LM391 National Semiconductor Corporation, LM391 Datasheet - Page 5

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

Application Hints
GENERALIZED AUDIO POWER AMP DESIGN
Givens
The power output and load impedance determine the power
supply requirements Output signal swing and current are
found from
Add 5 volts to the peak output swing (V
voltage to get the supplies i e
of I
ed voltage usually about 15% higher Supply voltage will
also rise 10% during high line conditions
max supplies
The input sensitivity and output power specs determine the
required gain
Normally the gain is set between 20 and 200 for a 25 watt
8 ohm amplifier this results in a sensitivity of 710 mV and 71
mV respectively The higher the gain the higher the THD
as can be seen from the characteristics curves Higher gain
also results in more hum and noise at the output
The desired input impedance is set by R
can cause board layout problems and DC offsets at the out-
put The bandwidth requirements determine the size of C
and C
The output transistors and drivers must have a breakdown
voltage greater than the voltage determined by equation (3)
The current gain of the drive and output device must be high
enough to supply I
The power transistors must be able to dissipate approxi-
mately 40% of the maximum output power the drivers must
dissipate this amount divided by the current gain of the out-
puts See the output transistor selection guide Table A
peak
C
as indicated in the external component listing
The regulation of the supply determines the unload-
Power Output
Load Impedance
Input Sensitivity
Input Impedance
Bandwidth
g
A
Opeak
(V
V t
V
Opeak
Opeak
I
Opeak
P
with 5 mA of drive from the LM391
V
O
IN
a
e
R
e
L
5) (1
e
2 R
g
2 P
V
V
R
L
(V
a
INRMS
ORMS
L
P
O
OP
O
regulation) (1 1)
IN
a
OP
Very high values
5V) at a current
) for transistor
(1)
(2)
(3)
(4)
f
5
To prevent thermal runaway of the AB bias current the fol-
lowing equation must be valid
where
Often the value of R
is rearranged to be
The maximum average power dissipation in each output
transistor is
The power dissipation in the driver transistor is
Heat sink requirements are found using the following formu-
las
where
tion to ambient in C W
R
V
equation (3)
K is the temperature coefficient of the driver base-emitter
voltage typically 2 mV C
T
T
most mountings
JA
jMAX
AMAX
JA
SA
JC
CS
min
CEQMAX
E
is the emitter degeneration resistance in ohms
is the thermal resistance of the driver transistor junc-
is thermal resistance junction to ambient
is thermal resistance junction to case
is thermal resistance case to sink typically 1 C W for
is thermal resistance sink to ambient
is that of the output transistor
is the maximum transistor junction temperature
is the maximum ambient temperature
is the highest possible value of one supply from
P
R
SA s JA
DRIVER(MAX)
P
JA s
E t
JA s
E
DMAX
is to be determined and equation (5)
T
JA
R
JMAX
V
e
E
CEQMAX
(V
b
(
MIN
0 4 P
CEQMAX
MIN
P
b
e
JC
D
a
OMAX
T
P
b
a
AMAX
1
DMAX
(K)
MIN
1)
) K
CS
(10)
(5)
(6)
(7)
(8)
(9)