TL H 7848
LM390
1W
Battery
Operated
Audio
Power
Amplifier
April 1995
LM390 1W Battery Operated Audio Power Amplifier
General Description
The LM390 Power Audio Amplifier is optimized for 6V 7 5V
9V operation into low impedance loads The gain is internal-
ly set at 20 to keep the external part count low but the
addition of an external resistor and capacitor between pins
2 and 6 wil increase the gain to any value up to 200 The
inputs are ground referenced while the output is automati-
cally biased to one half the supply voltage
Features
Y
Battery operation
Y
1W output power
Y
Minimum external parts
Y
Excellent supply rejection
Y
Ground referenced input
Y
Self-centering output quiescent voltage
Y
Variable voltage gain
Y
Low distortion
Y
Fourteen pin dual-in-line package
Applications
Y
AM-FM radio amplifiers
Y
Portable tape player amplifiers
Y
Intercoms
Y
TV sound systems
Y
Lamp drivers
Y
Line drivers
Y
Ultrasonic drivers
Y
Small servo drivers
Y
Power converters
Equivalent Schematic and Connection Diagrams
TL H 7848 1
Dual-In-Line Package
TL H 7848 2
Order Number LM390N
See NS Package Number N14A
C1995 National Semiconductor Corporation
RRD-B30M115 Printed in U S A
Absolute Maximum Ratings
If Military Aerospace specified devices are required
please contact the National Semiconductor Sales
Office Distributors for availability and specifications
Supply Voltage
10V
Package Dissipation 14-Pin DIP (Note 1)
8 3W
Input Voltage
g
0 4V
Storage Temperature
b
65 C to
a
150 C
Operating Temperature
0 C to
a
70 C
Junction Temperature
150 C
Lead Temperature (Soldering 10 sec )
260 C
Thermal Resistance
i
JC
30 C W
i
JA
79 C W
Electrical Characteristics
T
A
e
25 C
(Figure 1)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
V
S
Operating Supply Voltage
4
9
V
I
Q
Quiescent Current
V
S
e
6V V
IN
e
0
10
20
mA
P
OUT
Output Power
V
S
e
6V R
L
e
4X THD
e
10%
0 8
1 0
W
A
V
Voltage Gain
V
S
e
6V f
e
1 kHz
23
26
30
dB
10 mF from Pin 2 to 6
46
dB
BW
Bandwidth
V
S
e
6V Pins 2 and 6 Open
300
kHz
THD
Total Harmonic Distortion
V
S
e
6V R
L
e
4X P
OUT
e
500 mW
0 2
1
%
f
e
1 kHz Pins 2 and 6 Open
PSRR
Power Supply Rejection Ratio
V
S
e
6V f
e
1 kHz C
BYPASS
e
10 mF
Pins 2 and 6 Open Referred to Output
50
dB
(Note 2)
R
IN
Input Resistance
10
50
kX
I
BIAS
Input Bias Current
V
S
e
6V Pins 7 and 8 Open
250
nA
Note 1
Pins 3 4 5 10 11 12 at 25 C Above 25 C case derate at 15 C W junction to case or 85 C W junction to ambient
Note 2
If load and bypass capacitor are returned to V
S
(Figure 2) rather than ground (Figure 1) PSRR is typically 30 dB
Typical Performance Characteristics
vs Ambient Temperature
Maximum Device Dissipation
Supply Voltage
Quiescent Supply Current vs
Frequency
(Referred to the Output) vs
Power Supply Rejection Ratio
Swing vs Supply Voltage
Peak-to-Peak Output Voltage
Voltage Gain vs Frequency
Distortion vs Frequency
TL H 7848 5
2
Typical Performance Characteristics
(Continued)
Distortion vs Output Power
Output Power 4X Load
Device Dissipation vs
Output Power 8X Load
Device Dissipation vs
TL H 7848 6
Application Hints
Gain Control
To make the LM390 a more versatile amplifier two pins (2
and 6) are provided for gain control With pins 2 and 6 open
the 1 35 kX resistor sets the gain at 20 (26 dB) If a capaci-
tor is put from pin 2 to 6 bypassing the 1 35 kX resistor the
gain will go up to 200 (46 dB) If a resistor is placed in series
with the capacitor the gain can be set to any value from 20
to 200 A low frequency pole in the gain response is caused
by the capacitor working against the external resistor in se-
ries with the 150X internal resistor If the capacitor is elimi-
nated and a resistor connects pin 2 to 6 then the output dc
level may shift due to the additional dc gain Gain control
can also be done by capacitively coupling a resistor (or
FET) from pin 6 to ground as in
Figure 7
Additional external components can be placed in parallel
