LM390N National Semiconductor, LM390N Datasheet - Page 3

• Manufacturer Part Number: LM390N
• Description: LM390 1W Battery Operated Audio Power Amplifier
• Manufacturer: National Semiconductor

www.DataSheet4U.com
Typical Performance Characteristics
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 k
tor is put from pin 2 to 6 bypassing the 1 35 k
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 150
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 k
tor) For 6 dB effective bass boost R j 15 k
value for good stable operation is R
open If pins 2 and 6 are bypassed then R as low as 2 k
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 k
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 k
additional offset (about 2 5 mV at the input 50 mV at the
output) If the dc source resistance is less than 10 k
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 k
resistor between pins 2 and 6) it is necessary to
Distortion vs Output Power
resistor sets the gain at 20 (26 dB) If a capaci-
resistor The base current of the input transis-
internal resistor If the capacitor is elimi-
it will contribute very little
e
10 k
resistor the
the lowest
if pin 2 is
Device Dissipation vs
Output Power 4
resis-
then
(Continued)
3
Figure 2 )
bypass the unused input preventing degradation of gain
and possible instabilities This is done with a 0 1 F 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
output transistor The maximum output current divided by
beta is the value required for the current in R1 and R2
Good design values are V
Example 0 8 watt into 4
To keep the current in R2 constant during positive swing
capacitor C
R1 and R2 above the supply maintaining a constant voltage
across R2 To minimize the value of C
due to C
due to the output coupling capacitor and the load This
gives
Example for 100 Hz pole and R
C
the same current C
4 as in Figure 4
For reduced component count the load can replace R1 The
value of (R1
is both the coupling and the bootstrapping capacitor (see
B
e
a
Load
16 F if R1 is made a diode and R2 increased to give
R2 set the amount of base current available to the
(R1
B
and R1 and R2 is usually set equal to the pole
B
a
a
is added As the output swings positive C
(R1
R2) is the same so R2 is increased Now C
I
R2)
O MAX
a
B
e
R2)
C
can be decreased by about a factor of
100
B
e
j
e
BE
load with V
Device Dissipation vs
Output Power 8
4C
2 P
(6 2)
R
O
e
O
c
L
O
(V
0 632
L
0 7V and
j
e
S
e
b
I
C
25
O MAX
2)
632 mA
c
4
0 7
S
B
b
e
R1
C
V
6V
e
c
BE
e
O
e
364
e
Load
R2 The pole
400 F and
100
TL H 7848 – 6
B
lifts
B