TL H 7146
LM391
Audio
Power
Driver
December 1994
LM391 Audio Power Driver
General Description
The LM391 audio power driver is designed to drive external
power transistors in 10 to 100 watt power amplifier designs
High power supply voltage operation and true high fidelity
performance distinguish this IC The LM391 is internally pro-
tected for output faults and thermal overloads circuitry pro-
viding output transistor protection is user programmable
Features
Y
High Supply Voltage
g
50V max
Y
Low Distortion
0 01%
Y
Low Input Noise
3 mV
Y
High Supply Rejection
90 dB
Y
Gain and Bandwidth Selectable
Y
Dual Slope SOA Protection
Y
Shutdown Pin
Equivalent Schematic and Connection Diagram
TL H 7146 1
Dual-In-Line Package
TL H 7146 2
Top View
Order Number LM391N-100
See NS Package Number N16A
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
LM391N-100
g
50V or
a
100V
Input Voltage
Supply Voltage less 5V
Shutdown Current (Pin 14)
1 mA
Package Dissipation (Note 1)
1 39W
Storage Temperature
b
65 C to
a
150 C
Operating Temperature
0 C to
a
70 C
Lead Temp (Soldering 10 sec )
260 C
Thermal Resistance
i
JC
20 C W
i
JA
63 C W
Electrical Characteristics
T
A
e
25 C (The following are for V
a
e
90% V
a
MAX
and V
b
e
90% V
b
MAX
)
Parameter
Conditions
Min
Typ
Max
Units
Quiescent Current
Current in Pin 15
mA
LM391N-100
V
IN
e
0
5
6
Output Swing
Positive
V
a
b
7
V
a
b
5
V
Negative
V
b
a
7
V
b
a
5
V
Drive Current
Source (Pin 8)
5
mA
Sink (Pin 5)
5
mA
Noise (20 Hz 20 kHz)
Input Referred
3
m
V
Supply Rejection
Input Referred
70
90
dB
Total Harmonic Distortion
f
e
1 kHz
0 01
%
f
e
20 kHz
0 10
0 25
%
Intermodulation Distortion
60 Hz 7 kHz 4 1
0 01
%
Open Loop Gain
f
e
1 kHz
1000
5500
V V
Input Bias Current
0 1
1 0
m
A
Input Offset Voltage
5
20
mV
Positive Current Limit V
BE
Pin 10 9
650
mV
Negative Current Limit V
BE
Pin 9 13
650
mV
Positive Current Limit Bias Current
Pin 10
10
100
m
A
Negative Current Limit Bias Current
Pin 13
10
100
m
A
Pin 14 Current Comments
Minimum pin 14 current required for shutdown is 0 5 mA and must not exceed 1 mA
Maximum pin 14 current for amplifier not shut down is 0 05 mA
The typical shutdown switch point current is 0 2 mA
Note 1
For operation in ambient temperatures above 25 C the device must be derated based on a 150 C maximum junction temperature and a thermal resistance
of 90 C W junction to ambient
Typical Applications
TL H 7146 3
FIGURE 1 LM391 with External Components
Protection Circuitry Not Shown
2
Typical Performance Characteristics
Output Power vs Supply Voltage
Frequency (R
L
e
8X)
Total Harmonic Distortion vs
Frequency (R
L
e
4X)
Total Harmonic Distortion vs
Open Loop Gain vs Frequency
Rejection vs Frequency
Input Referred Power Supply
AB Bias Current
Total Harmonic Distortion vs
TL H 7146 4
Pin Descriptions
Pin No
Pin Name
Comments
1
a
Input
Audio input
2
b
Input
Feedback input
3
Compensation
Sets the dominant pole
4
Ripple Filter
Improves negative supply rejection
5
Sink Output
Drives output devices and is emitter of AB bias V
BE
multiplier
6
BIAS
Base of V
BE
multiplier
7
BIAS
Collector of V
BE
multiplier
8
Source Output
Drives output devices
9
Output Sense
Biases the IC and is used in protection circuits
10
a
Current Limit
Base of positive side protection circuit transistor
11
a
SOA Diode
Diode used for dual slope SOA protection
12
b
SOA Diode
Diode used for dual slope SOA protection
13
b
Current Limit
Base of negative side protection circuit transistor
14
Shutdown
Shuts off amplifier when current is pulled out of pin
15
V
a
Positive supply
16
V
b
Negative supply
3
External Components
(Figure 1)
Component
Typical Value
Comments
C
IN
1 mF
Input coupling capacitor sets a low frequency pole with R
IN
f
L
e
1
2
q
R
IN
C
IN
R
IN
100k
Sets input impedance and DC bias to input
R
f2
100k
Feedback resistor for minimum offset voltage at the output this should be equal to R
IN
R
f1
5 1k
Feedback resistor that works with R
f2
to set the voltage gain
A
V
e
1
a
R
f2
R
f1
