Designing Class D Audio Amp, not sure how to hook up VSS to ground. - Page 1
Forum section: electronics / projects

author: LittleRain
author_rank: Contributor
timestamp: August 16, 2020, 11:32:22 pm
content: Just designing a Class D Audio amplifier, and I'm pretty much sticking with the datasheet's schematic, but it has 2 ground planes which I'm not really sure how they connect.The amp IC I'm using is the TDA8954, and VDD is 41V.As seen in the schematic, the only place VSS and SGND seem to be connected is via a 470uf cap, is that right, or am I missing something?Another thing I don't understand are the resistors up there, specifically RVDDA and RVSSA, they don't seem to be doing anything, what is your take on that?Oh and also the CAP CVSSP3, non-electrolytics are pretty expensive so I plan to use an electrolytic, but would I hook up positive to VSS, and negative to ground?Thanks for any help you can provide.

author: ArthurDent
author_rank: Super Contributor
timestamp: August 16, 2020, 11:44:08 pm
content: The amp in this schematic is designed to operate from a split supply, which is very common for audio amps. There is only one ground (SGND) and a plus supply (VDD) and a minus supply (VSS) with a equal but negative voltage (+/-41VDC). If you were to connect VSS to SGND you would be shorting out the negative supply.NOTE: The NXP datasheet where you got your schematic goes into great detail on how to mount and connect the heatsink and ground connections. You need to read and understand what the datasheet is saying. Do not use polarized capacitors for the any of the specified caps except for the two 470 uF caps which are just bulk power supply caps and are polarized.

author: LittleRain
author_rank: Contributor
timestamp: August 17, 2020, 01:18:50 am
content: Ah well that makes a lot more sense.So I just designed a regular non-split PSU for this, I take it I will have to redesign it? Or is there a way to convert it to split supply on the amps board?https://sound-au.com/project15.htmAnd if I do have to redesign it, lets use the link above for reference(Figure 3 - Complete Dual Capacitance Multiplier (Darlington Pair)), my PSU does not have a transformer, I am just rectifying 120V AC, then going to a buck converter, which ends in the capacitance multiplier(thanks Dave, I had thought I had finished my PSU, and then your 5 year old video pops up in my feed, creating more work for me!).Where would my common ground point be? Could the earth pin on the AC connector be used as a common ground, or is that just for grounding things like the case?This is my first time working with AC, so a lot of this PSU design is new to me.Edit: I'll just post my PSU schematic, so where would the common ground point be? My guess is I'll have to add an inverted buck controller for the minus voltage, but everywhere I see they are using a transformer for common ground.As for capacitor, with minimum specs of 470uf, rated at 50V, (which I can't seem to find), a Film Capacitor is about $60 CAD, or is there a better type cap for the job?I did read the heat sink requirements, well, I started to, was going to wait until I started routing, but yes I'll read more.Started this as a side project to add to my PCB order, so I've kinda been rushing it, I really want to order the boards for my main project.