Page 1
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 protected for output faults and thermal overloads; circuitry providing output transistor protection is user programmable.
Equivalent Schematic and Connection Diagram
Features
Y
High Supply Voltage
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
December 1994
g
50V max
LM391 Audio Power Driver
Dual-In-Line Package
TL/H/7146– 1
Top View
TL/H/7146– 2
Order Number LM391N-100
See NS Package Number N16A
C
1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.
TL/H/7146
Page 2
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 ora100V
Input Voltage Supply Voltage less 5V
Shutdown Current (Pin 14) 1 mA
Electrical Characteristics T
e
25§C (The following are for V
A
Parameter Conditions Min Typ Max Units
Quiescent Current Current in Pin 15
LM391N-100 V
e
056
IN
Output Swing Positive V
Negative V
Drive Current Source (Pin 8) 5 mA
Sink (Pin 5) 5 mA
Noise (20 Hz–20 kHz) Input Referred 3 mV
Supply Rejection Input Referred 70 90 dB
Total Harmonic Distortion fe1 kHz 0.01 %
e
f
20 kHz 0.10 0.25 %
Intermodulation Distortion 60 Hz, 7 kHz, 4:1 0.01 %
Open Loop Gain fe1 kHz 1000 5500 V/V
Input Bias Current 0.1 1.0 mA
Input Offset Voltage 5 20 mV
Positive Current Limit V
Negative Current Limit V
BE
BE
Pin 10–9 650 mV
Pin 9–13 650 mV
Positive Current Limit Bias Current Pin 10 10 100 mA
Negative Current Limit Bias Current Pin 13 10 100 mA
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
C/W junction to ambient.
of 90
§
Package Dissipation (Note 1) 1.39W
Storage Temperature
b
65§Ctoa150§C
Operating Temperature 0§Ctoa70§C
Lead Temp. (Soldering, 10 sec.) 260§C
Thermal Resistance
i
JC
i
JA
a
e
90% V
a
MAX
and V
b
e
90% V
b
20§C/W
63§C/W
MAX
.)
mA
a
b
7V
b
a
7V
a
b
5V
b
a
5V
Typical Applications
FIGURE 1. LM391 with External ComponentsÐProtection Circuitry Not Shown
TL/H/7146– 3
2
Page 3
Typical Performance Characteristics
Output Power vs Supply Voltage Frequency (R
Total Harmonic Distortion vs
e
8X)
L
Input Referred Power Supply
Open Loop Gain vs Frequency Rejection vs Frequency
Total Harmonic Distortion vs
Frequency (R
e
4X)
L
Total Harmonic Distortion vs
AB Bias Current
TL/H/7146– 4
Pin Descriptions
Pin No. Pin Name Comments
1
2
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
6 BIAS Base of V
7 BIAS Collector of V
8 Source Output Drives output devices
9 Output Sense Biases the IC and is used in protection circuits
10
11
12
13
14 Shutdown Shuts off amplifier when current is pulled out of pin
15 V
16 V
a
Input Audio input
b
Input Feedback input
multiplier
BE
multiplier
BE
a
Current Limit Base of positive side protection circuit transistor
a
SOA Diode Diode used for dual slope SOA protection
b
SOA Diode Diode used for dual slope SOA protection
b
Current Limit Base of negative side protection circuit transistor
a
b
Positive supply
Negative supply
3
multiplier
BE
Page 4
External Components (Figure 1)
Component Typical Value Comments
C
IN
R
IN
R
f
2
R
f
1
C
f
C
C
R
A
R
B
C
AB
C
R
R
eb
R
O
C
O
R
E
R
TH
C2,C
X
L
Ê
2
1 mF Input coupling capacitor sets a low frequency pole with RIN.
1
e
f
L
2qRINC
IN
100k Sets input impedance and DC bias to input.
100k Feedback resistor; for minimum offset voltage at the output this should be equal to RIN.
5.1k Feedback resistor that works with R
R
f
A
e1a
V
2
R
f
1
to set the voltage gain.
f
2
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
1
e
f
L
2qR
C
f
f
1
.
f
1
5 pF Compensation capacitor. Sets gain bandwidth product and a high frequency pole.
1
e
GBW
2q5000C
Max fhfor stable design&500 kHz.
GBW
e
,f
h
A
C
V
3.9k AB bias resistor.
10k AB bias potentiometer. Adjust to set bias current in the output stage.
0.1 mF Bypass capacitor for bias. This improves high frequency distortion and transient response.
5 pF Ripple capacitor. This improves negative supply rejection at midband and high frequencies.
C
, if used, must equal CC.