C
f
10 mF
Feedback capacitor This reduces the gain to unity at DC for minimum offset voltage at the
output Also sets a low frequency pole with R
f1
f
L
e
1
2
q
R
f1
C
f
C
C
5 pF
Compensation capacitor Sets gain bandwidth product and a high frequency pole
GBW
e
1
2
q
5000C
C
f
h
e
GBW
A
V
Max f
h
for stable design
500 kHz
R
A
3 9k
AB bias resistor
R
B
10k
AB bias potentiometer Adjust to set bias current in the output stage
C
AB
0 1 mF
Bypass capacitor for bias This improves high frequency distortion and transient response
C
R
5 pF
Ripple capacitor This improves negative supply rejection at midband and high frequencies
C
R
if used must equal C
C
R
eb
100X
Bleed resistor This removes stored charge in output transistors
R
O
2 7X
Output compensation resistor This resistor and C
O
compensate the output stage This value
will vary slightly for different output devices
C
O
0 1 mF
Output compensation capacitor This works with R
O
to form a zero that cancels f
b
of the
output power transistors
R
E
0 3X
Emitter degeneration resistor This resistor gives thermal stability to the output stage
quiescent current IRC PW5 type
R
TH
39k
Shutdown resistor Sets the amount of current pulled out of pin 14 during shutdown
C
2
C
2
1000 pF
Compensation capacitors for protection circuitry
X
L
10X
ll
5 mH
Used to isolate capacitive loads usually 20 turns of wire wrapped around a 10X 2W resistor
4
Application Hints
GENERALIZED AUDIO POWER AMP DESIGN
Givens
Power Output
Load Impedance
Input Sensitivity
Input Impedance
Bandwidth
The power output and load impedance determine the power
supply requirements Output signal swing and current are
found from
V
Opeak
e
0
2 R
L
P
O
(1)
I
Opeak
e
0
2 P
O
R
L
(2)
Add 5 volts to the peak output swing (V
OP
) for transistor
voltage to get the supplies i e
g
(V
OP
a
5V) at a current
of I
peak
The regulation of the supply determines the unload-
ed voltage usually about 15% higher Supply voltage will
also rise 10% during high line conditions
max supplies
g
(V
Opeak
a
5) (1
a
regulation) (1 1)
(3)
The input sensitivity and output power specs determine the
required gain
A
V
t
0
P
O
R
L
V
IN
e
V
ORMS
V
INRMS
(4)
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
IN
Very high values
can cause board layout problems and DC offsets at the out-
put The bandwidth requirements determine the size of C
f
and C
C
as indicated in the external component listing
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
Opeak
with 5 mA of drive from the LM391
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
To prevent thermal runaway of the AB bias current the fol-
lowing equation must be valid
i
JA
s
R
E
(b
MIN
a
1)
V
CEQMAX
(K)
(5)
where
i
JA
is the thermal resistance of the driver transistor junc-
tion to ambient in C W
R
E
is the emitter degeneration resistance in ohms
b
min
is that of the output transistor
V
CEQMAX
is the highest possible value of one supply from
equation (3)
K is the temperature coefficient of the driver base-emitter
voltage typically 2 mV C
Often the value of R
E
is to be determined and equation (5)
is rearranged to be
R
E
t
i
JA
(V
CEQMAX
) K
b
MIN
a
1
(6)
The maximum average power dissipation in each output
transistor is
(7)
P
DMAX
e
0 4 P
OMAX
The power dissipation in the driver transistor is
P
DRIVER(MAX)
e
P
DMAX
b
MIN
(8)
Heat sink requirements are found using the following formu-
las
i
JA
s
T
JMAX
b
T
AMAX
P
D
(9)
(10)
i
SA
s
i
JA
b
i
JC
b
i
CS
where
T
jMAX
is the maximum transistor junction temperature
T
AMAX
is the maximum ambient temperature
i
JA
is thermal resistance junction to ambient
i
SA
is thermal resistance sink to ambient
i
JC
is thermal resistance junction to case
i
CS
is thermal resistance case to sink typically 1 C W for
most mountings
5
Application Hints
(Continued)
PROTECTION CIRCUITRY
The protection circuits of the LM391 are very flexible and
should be tailored to the output transistor's safe operating
area The protection V-I characteristics circuitry and resis-
