are of 1V. I want to resolve this issue in my next iteration but I am not sure who is the culprit. I have been using these capacitors near to power pin of LTE Module.

  1. 470uF Aluminium Polymer 11mohm 16V X 1
  2. 100uF Tantalum 100mohm 16V X 2
  3. 100nF Ceramic Capacitor X 1
  4. 33pF Ceramic Capacitor X 1

The pictures of the noise on power pins of LTE Module are:

enter image description here

enter image description here

I want help in understanding what I did wrong. I think I have followed the datasheets and took measures to avoid any problem after PCB fabrication. Please help.

\$\endgroup\$
  • 1
    \$\begingroup\$ Please show your probe setup. Probing with a large inductance ground lead will cause red herring oscillations. \$\endgroup\$ – Kevin Kruse Mar 21 at 13:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.