with the internal feedback resistors to tailor the gain and
frequency response for individual applications For example
we can compensate poor speaker bass response by fre-
quency shaping the feedback path This is done with a se-
ries RC from pin 6 to 13 (paralleling the internal 15 kX resis-
tor) For 6 dB effective bass boost R j 15 kX the lowest
value for good stable operation is R
e
10 kX if pin 2 is
open If pins 2 and 6 are bypassed then R as low as 2 kX
can be used This restriction is because the amplifier is only
compensated for closed-loop gains greater than 9 V V
Input Biasing
The schematic shows that both inputs are biased to ground
with a 50 kX resistor The base current of the input transis-
tors is about 250 nA so the inputs are at about 12 5 mV
when left open If the dc source resistance driving the
LM390 is higher than 250 kX it will contribute very little
additional offset (about 2 5 mV at the input 50 mV at the
output) If the dc source resistance is less than 10 kX then
shorting the unused input to ground will keep the offset low
(about 2 5 mV at the input 50 mV at the output) For dc
source resistances between these values we can eliminate
excess offset by putting a resistor from the unused input to
ground equal in value to the dc source resistance Of
course all offset problems are eliminated if the input is ca-
pacitively coupled
When using the LM390 with higher gains (bypassing the
1 35 kX resistor between pins 2 and 6) it is necessary to
bypass the unused input preventing degradation of gain
and possible instabilities This is done with a 0 1 mF capaci-
tor or a short to ground depending on the dc source resist-
ance on the driven input
Bootstrapping
The base of the output transistor of the LM390 is brought
out to pin 9 for Bootstrapping The output stage of the am-
plifier during positive swing is shown in
Figure 3 with its
external circuitry
R1
a
R2 set the amount of base current available to the
output transistor The maximum output current divided by
beta is the value required for the current in R1 and R2
(R1
a
R2)
e
b
O
(V
S
2)
b
V
BE
I
O MAX
Good design values are V
BE
e
0 7V and b
O
e
100
Example 0 8 watt into 4X load with V
S
e
6V
I
O MAX
e
0
2 P
O
R
L
e
632 mA
(R1
a
R2)
e
100
(6 2)
b
0 7
0 632
J
e
364X
To keep the current in R2 constant during positive swing
capacitor C
B
is added As the output swings positive C
B
lifts
R1 and R2 above the supply maintaining a constant voltage
across R2 To minimize the value of C
B
R1
e
R2 The pole
due to C
B
and R1 and R2 is usually set equal to the pole
due to the output coupling capacitor and the load This
gives
C
B
j
4C
c
b
O
j
C
c
25
Example for 100 Hz pole and R
L
e
4X C
c
e
400 mF and
C
B
e
16 mF if R1 is made a diode and R2 increased to give
the same current C
B
can be decreased by about a factor of
4 as in
Figure 4
For reduced component count the load can replace R1 The
value of (R1
a
R2) is the same so R2 is increased Now C
B
is both the coupling and the bootstrapping capacitor (see
Figure 2 )
3
Typical Applications
TL H 7848 3
FIGURE 1 Load Returned to Ground
(Amplifier with Gain
e
20)
TL H 7848 4
FIGURE 2 Load Returned to Supply
(Amplifier with Gain
e
20)
TL H 7848 7
FIGURE 3
TL H 7848 8
FIGURE 4 Amplifier with Gain
e
200 and Minimum C
B
TL H 7848 9
FIGURE 5 2 5W Bridge Amplifier
4
Typical Applications
(Continued)
TL H 7848 10
FIGURE 6(a) Amplifier with Bass Boost
TL H 7848 11
FIGURE 6(b) Frequency Response
with Bass Boost
TL H 7848 12
FIGURE 7 Intercom
TL H 7848 13
FIGURE 8 AM Radio Power Amplifier
Note 1
Twist supply lead and supply ground very tightly
Note 2
Twist speaker lead and ground very tightly
Note 3
Ferrite bead is Ferroxcube K5-001-001 3B with 3 turns of wire
Note 4
R1C1 band limits input signals
Note 5
All components must be spaced very close to IC
5
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