R
100X Bleed resistor. This removes stored charge in output transistors.
2.7X Output compensation resistor. This resistor and COcompensate the output stage. This value
will vary slightly for different output devices.
0.1 mF Output compensation capacitor. This works with ROto form a zero that cancels fbof the
output power transistors.
0.3X Emitter degeneration resistor. This resistor gives thermal stability to the output stage
quiescent current. IRC PW5 type.
39k Shutdown resistor. Sets the amount of current pulled out of pin 14 during shutdown.
1000 pF Compensation capacitors for protection circuitry.
10Xll5 m H Used to isolate capacitive loads, usually 20 turns of wire wrapped around a 10X, 2W resistor.
4
Page 5
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:
e
V
Opeak
I
Add 5 volts to the peak output swing (V
voltage to get the supplies, i.e.,
of I
. The regulation of the supply determines the unload-
peak
ed voltage, usually about 15% higher. Supply voltage will
also rise 10% during high line conditions.
&
max supplies
g
(V
Opeak
The input sensitivity and output power specs determine the
required gain.
t
A
V
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 output. The bandwidth requirements determine the size of C
and CCas 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
The power transistors must be able to dissipate approxi-
Opeak
mately 40% of the maximum output power; the drivers must
dissipate this amount divided by the current gain of the outputs. See the output transistor selection guide, Table A.
2RLP
0
O
2P
O
e
Opeak
POR
0
R
0
L
) for transistor
OP
a
g
(V
5V) at a current
OP
a
5) (1aregulation) (1.1) (3)
V
L
ORMS
e
V
V
IN
INRMS
. Very high values
IN
with 5 mA of drive from the LM391.
(1)
(2)
(4)
To prevent thermal runaway of the AB bias current the following equation must be valid:
V
CEQMAX
MIN
a
1)
(K)
RE(b
s
i
JA
where:
is the thermal resistance of the driver transistor, junc-
i
JA
tion to ambient, in
C/W.
§
REis the emitter degeneration resistance in ohms.
b
is that of the output transistor.
min
V
equation (3).
is the highest possible value of one supply from
CEQMAX
K is the temperature coefficient of the driver base-emitter
voltage, typically 2 mV/
C.
§
Often the value of REis to be determined and equation (5)
is rearranged to be:
iJA(V
t
R
E
b
CEQMAX
a
MIN
)K
1
The maximum average power dissipation in each output
transistor is:
e
0.4 P
DMAX
OMAX
The power dissipation in the driver transistor is:
P
DMAX
P
DRIVER(MAX)
e
b
MIN
Heat sink requirements are found using the following formulas:
b
T
s
i
JA
s
i
SA
JMAX
JA
T
AMAX
P
D
b
b
i
i
JC
CS
where:
T
is the maximum transistor junction temperature.
f
jMAX
is the maximum ambient temperature.
T
AMAX
iJAis thermal resistance junction to ambient.
iSAis thermal resistance sink to ambient.
iJCis thermal resistance junction to case.
iCSis thermal resistance case to sink, typically 1§C/W for
most mountings.
(5)
(6)
(7)P
(8)
(9)
(10)i
5
Page 6
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 resistor formulas are described below. The diodes from the output to each supply prevent the output voltage from exceeding the supplies and harming the output transistors. The output 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 amplifier. 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
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
So:
e
R
a 30V rating.
e
2RC
R
30k. Solving for C gives 16.7 mF. Use Ce20 mF with
Protection Characteristics
e
Max V
1mA
a
TL/H/7146– 6
Protection Circuit Resistor Formulas (V
Type of Protection RE,R
Current Limit R
Single Slope SOA
Protection
Dual Slope SOA
Protection R
e
(V
Va)
B
Note: w is the current limit VBEvoltage, 650 mV. Assumptions: V
transistors.
Ê
w
e
E
I
L
w
e
R
E
I
L
w
e
E
I
L
TL/H/7146– 5
R
R
a
ll
w,V
a
e
)
V
B
R1,R
Ê
1
Not Required Short Not Required
b
V
w
M
e
R
1
2
w
#
b
V
w
M
e
R
1
2
w
#
ll
w.Vais the load supply voltage. VMis the maximum rated VCEof the output
M
6
R2,R
Ê
2
J
J
1kX Not Required
1kX R
R3,R
e
R
3
2
Ê
I
R
Ð
L
Ê
3
a
V
b
1
b
w
E
(
Page 7
Application Hints (Continued)
TRANSIENT INTERMODULATION DISTORTION
There has been a lot of interest in recent years about transient 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. Puta1MXresistor 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
Ý
1, and V
Ý
2 is disconnected.
