# Basic Qs on LM3886 power supply design

Discussion in 'Electronic Design' started by Mahendra Reddy, Jul 21, 2003.

1. ### Mahendra ReddyGuest

I have some bsic questions in designing power supply for LM3886
only for this requirement but in general I need to understand these
things. Sorry for asking so many basic questions.

LM3886 datasheet page 16 says it requires +/-21V at 4.5A for 40W into

1.) Does it mean it dissipates 42V(+/-21V dual supply) * 4.5A = 189
watts? i.e Pd = V * I

2.) To calculate power dissipation in the above equation is it correct
to consider +/-21V(dual supply) as 42V? If I need to consider 21V in P
= V*I then what should be the voltage if I use 42V single supply
configuration of LM3886? Power disspation and current requirement
should be more or less same in +/-21V dual supply and 42V single
supply. Am I correct?

3.) Is 4.5A at +/-21V equal to 2.25A on +21V supply and 2.25A on -21V
supply? If I use ammeter to measure the current drawn by the amplifier
I see some amps drawn on +V supply and some amps drawn on -V supply.
So is it sufficient to use a 18V-0V-18V transformer supplying 2.5 amps
to get approx +/-21V and 4.5amps(2.25 at +21V and another 2.25 at
-21V) required for 40W into 4 ohms in dual supply configuration?

4.)What is continuous output power and what is Instantaneous peak
output power of an amplifier?

5.)What if I give required voltage but the trnasformer delivering less
current(for example 1A against the required 4.5amps) and turn up the
volume to maximum? Will it distort the signal? or simply doesn't sound
loud?

Mahendra Reddy

3. ### R.LeggGuest

4.47 Apk from the supply into the load to produce 40W RMS.
3.16A/2 = 1.58Arms from each rail. ' Irms = Ipk x (D/2)^.5 '

18V peak is required into the load, so they are telling you that the
output saturation voltage of the amplifier is 3V @ 4.5A.
1.58A x 21V x 2rails
66.4W total from the supply
The amplifier dissipates 66.4 - 40 = 26.4W
1.58A x 42V x 1rail = 66.4W
66.4W total from the supply
If the supply is not current limited below 4.5A and filter caps are
large enough, (assume 10% droop), 18-0-18V transformer will produce
roughly 27V average.

This will increase power consumption to
1.58A x 27V x 2rails = 86W
For the same 40W into 4R output.

Power dissipation in the amplifier would be 86 - 40 = 46W
40Wrms output, as you are requesting, and 4.5^2 x 4R = 80W peak
LM3886 is thermally limited below around 90W output, using the
specified supply voltages, according to the data sheet.

The transformer will not current limit at it's rating, it will simply
overheat if overloaded, unless the LM3886 overheats first and limits
at a lower output power level.

Full power output is not the worst case power dissipation condition
for a linear amplifier.

RL