National Semiconductor LM380 User Manual

LM380 Power Audio
Amplifier
INTRODUCTION
The LM380 is a power audio amplifier intended for consum-
er applications It features an internally fixed gain of 50
(34 dB) and an output which automatically centers itself at
one-half of the supply voltage A unique input stage allows
inputs to be ground referenced or AC coupled as required
The output stage of the LM380 is protected with both short
circuit current limiting and thermal shutdown circuitry All of
these internally provided features result in a minimum exter-
nal parts count integrated circuit for audio applications
This paper describes the circuit operation of the LM380 its
power handling capability methods of volume and tone con-
trol distortion and various application circuits such as a
bridge amplifier a power supply splitter and a high input
impedance audio amplifier
CIRCUIT DESCRIPTION
Figure 1 shows a simplified circuit schematic of the LM380
The input stage is a PNP emitter-follower driving a PNP dif-
ferential pair with a slave current-source load The PNP
Parameter
Power Output (rms)
Gain
Output Voltage Swing
Input Resistance
Total Harmonic Distortion
Power Supply Rejection
Supply Voltage Range
Bandwidth
Quiescent Output Voltage
Quiescent Supply Current
Short Circuit Current
Note 1 V 18V T 25 C unless otherwise specified
e e
S A
Note 2 Rejection ratio referred to output
Note 3 With device Pins 3 4 5 10 11 12 soldered into a
Note 4 If oscillation exists under some load conditions add a 2 7X resistor and 0 1 mF series network from Pin 8 to ground
Note 5 C 0 47 mF on Pin 1
e
bypass
Note 6 Pins 3 4 5 10 11 12 at 50 C derates 25 C W above 50 C case
C 1995 National Semiconductor Corporation
TABLE I Electrical Characteristics (Note 1)
Conditions
8X loads 3% T H D (Notes 3 4)
8X load
P 1W (Notes 4 5)
e
o
C 5 mF 120 Hz
e e
bypass
(Note 2)
P 2W R 8X
e e
o L
epoxy glass board with 2 ounce copper foil with a minimum surface of six square inches
TL H 7380
National Semiconductor
Application Note 69
December 1972
input is chosen to reference the input to ground thus en-
abling the input transducer to be directly coupled
The output is biased to half the supply voltage by resistor
ratio R R Negative DC feedback through resistor R
1 2
balances the differential stage with the output at half supply
since R 2 R ( Figure 1 )
e
1 2
The second stage is a common emitter voltage gain amplifi-
er with a current-source load Internal compensation is pro-
vided by the pole-splitting capacitor C Pole-splitting com-
pensation is used to preserve wide power bandwidth
(100 kHz at 2W 8X) The output is a quasi-complementary
pair emitter-follower
The amplifier gain is internally fixed to 34 dB or 50 This is
accomplished by the internal feedback network R
gain is twice that of the ratio R
2
source which provides the full differential gain of the input
stage
Min Typ
2 5
40 50
14
150k
0 2
38
8
100k
8 9
7
1 3
2
– R The
2 3
R due to the slave current-
3
Max Units
Wrms
60 V V
V
p-p
X
%
dB
22 V
Hz
10 V
25 mA
A
RRD-B30M115 Printed in U S A