AP3031 Backlight boost converter EMI - Page 1
Forum section: electronics / projects

author: cheeseburger
author_rank: Contributor
timestamp: March 26, 2025, 12:22:06 am
content: I have a Backlight boost converter designed to drive a Backlight with the following specifications:If=160mA, Vf=8.4V-9.9VThe boost converter works, but internal EMI testing showed that this circuit part isn't optimal.Any recommendation what I could improve? Maybe another boost converter part?I wasn't able to find a reference board for the AP3031.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: March 26, 2025, 08:04:48 am
content: The schematics and the layout both look fine the to me, no glaring issues at a glance.What is the stackup like? Is it a 2 layer board?

author: cheeseburger
author_rank: Contributor
timestamp: March 26, 2025, 08:45:58 am
content: Stackup is 4 Layer.Top (Red, Signal)In1 (GND)In2 (Power, 3.3V)Bottom (Blue, Signal)

author: eleguy
author_rank: Regular Contributor
timestamp: March 26, 2025, 01:02:54 pm
content: When you say EMI was not optimal do you mean radiated or conducted or both? Without knowing more, just as one assumption, maybe that little cap just does not have enough capacitance left at the output voltage. I would try next to add hefty capacitance to the output and check what it does.Quickly calculated, allowing 5% ripple at output, 10V, 160mA, 1MHz gave me ~1uF as effective capacitance. You designed half of that. An addition, in practice capacitance will be something much lower at target bias voltage. Murata for example do publish these figures. I would not use random caps in sensitive "things" without having the data available, tested before or then slammed 10x more or so (sometimes that is not enough either).Which layer did you route LED+ back to CHIP's pin 5 and how does the reference layer of that trace look like? LED+ "left" via is also pretty close to potentially high impedance trace TP_INT_CM that could catch some of the switching fields. And if it does then the next traces are too close to that -> you would be in trouble. Do you have solid GND plane (In1) in this area without any "holes".

author: Terry Bites
author_rank: Super Contributor
timestamp: March 26, 2025, 04:39:51 pm
content: Current loop areas must be kept to a minimum. You might want to take a look at that.This is an obsolete part, do you really want to design it in?

author: cheeseburger
author_rank: Contributor
timestamp: March 26, 2025, 10:17:55 pm
content: Quote from: eleguy on March 26, 2025, 01:02:54 pmThe datasheet recommends 0.47uF. But this is a good point to try more nevertheless!The EMI measurement was done with a simple Near-field probe. In the meantime, comparison showed that other Backlight boost converters are in the same ball-park like my design (e.g. Raspberry Pi Display)Quote from: eleguy on March 26, 2025, 01:02:54 pmThe exact PN is a CL21B474KBFNNNE. Capacitance seems to be pretty good at ~10V. See attachment.Quote from: eleguy on March 26, 2025, 01:02:54 pmLED+ is routed back on the bottom Layer. Also adding GND between the Interrupt line and LED+ is a good catch to change in the next Revision.Appreciate the detailed feedback!

author: cheeseburger
author_rank: Contributor
timestamp: March 26, 2025, 10:20:00 pm
content: Quote from: Terry Bites on March 26, 2025, 04:39:51 pmWhere did you find that? It is active on Digikey and Mouser. Also Diodes posted a PCN this year about additional Wafer sources.Do you have any recommendation for another Part?

author: Niklas
author_rank: Frequent Contributor
timestamp: March 27, 2025, 12:04:03 am
content: The EOLs mentioned were for older variants and the recommended replacement is the current part number. Might have been a move to halogen free plastic or something similar.LayoutNot possible to see where the LED output traces are going, but just looking at the regulator circuitry shows a few things.1 - decrease the size of the switch node copper pour to reduce the capacitive coupling to GND. Could affect some common mode noise.2 - Move the current shunt for feedback to the left side of the regulator. Also move the enable signal resistors furter away. The goal is to have a solid copper between the output capacitor, switcher ic GND and the input capacitors. Perhaps also move the diode furter down and the output capacitor closer to the input capacitors.3 - the 10 uF input capacitor has a really poor GND connection. Try to just patch a wire or component lead to the 100 nF cap just next to it.4 - is the input filter with a ferrite bead really doing anything at the switch frequency and the first 10+ harmonics? I suggest that you try a wire wound inductor instead. Additional capacitance on the supply side might also be needed.5 - near field probing is very sensitive to direction, position etc. Even small changes in position could have large impact on the measurements. A LISN to measure conducted emissions is a better option.6 - how is the loop with LEDs routed? On another layer, but do the traces have the same reference plane? In this case: are they routed on inner layer 2 with 3.3V / power, or on the bottom layer? Bottom layer is worse as you then have a shift in reference plane from GND to power. Routing on the power layer is probably better as both that layer and the top layer are referenced to the same layer (GND)

author: eleguy
author_rank: Regular Contributor
timestamp: March 27, 2025, 05:53:48 am
content: Quote from: cheeseburger on March 26, 2025, 10:17:55 pmThis, cap(s) and their VIA(s) are some likely areas where one could improve. (list not in importance order). I added quick example how close you should aim your cap and their via(s). Rule is more and closer. One is rarely enough and if it is far then the cap cannot react in the frequencies one would like as it has lot of lead inductance. Reference layer (GND) via(s) are the ones we are typically optimizing. Sometimes similar handling in the power input (3V3 in the example) as well.This is now all in very general. It would be helpful to know in which frequencies you have issues.To everyone interested in this are I recommend Signal and Power Integrity book by Eric Bogatin. Stuff is pretty well explained there. "Too Long I Dont Like To Read",https://youtu.be/ARwBwHZESOY?t=2134