tor formulas are described below The diodes from the out-
put to each supply prevent the output voltage from exceed-
ing the supplies and harming the output transistors The out-
put will do this if the protection circuitry is activated while
driving an inductive load
TURN-ON DELAY
It is often desirable to delay the turn-ON of the power ampli-
fier This is easily implemented by putting a resistor in series
with a capacitor from pin 14 to ground The value of the
Protection Circuitry with External Components
TL H 7146 5
resistor is set to limit the current to less than 1 mA (the
absolute maximum) This resistor with the capacitor gives a
time constant of RC The turn-ON delay is approximately 2
time constants
Example
Amplifier with maximum supply of 30V like the 20W 8X
example in the data sheet requiring a delay of 1 second
Time delay
e
2 RC
R
e
Max V
a
1 mA
So
R
e
30k Solving for C gives 16 7 mF Use C
e
20 mF with
a 30V rating
Protection Characteristics
TL H 7146 6
Protection Circuit Resistor Formulas (V
B
e
V
a
)
Type of Protection
R
E
R
R
1
R
1
R
2
R
2
R
3
R
3
Current Limit
R
E
e
w
I
L
Not Required
Short
Not Required
Single Slope SOA
R
E
e
w
I
L
R
1
e
R
2
V
M
b
w
w
J
1 kX
Not Required
Protection
Dual Slope SOA
Protection
R
E
e
w
I
L
R
1
e
R
2
V
M
b
w
w
J
1 kX
R
3
e
R
2
V
a
I
L
R
E
b
w
b
1
(
(V
B
e
V
a
)
Note w
is the current limit V
BE
voltage 650 mV Assumptions V
a
l l
w
V
M
l l
w
V
a
is the load supply voltage V
M
is the maximum rated V
CE
of the output
transistors
6
Application Hints
(Continued)
TRANSIENT INTERMODULATION DISTORTION
There has been a lot of interest in recent years about tran-
sient intermodulation distortion Matti Otala of University of
Oulu Oulu Finland has published several papers on the
subject The results of these investigations show that the
open loop pole of the power amplifier should be above 20
kHz
To do this with the LM391 is easy Put a 1 MX resistor from
pin 3 to the output and the open loop gain is reduced to
about 46 dB Now the open loop pole is at 30 kHz The
current in this resistor causes an offset in the input stage
that can be cancelled with a resistor from pin 4 to ground
The resistor from pin 4 to ground should be 910 kX rather
than 1 MX to insure that the shutdown circuitry will operate
correctly The slight difference in resistors results in about
15 mV of offset The 40W 8X amplifier schematic shows
the hookup of these two resistors
BRIDGE AMPLIFIER
A switch can be added to convert a stereo amplifer to a
single bridge amplifer The diagram below shows where the
switch and one resistor are added When operating in the
bridge mode the output load is connected between the two
outputs the input is V
IN
1 and V
IN
2 is disconnected
OSCILLATIONS
GROUNDING
Most power amplifiers work the first time they are turned on
They also tend to oscillate and have excess THD Most os-
cillation problems are due to inadequate supply bypassing
and or ground loops A 10 mF 50V electrolytic on each
power supply will stop supply-related oscillations However
if the signal ground is used for these bypass caps the THD
is usually excessive The signal ground must return to the
power supply alone as must the output load ground All
other grounds
bypass output R-C protection etc can tie
together and then return to supply This ground is called
high frequency ground On the 40W amplifier schematic all
the grounds are labeled
Capacitive loads can cause instabilities so they are isolated
from the amplifier with an inductor and resistor in the output
lead
AB BIAS CURRENT
To reduce distortion in the output stage all the transistors
are biased ON slightly This results in class AB operation
and reduces the crossover (notch) distortion of the class B
stage to a low level (see performance curve THD vs AB
bias) The potentiometer R
B
from pins 6 7 is adjusted to
give about 25 mA of current in the output stage This current
is usually monitored at the supply or by measuring the volt-
