Review my 12v to 5v buck convertor IC and circuit - Page 1
Forum section: electronics / projects

author: MrOmnos
author_rank: Frequent Contributor
timestamp: July 09, 2020, 08:06:34 pm
content: Hi all, I am designing a Esp32 I2C board for interfacing i2C sensors and led matrix displays. I want my board to work with 5-12v power supplies or battery. Hence, I need a buck converter on board. I am thinking of getting few pcbs made from a fab house. But I don't have any experience with designing switching regulators. I do not want my boards to go to waste so It would be really nice if you guys could take a look at my circuit to see if I need to add or remove anything. I looked at many ICs fixed and adjustable. I liked fixed ics because they seem to use few passive components. So I went with AP1509 fixed 5v version. I drew the circuit from the datasheet.Also I am thinking that it would be better if I could use 5v supply as well when in need. The ap1509 datasheet says Vin from 4.5 to 40v does that mean if I give 5v instead of 12v to vin the vout will still be 5v?Or would it be better to have the 5v input to be directly on the input side of AMS1117-3.3? If i put 5v input on the input side of AMS1117-3.3 what protections do I need to have for the AP1509? Hope to learn something from this. Thankyou !!

author: ocset
author_rank: Super Contributor
timestamp: July 09, 2020, 08:52:32 pm
content: https://www.diodes.com/assets/Datasheets/AP1509.pdf...if you want 5v out of ap1509, you have to put at least 7v in....datasheet says

author: aiq25
author_rank: Regular Contributor
timestamp: July 10, 2020, 05:23:31 pm
content: How much current are you looking to get out of the 5V buck?

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 10, 2020, 05:52:40 pm
content: You don't seem to have spent much time reviewing your schematics, why hasn't the 3V3 regulator got a ground connection ?

author: MrOmnos
author_rank: Frequent Contributor
timestamp: July 11, 2020, 04:29:13 pm
content: About 1.5 to 2A maximum but most of the times it will run at less than 1 Amps.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: July 11, 2020, 04:30:44 pm
content: I moved the the 3v3 schematic to get it in the screenshot. While moving the ground wire was left behind. Thank you for let me know. That would have been an oopsie.

author: MrOmnos
author_rank: Frequent Contributor
timestamp: July 11, 2020, 04:43:22 pm
content: "Internal switch. The voltage at this pin switches between (+VIN - VSAT) and approximately -0.5V with a duty cycle of approximately VOUT/VIN. Tominimize coupling to sensitive circuitry, the PCB copper area connected to this pin should be minimized."Can anyone be explain to mean what this actually means? The output pin is connected to output cap and from there to all the 5v components on board. Wouldn't that increase the PCB copper area connected to this pin? I am planning on having 5V plane on 4 layer pcb. Wouldn't that make the PCB area even more? What does this actually mean? How do I minimize the connected PCB copper area?

author: Niklas
author_rank: Frequent Contributor
timestamp: July 11, 2020, 10:01:10 pm
content: It means that the copper connected between the output pin, the diode and the inductor should be as small as possible. Place the components close to each other and don't connect them with an oversized polygon.The coupling mode is capacitive, where the output pin copper is one plate in the capacitor and the ground plane is the other one, and it is dependent on area and dV/dt of the switch output signal.

author: ocset
author_rank: Super Contributor
timestamp: July 12, 2020, 09:21:46 am
content: "Internal switch. The voltage at this pin switches between (+VIN - VSAT) and approximately -0.5V with a duty cycle of approximately VOUT/VIN. Tominimize coupling to sensitive circuitry, the PCB copper area connected to this pin should be minimized."..yes, it is the "switching node"...it has massive relevance in EMC as it can fail you on EMC. The switching node should be made as small in area as possible. I must admit sometimes i violate this when i desperately need it for heetsinking.........but i violate it and feel the fear.Attached is my SMPS PCB layout doc.(The switching node bangs up and down in voltage with very high dv/dt, and causes radiated emissions and capacitive emissions, which are a nightmare for your EMC test....keep it small......it is a devil within your SMPS..it is evil)You need to have some area to it...but if you can shield this with "quiet node" (eg ground or power rail) copper then its a bit better...ie kind of hide the switching node behind the quiet node copper

author: MrOmnos
author_rank: Frequent Contributor
timestamp: July 19, 2020, 04:05:01 pm
content: Hey, Treez. I have one question. I have added two grounds on my schematic now. GND is the overall ground and GNDA is the Switching loop ground that I am connecting using net ties as suggested in your doc. However I was wondering if GND pins of the switching IC should be connected to GND or GNDA? I am guessing GNDA. Also another question. Later on the board I am using input 12V to run motor using a mosfet. So, it is ok to ground the input ground and 5V and 3.3V ground together on a single plane?

author: ocset
author_rank: Super Contributor
timestamp: July 19, 2020, 08:50:49 pm
content: yes it should be ok with a nice low Z ground plane. But beware of your ground voltage drops.