IN
Typical Applications (Continued)
Bridge Circuit Diagram
OSCILLATIONS & GROUNDING
Most power amplifiers work the first time they are turned on.
They also tend to oscillate and have excess THD. Most oscillation 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 groundsÐbypass, 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
give about 25 mA of current in the output stage. This current
is usually monitored at the supply or by measuring the voltage across R
.
E
, from pins 6 –7 is adjusted to
B
Output Transistors Selection Guide
Table A.
Power
Output
20W@8X MJE711 MJE721 TIP42A TIP41A
@
30W
4X MJE171 MJE181 2N6490 2N6487
40W@8X MJE712 MJE722 2N5882 2N5880
60W@4X MJE172 MJE182
Driver Transistor Output Transistor
PNP NPN PNP NPN
D43C8 D42C8
D43C11 D42C11
7
TL/H/7146– 7
Page 8
Application Hints (Continued)
A 20W, 8X; 30W, 4X AMPLIFIER
Givens:
Power Output 20W into 8X
Input Sensitivity 1V Max
Input Impedance 100k
Bandwidth 20 Hz–20 kHz
Equations (1) and (2) give:
e
20W/8X V
30W/4X V
OP
OP
17.9V I
e
15.5V I
OP
OP
Therefore the supply required is:
g
23V@2.24A, reducing to . . .
g
21V@3.87A
With 15% regulation and high line we getg29V from equation (3).
Sensitivity and equation (4) set minimum gain:
20c8
0
t
A
V
e
12.65
1
We will use a gain of 20 with resulting sensitivity of 632 mV.
Letting R
For low DC offsets at the output we let R
for R
equal 100k gives the required input impedance.
IN
gives:
f
1
e
R
f
1
R
100k
20b1
e
100k
f
2
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. Therefore:
20
e
e
f
L
e
20kc5e100 kHz
f
h
4Hz
5
e
e
2.24A
3.87A
e
100k. Solving
f
2
30W into 4X
g
0.25 dB
Solving for C
The recommended value for C
larger. This gives a gain-bandwidth product of 6.4 MHz and
:
f
1
t
C
f
2qR
e
7.8 mF; use 10 mF
f
f
L
1
is 5 pF for gains of 20 or
C
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
are 60V, 60W transistors with a minimum beta of 30 at 4A.
, 3.87A. The TIP42, TIP41 complementary pair
Opeak
The driver transistor must supply the base drive given 5 mA
drive from the LM391. The MJE711, MJE721 complementary driver transistors are 60V devices with a minimum beta of
40 at 200 mA. The driver transistors should be much faster
(higher f
) than the output transistors to insure that the R-C
T
on the output will prevent instability.
To find the heat sink required for each output transistor we
use equations (7), (9), and (10):
s
i
JA
150§Cb55§C
D
12
s
7.9b2.1b1.0e4.8§C/W
SA
e
0.4 (30)e12W
e
7.9§C/W for T
AMAX
e
(7)P
55§C (9)
(10)i
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):
12
e
P
DRIVER(MAX)
e
400 mW
30
Using equation (9):
155b55
s
i
JA
0.4
e
237§C/W
8
Page 9
Application Hints (Continued)
Since the free air thermal resistance of the MJE711,
MJE721 is 100
information and equation (6) we can find the minimum value
of R
required to prevent thermal runaway.
E
We must now use the SOA data on the TIP42, TIP41 transistors 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
supply voltage, so we use the formulas in the table.
C/W, no heat sink is required. Using this
§
100 (30) (0.002)
t
R
E
30a1
D.C. SOA of TIP42, TIP41
Transistors
e
0.19X
B
is equal to the
TL/H/7146– 8
(6)
Typical Applications (Continued)
20W-8X, 30W-4X Amplifier with 1 Second Turn-ON Delay
The data points from the curve are:
Using the dual slope protection formulas:
Note that an R
schematic of this amplifier is below. If the output is shorted
the current will be 1.8A and V
AC, the average power is:
This power is greater than was used in the heat sink calculations, so the transistors will overheat for long-duration
shorts unless a larger heat sink is used.
e
V
M
R
e
R
3
short P
e
60V, V
1
1k
of 0.22X satisfies equation (6). The final
E
23V, I
B
0.65
e
R
E
3
R
2
60b0.65
e
1k
#
23
7(0.22)b0.65
#
e
(/2(1.8) (23)&21W
D
e
0.65
e
CE
L
0.22X
1k
Ê
e
e
3A, I
&
J
b
is 23V. Since the input is
7A
L
91k
&
1
24k
J
*Additional protection for LM391N; Schottky diodes and Rj100X.