age across R
E
Typical Applications
(Continued)
Bridge Circuit Diagram
TL H 7146 7
Output Transistors Selection Guide
Table A
Power
Driver Transistor
Output Transistor
Output
PNP
NPN
PNP
NPN
20W
8X
MJE711
MJE721
TIP42A
TIP41A
30W
4X
MJE171
MJE181
2N6490
2N6487
D43C8
D42C8
40W
8X
MJE712
MJE722
2N5882
2N5880
60W
4X
MJE172
MJE182
D43C11
D42C11
7
Application Hints
(Continued)
A 20W 8X 30W 4X AMPLIFIER
Givens
Power Output
20W into 8X
30W into 4X
Input Sensitivity
1V Max
Input Impedance
100k
Bandwidth
20 Hz 20 kHz
g
0 25 dB
Equations (1) and (2) give
20W 8X
V
OP
e
17 9V
I
OP
e
2 24A
30W 4X
V
OP
e
15 5V
I
OP
e
3 87A
Therefore the supply required is
g
23V
2 24A reducing to
g
21V
3 87A
With 15% regulation and high line we get
g
29V from equa-
tion (3)
Sensitivity and equation (4) set minimum gain
A
V
t
0
20
c
8
1
e
12 65
We will use a gain of 20 with resulting sensitivity of 632 mV
Letting R
IN
equal 100k gives the required input impedance
For low DC offsets at the output we let R
f2
e
100k Solving
for R
f1
gives
R
f2
e
100k
R
f1
e
100k
20
b
1
e
5 26k use 5 1k
The bandwidth requirement must be stated as a pole i e
the 3 dB frequency Five times away from a pole gives 0 17
dB down which is better than the required 0 25 dB There-
fore
f
L
e
20
5
e
4 Hz
f
h
e
20k
c
5
e
100 kHz
Solving for C
f
C
f
t
1
2
q
R
f1
f
L
e
7 8 mF use 10 mF
The recommended value for C
C
is 5 pF for gains of 20 or
larger This gives a gain-bandwidth product of 6 4 MHz and
a resulting bandwidth of 320 kHz better than required
The breakdown voltage requirement is set by the maximum
supply we need a minimum of 58V and will use 60V We
must now select a 60V power transistor with reasonable
beta at I
Opeak
3 87A The TIP42 TIP41 complementary pair
are 60V 60W transistors with a minimum beta of 30 at 4A
The driver transistor must supply the base drive given 5 mA
drive from the LM391 The MJE711 MJE721 complementa-
ry driver transistors are 60V devices with a minimum beta of
40 at 200 mA The driver transistors should be much faster
(higher f
T
) than the output transistors to insure that the R-C
on the output will prevent instability
To find the heat sink required for each output transistor we
use equations (7) (9) and (10)
(7)
P
D
e
0 4 (30)
e
12W
i
JA
s
150 C
b
55 C
12
e
7 9 C W for T
AMAX
e
55 C
(9)
(10)
i
SA
s
7 9
b
2 1
b
1 0
e
4 8 C W
If both transistors are mounted on one heat sink the thermal
resistance should be halved to 2 4 C W
The maximum average power dissipation in each driver is
found using equation (8)
P
DRIVER(MAX)
e
12
30
e
400 mW
Using equation (9)
i
JA
s
155
b
55
0 4
e
237 C W
8
Application Hints
(Continued)
Since the free air thermal resistance of the MJE711
MJE721 is 100 C W no heat sink is required Using this
information and equation (6) we can find the minimum value
of R
E
required to prevent thermal runaway
R
E
t
100 (30) (0 002)
30
a
1
e
0 19X
(6)
We must now use the SOA data on the TIP42 TIP41 tran-
sistors to set up the protection circuit Below is the SOA
curve with the 4X and 8X load lines Also shown are the
desired protection lines Note the value of V
B
is equal to the
supply voltage so we use the formulas in the table
D C SOA of TIP42 TIP41
Transistors
TL H 7146 8
The data points from the curve are
V
M
e
60V V
B
e
23V I
L
e
3A I
L
e
7A
Using the dual slope protection formulas
R
E
e
0 65
3
e
0 22X
R
2
e
1k
R
1
e
1k
60
b
0 65
0 65
J
91k
R
3
e
1k
23
7(0 22)
b
0 65
b
1
J
24k
Note that an R
E
of 0 22X satisfies equation (6) The final
schematic of this amplifier is below If the output is shorted
the current will be 1 8A and V
CE
is 23V Since the input is
AC the average power is
short P
D
e
(1 8) (23)
21W
This power is greater than was used in the heat sink calcula-
tions so the transistors will overheat for long-duration
shorts unless a larger heat sink is used
Typical Applications
(Continued)
20W-8X 30W-4X Amplifier with 1 Second Turn-ON Delay
TL H 7146 9
Additional protection for LM391N Schottky diodes and R j 100X
