LM391 National Semiconductor Corporation, LM391 Datasheet - Page 11

• Manufacturer Part Number: LM391
• Description: Audio Power Driver (discontinued)
• Manufacturer: National Semiconductor Corporation

Application Hints
The formulas for R
R
The formula for R
mula becomes V
V
get V
Now we must find R
Putting V
duces to
Typical Applications
Note All Grounds Should be Tied Together
Additional protection for LM391N Schottky diodes and R j 100
TH
2
R
High Frequency Ground
Input Ground
Speaker Ground
e
TH
is the additional voltage added to the supply voltage to
Only at Power Supply Ground
B
1k
V
e
e
R
TH
B
TH
3
R
1k
e
2
e b
V
0 76 R
I
11 (0 22)
b
R
L
(V
R
1
B
and R
3
V
E
e
B
E
B
R
now gives R
b
A
3
b
A
3
E
1k
R
47
and R
1
e
V
TH
b
80
a
b
and R
and
0 65
3A
0 65
)
into the appropriate formulas re-
0 65
b
1
e b
B
3
(Continued)
0 65
using the Thevenin formulas
e
2
b
TH
(47
do not change
0 22
25 55k
1
(Continued)
e
when the V
b
e
120k
30)
25 55k
e
e b
R
A
3
a
ll
17V
40W-8
R
in the for-
B
3
60W-4
11
The easiest way to solve these equations is to iterate with
standard values If we guess R
use 47k The Thevenin impedance comes out 26 7k which
is close enough to 25 55k
Now we will use equation (5) to determine the heat sinking
requirements of the drivers to insure thermal stability
This value is lower than we got with equation (9) so we will
use it in equation (10)
This is the required heat sink for each driver For low TIM
we add the 1 M
910k resistor from pin 4 to ground The complete schematic
is shown below
If the output is shorted the transistor voltage is about 28V
and the current is 5A Therefore the average power is
This is much larger than the power used to calculate the
heat sinks and the output transistors will overheat if the out-
put is shorted too long
Amplifier
JA s
short PD
SA s
resistor from pin 3 to the output and a
0 22 (20
57
40 (0 002)
b
e
6
a
b
(28) 5
A
3
1)
1
e
e
62k then R
e
50 C W
57 C W
70W
TL H 7146 – 13
B
3
e
47 12k
(10)
(5)