9
TL/H/7146– 9
Page 10
Application Hints (Continued)
A 40W/8X, 60W/4X AMPLIFIER
Given:
Power Output 40W/8X
Input Sensitivity 1V Max
Input Impedance 100k
Bandwidth 20 Hz–20 kHz
Equations (1) and (2) give:
e
40W/8X V
60W/4X V
OPeak
OPeak
25.3V I
e
21.9V I
OPeak
OPeak
Therefore the supply required is:
g
30.3V@3.16A, reducing to . . .
g
26.9V@5.48A
With 15% regulation and high line we getg38.3V using
equation (3).
The minimum gain from equation (4) is:
t
A
18
V
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.
e
R
5.1k R
f
1
e
R
100k C
f
The maximum supplies dictate using 80V devices. The
2
e
IN
e
f
100k C
10 mF
2N5882, 2N5880 pair are 80V, 160W transistors with a minimum beta of 40 at 2A and 20 at 6A. This corresponds to a
minimum beta of 22.5 at 5.5A (I
MJE722 driver pair are 80V transistors with a minimum beta
Opeak
of 50 at 250 mA. This output combination guarantees I
with 5 mA from the LM391.
Output transistor heat sink requirements are found using
equations (7), (9), and (10):
e
0.4 (60)e24W
D
200b55
s
i
JA
e
C/W for T
6.0
24
s
6.0b1.1b1.0e3.9§C/W
SA
§
AMAX
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:
24
e
e
1.2W
20
e
79§C/W
i
P
JA
DRIVER
150b55
s
1.2
60W/4X
g
0.25 dB
e
3.16A
e
5.48A
e
5pF
C
). The MJE712,
Opeak
e
55§C
(10)i
(7)P
(9)
(8)
(9)
Since a heat sink is required on the driver, we should investigate the output stage thermal stability at the same time to
optimize the design. If we find a value of R
the protection circuitry, we can then use equation (5) to find
that is good for
E
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 protection line.
SOA 2N5882, 2N5880
TL/H/7146– 10
The desired data points are:
e
V
80V V
M
B
e
47V I
e
L
3A I
Ê
e
11A
L
Since the break voltage is not equal to the supply, we will
use two resistors to replace R
and move VB.
3
Circuit Used
TL/H/7146– 11
Thevenin Equivalent
A
Where: R
V
TL/H/7146– 12
e
R
R
3
ll
TH
A
R
3
b
e
V
TH
A
B
Ð
a
R
R
3
3
B
3
(
10
Page 11
Application Hints (Continued)
The formulas for R
e
1k R
R
2
The formula for R
mula becomes V
e
R
R
TH
2
Ð
e
1k
Ð
is the additional voltage added to the supply voltage to
V
TH
get V
.
B
eb
V
TH
Now we must find R
Putting V
duces to:
,Vb, and RTHinto the appropriate formulas re-
TH
B
e
R
0.76 R
3
, and R2do not change:
E,R1
0.65
e
e
R
E
e
1k
1
now gives RTHwhen the Vain the for-
3
.
B
V
B
Ê
b
I
R
w
L
E
47
11 (0.22)b0.65
b
(V
Va)eb(47b30)eb17V
B
A
and R
3
A
3
0.22X
3A
b
80
0.65
e
0.65
b
1
120k
(
b
e
1
25.55k
(
B
using the Thevenin formulas.
3
and 25.55keR
A
3
ll
Typical Applications (Continued)
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 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:
B
R
3
This is much larger than the power used to calculate the
heat sinks and the output transistors will overheat if the output is shorted too long.
40W-8X, 60W-4X Amplifier
0.22 (20a1)
s
i
JA
40 (0.002)
s
57b6b1e50§C/W
SA
e
short PD
A
e
62k, then R
3
&
57
§
(/2(28) 5e70W
C/W
B
e
47.12k;
3
(5)
(10)i
*High Frequency Ground
**Input Ground
***Speaker Ground
Note: All Grounds Should be Tied Together
Only at Power Supply Ground.
²
Additional protection for LM391N; Schottky diodes and Rj100X.
TL/H/7146– 13
11
Page 12
Physical Dimensions inches (millimeters)
LM391 Audio Power Driver
Molded Dual-In-Line Package (N)
Order Number LM391N-100
NS Package Number N16A
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into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life
failure to perform, when properly used in accordance support device or system, or to affect its safety or
with instructions for use provided in the labeling, can effectiveness.
be reasonably expected to result in a significant injury
to the user.
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