9
Application Hints
(Continued)
A 40W 8X 60W 4X AMPLIFIER
Given
Power Output
40W 8X
60W 4X
Input Sensitivity
1V Max
Input Impedance
100k
Bandwidth
20 Hz 20 kHz
g
0 25 dB
Equations (1) and (2) give
40W 8X
V
OPeak
e
25 3V
I
OPeak
e
3 16A
60W 4X
V
OPeak
e
21 9V
I
OPeak
e
5 48A
Therefore the supply required is
g
30 3V
3 16A reducing to
g
26 9V
5 48A
With 15% regulation and high line we get
g
38 3V using
equation (3)
The minimum gain from equation (4) is
A
V
t
18
We select a gain of 20 resulting sensitivity is 900 mV
The input impedance and bandwidth are the same as the 20
watt amplifier so the components are the same
R
f1
e
5 1k
R
IN
e
100k
C
C
e
5 pF
R
f2
e
100k
C
f
e
10 mF
The maximum supplies dictate using 80V devices The
2N5882 2N5880 pair are 80V 160W transistors with a mini-
mum beta of 40 at 2A and 20 at 6A This corresponds to a
minimum beta of 22 5 at 5 5A (I
Opeak
)
The MJE712
MJE722 driver pair are 80V transistors with a minimum beta
of 50 at 250 mA This output combination guarantees I
Opeak
with 5 mA from the LM391
Output transistor heat sink requirements are found using
equations (7) (9) and (10)
(7)
P
D
e
0 4 (60)
e
24W
i
JA
s
200
b
55
24
e
6 0 C W for T
AMAX
e
55 C
(9)
(10)
i
SA
s
6 0
b
1 1
b
1 0
e
3 9 C W
For both output transistors on one heat sink the thermal
resistance should be 1 9 C W
Now using equation (8) we find the power dissipation in the
driver
P
DRIVER
e
24
20
e
1 2W
(8)
i
JA
s
150
b
55
1 2
e
79 C W
(9)
Since a heat sink is required on the driver we should inves-
tigate the output stage thermal stability at the same time to
optimize the design If we find a value of R
E
that is good for
the protection circuitry we can then use equation (5) to find
the heat sink required for the drivers
The SOA characteristics of the 2N5882 2N5880 transistors
are shown in the following curve along with a desired pro-
tection line
SOA 2N5882 2N5880
TL H 7146 10
The desired data points are
V
M
e
80V
V
B
e
47V
I
L
e
3A
I
L
e
11A
Since the break voltage is not equal to the supply we will
use two resistors to replace R
3
and move V
B
Circuit Used
TL H 7146 11
Thevenin Equivalent
Where R
TH
e
R
A
3
ll
R
B
3
V
TH
e
V
b
R
A
3
R
A
3
a
R
B
3
(
TL H 7146 12
10
Application Hints
(Continued)
The formulas for R
E
R
1
and R
2
do not change
R
E
e
0 65
3A
e
0 22X
R
2
e
1k
R
1
e
1k
80
b
0 65
0 65
e
120k
The formula for R
3
now gives R
TH
when the V
a
in the for-
mula becomes V
B
R
TH
e
R
2
V
B
I
L
R
E
b
w
b
1
(
e
1k
47
11 (0 22)
b
0 65
b
1
(
e
25 55k
V
TH
is the additional voltage added to the supply voltage to
get V
B
V
TH
e b
(V
B
b
V
a
)
e b
(47
b
30)
e b
17V
Now we must find R
A
3
and R
B
3
using the Thevenin formulas
Putting V
TH
V
b
and R
TH
into the appropriate formulas re-
duces to
R
B
3
e
0 76 R
A
3
and
25 55k
e
R
A
3
ll
R
B
3
The easiest way to solve these equations is to iterate with
standard values If we guess R
A
3
e
62k then R
B
3
e
47 12k
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
i
JA
s
0 22 (20
a
1)
40 (0 002)
57 C W
(5)
This value is lower than we got with equation (9) so we will
use it in equation (10)
(10)
i
SA
s
57
b
6
b
1
e
50 C W
This is the required heat sink for each driver For low TIM
we add the 1 MX resistor from pin 3 to the output and a
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
short PD
e
(28) 5
e
70W
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
Typical Applications
(Continued)
40W-8X 60W-4X Amplifier
High Frequency Ground
Input Ground
Speaker Ground
TL H 7146 13
Note All Grounds Should be Tied Together
Only at Power Supply Ground
Additional protection for LM391N Schottky diodes and R j 100X
11
LM391
Audio
Power
Driver
Physical Dimensions
inches (millimeters)
Molded Dual-In-Line Package (N)
Order Number LM391N-100
NS Package Number N16A
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with instructions for use provided in the labeling can
effectiveness
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