Choosing the right substitutes for reference schematic components - Page 1
Forum section: electronics / beginners

author: GeMeMe
author_rank: Newbie
timestamp: August 06, 2021, 09:44:46 am
content: I need to design a LED boost driver circuit for a LCD backlight and am a bit out of my depth with selecting the components.The datasheet of the backlight module I’m using (https://www.lcd-module.de/deu/pdf/grafik/dogxl160-7.pdf;EA LED78x64-W variant) contains a reference circuit with a CAT4238TD-GT3 regulator IC (see page 2, bottom right). The datasheet of this regulator (https://www.mouser.de/datasheet/2/308/1/CAT4238_D-2310289.pdf) also contains a reference circuit (see top of page 2), which does not completely agree with the backlight's datasheet.The backlight consists of 9 white LEDs in series, with Vf 8.8V, Imax 20mA. My input voltage is 5V @ 200mA.I want to have the PCB manufactured by JLCPCB Assembly and thus can only use components from their catalogue. I therefore need to find substitutes for some components specified by the reference schematics, which are:1. L/Inductor:- Regulator datasheet specifies: 47uH, lower series resistance preferred, current rating >= 300mA recommended, e.g. CDC5D23B−470 (47uH, 490mA, 420mΩ)- Backlight datasheet specifies: 47uH, e.g. Coilcraft DO1608C-473M (47uH, 500mA, 640mΩ) or WE-PD4S (47uH, 500mA, 640mΩ)I chose SLF6028T-470MR59-PF (47uH, 590mA, 252mΩ),https://product.tdk.com/system/files/dam/doc/product/inductor/inductor/smd/catalog/inductor_commercial_power_slf6028_en.pdf2. D/Schottky diode:- Regulator datasheet: Vf as low as possible, response time suitable for 1MHz (?), e.g. CMDSH05-4 (Vf 470mV, Vrrm 40V, If 500mA, Ifsm 10A, Ir 30uA)- Backlight datasheet: e.g. MBR0540 (Vf 510mV, Vrrm 40V, If 500mA, Ifsm 5.5A, Ir 20uA)I chose PMEG4005EH,115 (Vf 470mV, Vrrm 40V, If 500mA, Ifsm 10A, Ir 20uA),https://www.mouser.de/datasheet/2/916/PMEGXX05EH_EJ_SER-1320570.pdf3. C1:- Regulator datasheet: 4.7uF- Backlight datasheet: 100nF + 1uF in parallelMy final schematic below.Questions:- Some inductors have a frequency rating (e.g. 14 MHz). Something to take into account in my case?- Schottky diode datasheets generally don’t seem to mention a ‘response time’, yet the regulator’s datasheet gives a specification. What am I to make of this?- Datasheets disagree on the input capacitors. What could the reason be? Which version should I go with?- Are my substitutions for L and D suitable? I’m not an EE and my understanding of the matter is superficial at best. All I can do is compare specs.- The regulator’s datasheet also contains a recommended PCB layout. How crucial is this? How closely should I follow component placement, track lengths/widths etc.?- Am I missing some subtle but crucial detail in all of this? I’m worried to have the PCBs manufactured, only to then realize I made some stupid mistake. I’m not set-up for SMD prototyping, unfortunately.Any help is greatly appreciated!

author: Jwillis
author_rank: Super Contributor
timestamp: August 08, 2021, 05:26:38 am
content: Inductors inductance should match , current capability should be equal or greater than and resistance will have little effect . The max frequency is where the inductors value changes significantly or drops off .Schottky diode reverse voltage and current capability are most important . The dv/dt is the diodes recovery in Volts per nanosecond . The diodes max frequency is when the recovery becomes to slow and can no longer block reverse current properly.Capacitor value should be equal or greater when used for decoupling . A smaller value capacitor between 0.01 µF to 0.1 µF in parallel helps decouple high frequency noise .The larger capacitor between 1 µF to 100 µF decouple low frequency noise .

author: GeMeMe
author_rank: Newbie
timestamp: August 08, 2021, 08:13:38 am
content: Thanks for taking the time, Jwillis!Quote from: Jwillis on August 08, 2021, 05:26:38 amBased on that, my chosen substitute should work, right? The regulator drives the inductor with 1MHz (if I understand this correctly), so a 14MHz-rated inductor should suffice, right?Quote from: Jwillis on August 08, 2021, 05:26:38 amSeeing that the datasheets of schottky diodes rarely even mention recovery time, should I assume that these diodes are probably fast enough for all general purposes, such as mine? Or should I look for a diode with a specified recovery time to be safe?Quote from: Jwillis on August 08, 2021, 05:26:38 amThen perhaps I should go with the double capacitor variant, assuming that the backlight manufacturer had good reason to give this recommendation.What about the recommended PCB layout?

