L6981C50DR power module pcb design

[aferjani]

Hi all,

I selected the L6981C50DR for my DC DC buck converter with the following external components and schematic for my application:

• Cin: GRT31CR61H106KE01L Multilayer Ceramic Capacitors MLCC - SMD/SMT 10 uF 50 VDC 10% 1206 X5R AEC-Q200 RoHS Compliant
• CinA: GCM31CR71H105KA55L Multilayer Ceramic Capacitors MLCC - SMD/SMT 1 uF 50 VDC 10% 1206 X7R AEC-Q200 RoHS Compliant
• CvCC: same as CinA
• CBoot: GRT188R71H104KE13D Multilayer Ceramic Capacitors MLCC - SMD/SMT 0.1 uF 50 VDC 10% 0603 X7R AEC-Q200 RoHS Compliant
• L: MSS1038T-333MLC Power Inductors - SMD 33uH Shld 20% 2.3A 93mOhms AECQ2
• Cout: GRJ32EC71E226KE11L Multilayer Ceramic Capacitors MLCC - SMD/SMT 22 uF 25 VDC 10% 1210 X7S

My system is powered by a smart battery and I want to use the converter to convert the typ 12.8V from the smart battery to 5V to then feed it to the esp32-c3-devkitm-1. Experimental values showed that the output current is at around 120mA with spikes close to 300mA, I used Iou_max = 500 mA in my calculations to determine the capacitance for the input and output capacitors and also the inductance.

Now I am proceeding with the layout of the PCB on which I will assemble all the parts, could you please suggest a guide or do you have some advices regarding the desing of the PCB?

I am new to this field and especially power modules design area, any remarks or advices would be highly appreciated.

 

[Peter BENSCH]

Well, the usual rules for switching controllers must be observed, which can already be found in the relevant reference manuals. However, information on similar switching regulators can also be found in application notes, which are also generally applicable here:

The layout of switching DC-DC converters is very important to minimise noise and interference. Power-generating portions of the layout are the main cause of noise and so high switching current loop areas should be kept as small as possible and lead lengths as short as possible.

High impedance paths (in particular the feedback connections) are susceptible to interference, so they should be as far as possible from the high current paths. Particularly with open (non-magnetically shielded) coils, the feedback pin should not be routed under the coil under any circumstances, as the coil's magnetic field will otherwise induce voltages in the feedback path, which can lead to severe instability of the DC/DC.

The input and output loops must be minimised to avoid radiation and high frequency resonance problems.

You can take a look at the layout of the STEVAL-L6981CDR as an example, whose GERBER data can be found in the CAD Resources tab.

Hope that helps?

Good luck!
/Peter

[aferjani]

Thanks Peter! For starters I used the guide found in the below link:
https://fscdn.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/converter_pcb_layout_appli-e.pdf
But I wanted to look for a more specific one oriented to my selected regulator which I was not able to find, up until now the above is my reference guide for placement and some calculations.

I proceeded with checking the gerber files like you suggested which confused me a little bit. 

As stated in my post, I am fairly new to this. 

I already ordered the parts and when it comes to designing the PCB I couldn't help but wonder is it normal to hand solder small components like the IC itself or the capacitors with very small dimensions like the ones I mentioned in the post desctription ? or is this something for a PCB manufaturer to do for me ? 

To draw the complete picture, I am in the process of devolopping a simple embedded system with a couple of stepper drivers to move a small surface around 10*10 cm along the x and y axis and later a tilt mechanism will be added. All the system is powered by a smart battery that gives typ 12.8V so to account for different voltage level, for example 5V to the esp32-c3-devkitm-1 and directly all the voltage to the motors. I am using the mentioned converter to bring that typ 12.8V from the smart battery down to 5V to feed to the mentioned motherboard.

For the converter I started with a ready one found in the link below and now I want to build my own and later integrate it in the same PCB where all the connections and small parts, stepper drivers, connections to sensors and all other compoents etc, are mounted on it.
https://protosupplies.com/product/lm2596s-adjustable-dc-dc-step-down-sy-dc3003-module/

This is basically the description for the system I am building for my master thesis, it will be used in an oriented manner so I am building it specifically to a certain application.

Any advices or instructions are highly appreciated.

[Peter BENSCH]

Well, it's a question of tools (electronically controlled soldering iron, fine soldering tip, possibly magnifying glasses) and a steady hand whether you can solder it yourself or have it done by a PCB service. I know people who still solder 0402 by hand.

As far as the layout is concerned, you already have a lot of information, so you can't really go wrong if you pay attention to it.

Good luck!
/Peter

[aferjani]

Thanks Peter! A last follow up, regarding the gerber files you were referring to. A couple of them are for solder mask and paste mask. are those only relevant when a manufacturer will do the assembly or should I pay attention to this when ordering my PCB without the parts mounted on it? Or maybe, if I solder them myself which I will do at least try it, I just need to apply some paste mask ?

I would really appreciate a good guide or tutorial tailored towards this topic.

[Peter BENSCH]

If you want to hand-solder them, you should only order them with a solder mask. A paste mask only makes sense for immediate assembly.

Regards
/Peter