lm380 question

[jufreese]

im making a small amp out of a few lm380's for a school project. I have seen a couple circuits for using the lm380 but my question is why do you need a cap between pin 1 and ground? Also, what size cap should I use at the output? Do i need to stabilize with a 2 ohm resistor and cap in parallel with my speakers?

Any help would be great thanks!

[gmphadte]

That RC across the speaker is a necessity.

LM380 is history and literature will not be available. Also that the IC was intended for use in wireless sets, and not for music.
Better go for newer products.

Gajanan Phadte

[gootee]

The LM380 "2.5 Watt Audio Power Amplifier" appears to be alive and well, and in full production, with free samples also available:

LM380 Product Page:
http://www.national.com/mpf/LM/LM380.html

LM380 Datasheet:
http://www.national.com/ds.cgi/LM/LM380.pdf

AN-69: "LM380 Power Audio Amplifier":
http://www.national.com/an/AN/AN-69.pdf

AN-146: "FM Remote Speaker System":
http://www.national.com/an/AN/AN-146.pdf

"A selected part for more power on higher supply voltages is available as the LM384. For more information see AN-69."

[gootee]

The capacitor from Pin 1 to ground appears to provide supply decoupling and to set the power supply rejection ratio (PSRR). The datasheet and AN-69 both say that 5uF from pin 1 ("bypass" pin) to ground provides 38 dB PSRR, referred to the output. See also the plot entitled "Supply Decoupling Versus Frequency". But the bypass capacitance also appears to affect the distortion level (%THD), the specs for which are given with Cbypass=0.47uF, which would provide only 15dB to 20dB PSRR at 100Hz to 120Hz. i.e. You might want to experiment with the capacitor's value and dielectric type.

[jufreese]

and the 51 pF cap between inputs for the bridge config?

[gootee]

Originally posted by jufreese
and the 51 pF cap between inputs for the bridge config?

That's probably for high-frequency stability, and/or RF rejection.

Don't forget to also put 0.1 uF from right at each supply pin to ground. It might be better with some larger cap in parallel with each of those 0.1 uF caps, too. Maybe try 10uF electrolytic. If in doubt, use an X7R ceramic type for the 0.1uF, there.

[gootee]

Do you have heatsinks?

[gootee]

What type of amplifier configuration are you thinking of using, anyway? Or, do you have a schematic that you can post?

What kind of specs are you going for? And what supply voltage are you going to use?

[jufreese]

here is a circuit that i am going to use with:
Cbypass=4.7uF
Vs= 18-20v

this circuit will give me 4 watts for my sub, and then i can make an identical circuit for my left and right speakers. Then i will add my low and high pass filters in front of the inputs.

[gootee]

At least you're 'a man with a plan'. That's good.

Don't forget to also connect 0.1 uF capacitors from each power supply pin to ground.

But first, maybe you had better check to see if that circuit is going to work well-enough, for you.

Uh oh. I hope that you have also looked at appnote AN-69, and not just the LM380 datasheet.

The schematic you posted is also Figure 14, in AN-69. On that same AN-69 page are Figures 15A and 15B, which show the relationships between total output power, per-device heat dissipation, power supply voltage, and distortion, with 8 Ohm and 16 Ohmn loads, for the bridged configuration shown on your schematic.

I hope that your sub-woofer is a 16-Ohm speaker. Otherwise, according to Figure 15A, at 8 Ohms it won't do 4 Watts unless your supply is 12V and distortion is 10%, apparently, because that's the only place the curves even reach 4 Watts output power, with <= 10% distortion. The reason the curves are that way is because otherwise the device gets too hot. (Keep in mind that you might be able to just add some more LM380 chips to 'share the load' somehow, if necessary, to help with any overheating tendencies, if you really want or need that output power level.)

Of course, I could fairly-easily have made a mistake, above. So study AN-69. ALSO, since none of the LM380 docs have anything about the arithmetic needed for configuring bridged, paralleled, or bridged-and-paralleled amplifiers, you'll 'probably' want to go to national.com and download AN-1192, which has good examples that you could probably also apply to the LM380.

Unless you have a bucketful of free LM380 chips, you might also want to think about using a higher-power chipamp, since your desired specs might necessitate fighting for the edge of the SOA (Safe Operating Area) of the LM380. National.com and ti.com both have fairly-good selection tables that you will probably enjoy perusing.