PS Pin Capacitance LM3886

Puffin

Member
2006-02-19 8:15 am
U.K
It seems that the PS pin capacitance is suggested to be between 1000 and 2200uf for a single 3886. I have built a bridged LM3886 and after re-reading the notes it seems that they suggest 4700uf for bridged.

Is the reccommendation an absolute i.e is there any detriment to using higher than 2200uf (or 4700). The reason I ask is that I have some good quality 10,000uf caps. I will give them a go and see how they sound.

I would however be intersted to know if the extra capacitance will just be wasted or whether it will have any other detrimental effect ?
 
Hi,
when suggesting the smoothing capacitance requirement for a bridged amp I argued that bridged needed 4times as much capacitance as a single amp into the same load. Most came back and said "rubbish".
Try four or more times the local decoupling. it might help.
But remember the fast caps are the ones that go on the pins. The electrolytics can be a little further away.
 
AndrewT said:
Hi,
when suggesting the smoothing capacitance requirement for a bridged amp I argued that bridged needed 4times as much capacitance as a single amp into the same load.


I agree , It's simple math


1 Chipamp into 8 ohm = xAmps
1 Chipamp into 4 ohm = xAmps x 2

2 chipamps into 8ohms bridged(each chipamp "sees" 4ohms) =

xAmps x 4

So your current has multiplied by 4

C = 0.7(I)/ΔE(f)

where C = capacitance in farads, I = dc load current in amperes, ΔE = peak-to-peak ripple voltage, f = ripple frequency (generally 120 Hz for full-wave or 60 Hz for half-wave), and 0.7 is the complement of the rectifier-current duty cycle, which is assumed to be 0.3


http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=9809

IT all depends on the ripple voltage you are happy with at high loads (high volume)

You do have SOME play with music because you don't have a contstant current pull , but If you want to quote an RMS voltage these calculations are important.

am I overlooking something?