author: Jwillis
author_rank: Super Contributor
timestamp: August 08, 2021, 06:06:36 pm
content: Yes your inductor would workSchottky diode recovery is in the picoseconds to nanoseconds , so it's rarely considered a factor . This makes them ideal for high frequency applications. But keep in mine that they have low reverse bias voltages compared to regular diodes and are not considered ideal for high voltage rectification . Even when nearing the rated reverse voltage they can leak current substantially . But in your application they do well.Your PCB layout recommendations is in thecat4238datasheet . Keep components as close together as possible to minimize any parasitic inductance , resistance , capacitance and other electromagnetic anomalies .Remember when doing electronics is to not design your circuits with too tight of tolerances . Many factors can have an effect on your circuit that can be difficult to mitigate . Electrical Parasitic's , Ambient temperatures , Electromagnetic Interference and even physical shock . This is why you will never see components that have perfectly exact tolerances . Keep the devices within RECOMMENDED OPERATING CONDITIONS and it will work as designed .

author: GeMeMe
author_rank: Newbie
timestamp: August 09, 2021, 07:42:15 am
content: Again, thanks for your help.Quote from: Jwillis on August 08, 2021, 06:06:36 pmI guess I'll need to do some reading-up on parasitic phenomena.One last thing: The reference PCB layout has GND as a separate trace. Would it be advisable to use a ground plane instead?

author: Jwillis
author_rank: Super Contributor
timestamp: August 10, 2021, 03:08:39 am
content: Quote from: GeMeMe on August 09, 2021, 07:42:15 amThe layout they have is for single sided copper trace . You could have a ground plane on the opposite side if you like but that means adding Vias to tie your ground plane to the components . Normally you would use large ground plains to dissipate excess heat but in this application I doubt that will be an issue . A lot of extra fussing about for little gain . Best to keep traces as direct and simple as possible .

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 10, 2021, 09:12:31 am
content: hi,do me a favor, each time you see some uF capacitor on voltage regulator or dcdc output, put 0.1uF and 0.01uF next to him, your life will changeand take care when layout-ing at GND layout (VCC also) recommended by the datasheet, if he doesn't do recommend some layout, inspiration from other ic's from other manufacturer can do fineand others already pointed the right directions for L/D

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 11, 2021, 10:22:49 am
content: Quote from: GeMeMe on August 06, 2021, 09:44:46 am- inductor rated frequency rating may be important if you are working at the edge. You may check the working frequency of your DC/DC to be sure, but I guess it isn't larger than 1 MHz. As I can guess from for 47 uH value - it is somewhere in the 100-400 kHz region, so > 4 MHz rated inductors are fine.- 'response time' is not a property of a Shottky diode. They are fast enough. 'Trr' parameter became valuable if you are using fast/superfast diodes instead of Shottky.- PCB layout is crucial for EMI and stability.- I haven't looked at any other details of your design, so I can't tell more about it. Usually, it is reasonable to follow datasheet recommendations about everything - PCB layout, parts, etc.About 4,7 uF and 1 uF caps - usually the larger - the better. The datasheet gives minimal required values for caps, so you could make a cheap economic design, but you may use 2 x 4.7 uF caps, or 10 uF caps, etc. This rule is not applied to inductor inductance because it strictly relies on working current and input-output voltage. But you may use inductors with a much larger rated current until it becomes too large - so large that a frequency rating is too low. It is fine to have a rated frequency ten times higher than working frequency (taking into account all harmonics).

author: GeMeMe
author_rank: Newbie
timestamp: August 12, 2021, 08:28:37 am
content: Quote from: Jwillis on August 10, 2021, 03:08:39 amMy current PCB design actually has the LED driver components on the back side, because there is not enough room for the coil under the LCD. So the ground plane would be on the same side. Could this cause problems? There is also a buck converter and logic level converter on the same board and having a ground plane is simply convenient.

author: GeMeMe
author_rank: Newbie
timestamp: August 12, 2021, 08:30:19 am
content: Quote from: perieanuo on August 10, 2021, 09:12:31 amThanks for the suggestion!

author: GeMeMe
author_rank: Newbie
timestamp: August 12, 2021, 08:31:05 am
content: Thanks for all the input, Vovk_Z!Quote from: Vovk_Z on August 11, 2021, 10:22:49 amThe datasheet says 1MHz. There should be room enough.Quote from: Vovk_Z on August 11, 2021, 10:22:49 amThat would explain my confusion. Good to know.Quote from: Vovk_Z on August 11, 2021, 10:22:49 amThat's what I thought, too. But the backlight datasheet specifies SMALLER caps (1uF+0.1uF) than the regulator's datasheet (4.7uF), for some reason.So, I should use a larger cap than the regulator’s datasheet recommends (or 2x 4.7uF), but add two smaller caps of 100nF and 10nF for noise filtering, as well (as perieanuo suggested)?Just to make sure, we’re only talking about the capacitor on the input side, right? The 220nF on the output is fine?