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LM1896/LM2896 Dual Audio Power Amplifier
LM1896/LM2896 Dual Audio Power Amplifier
February 1995
General Description
The LM1896 is a high performance 6V stereo power amplifier designed to deliver 1 watt/channel into 4X or 2 watts
bridged monaural into 8X. Utilizing a unique patented compensation scheme, the LM1896 is ideal for sensitive AM
radio applications. This new circuit technique exhibits lower
wideband noise, lower distortion, and less AM radiation than
conventional designs. The amplifier’s wide supply range
(3V–9V) is ideal for battery operation. For higher supplies
l
(V
9V) the LM2896 is available in an 11-lead single-in-
S
line package. The LM2896 package has been redesigned,
resulting in the slightly degraded thermal characteristics
shown in the figure Device Dissipation vs Ambient Temperature.
Typical Applications
Features
Y
Low AM radiation
Y
Low noise
Y
3V, 4X, stereo P
Y
Wide supply operation 3V–15V (LM2896)
Y
Low distortion
Y
No turn on ‘‘pop’’
Y
Adjustable voltage gain and bandwidth
Y
Smooth waveform clipping
Y
e
P
9W bridged, LM2896
o
o
e
250 mW
Applications
Y
Compact AM-FM radios
Y
Stereo tape recorders and players
Y
High power portable stereos
FIGURE 1. LM2896 in Bridge Configuration (A
Order Number LM1896N Order Number LM2896P
e
400, BWe20 kHz)
V
TL/H/7920– 1
See NS Package Number N14A See NS Package Number P11A
C
1995 National Semiconductor Corporation RRD-B30M115/Printed in U. S. A.
TL/H/7920
Absolute Maximum Ratings
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage
LM1896 V
LM2896 V
Operating Temperature (Note 1) 0§Ctoa70§C
Storage Temperature
b
e
12V
S
e
18V
S
65§Ctoa150§C
Junction Temperature 150
Lead Temperature (Soldering, 10 sec.) 260§C
Thermal Resistance
i
(DIP) 30§C/W
JC
i
(DIP) 137§C/W
JA
i
(SIP) 10§C/W
JC
i
(SIP) 55§C/W
JA
C
§
Electrical Characteristics
Unless otherwise specified, T
e
T
25§C, V
TAB
Parameter Conditions
e
S
12V and R
e
25§C, A
A
e
L
V
8X. Test circuit shown in
e
200 (46 dB). For the LM1896; V
Figure 2
.
S
e
6V and R
e
4X. For LM2896,
L
LM1896 LM2896
Units
Min Typ Max Min Typ Max
Supply Current P
e
0W, Dual Mode 15 25 25 40 mA
o
Operating Supply Voltage 3 10 3 15 V
Output Power THDe10%, fe1 kHz
LM1896N-1 V
LM1896N-2 V
LM2896P-1 V
LM2896P-2 V
e
S
e
S
e
V
S
e
S
e
S
e
V
S
e
V
S
6V, R
6V, R
9V, R
12V, R
12V, R
9V, R
9V, R
e
4X Dual Mode 0.9 1.1 W/ch
L
e
8X Bridge Mode T
L
e
8X Dual Mode ( 1.3 W/ch
L
e
8X Dual Mode 2.0 2.5 W/ch
L
e
8X Bridge Mode
L
e
4X Bridge Mode 7.8 W
L
e
4X Dual Mode * 2.5 W/ch
L
e
25§C 1.8 2.1 W
A
e
T
25§C
TAB
7.2 9.0 W
Distortion fe1 kHz
e
P
50 mW 0.09 0.09 %
o
e
P
0.5W 0.11 0.11 %
o
e
P
1W 0.14 %
o
Power Supply Rejection C
Ratio (PSRR) Output Referred, V
Channel Separation C
e
100 mF, fe1 kHz, C
BY
e
100 mF, fe1 kHz, C
BY
Output Referred
RIPPLE
Noise Equivalent Input Noise R
e
C
0.1 mF, BWe20b20 kHz 1.4 1.4 mV
IN
CCIR/ARM 1.4 1.4 mV
S
IN
e
IN
e
e
250 mV
e
0,
0.1 mF
0.1 mF
b40b
b50b
54
64
b40b
b50b
54 dB
64 dB
Wideband 2.0 2.0 mV
DC Output Level 2.8 3 3.2 5.6 6 6.4 V
Input Impedance 50 100 350 50 100 350 kX
Input Offset Voltage 5 5 mV
Voltage Difference LM1896N-2, LM2896P-2
between Outputs
10 20 10 20 mV
Input Bias Current 120 120 nA
Note 1: For operation at ambient temperature greater than 25§C, the LM1896/LM2896 must be derated based on a maximum 150§C junction temperature using a
thermal resistance which depends upon mounting techniques.
2
Typical Performance Curves
LM2896 Device Dissipation
vs Ambient Temperature
LM1896 Maximum Device
Dissipation vs Ambient
Temperature
b
3 dB Bandwidth vs Voltage
Gain for Stable Operation
THD and Gain vs Frequency
e
A
54 dB, BWe30 kHz
V
THD and Gain vs Frequency
e
A
40 dB, BWe20 kHz
V
Power Supply Rejection Ratio
(Referred to the Output)
vs Frequency
THD and Gain vs Frequency
e
A
54 dB, BWe5 kHz
V
THD and Gain vs Frequency
e
A
34 dB, BWe50 kHz
V
Channel Separation (Referred
to the Output) vs Frequency
THD and Gain vs Frequency
e
A
46 dB, BWe50 kHz
V
AM Recovered Audio and Noise
vs Field Strength for Different
Speaker Lead Placement
Power Output vs
Supply Voltage
TL/H/7920– 2
3
Typical Performance Curves (Continued)
Total Harmonic Distortion
vs Power Output
Power Dissipation vs
Power Output R
Equivalent Schematic
e
4X
L
Power Dissipation vs
Power Output R
e
L
TL/H/7920– 3
8X
6, 9 No connection on LM1896 TL/H/7920– 4
( ) indicates pin number for LM2896
Connection Diagrams
Single-In-Line Package
Dual-In-Line Package
Top View
TL/H/7920– 5
TL/H/7920– 6
Top View
4
Typical Applications (Continued)
6, 9 No connection on LM1896
( ) Indicates pin number for LM2896
FIGURE 2. Stereo Amplifier with A
TL/H/7920– 7
e
200, BWe30 kHz
V
TL/H/7920– 8
External Components
Components Comments
1. R2, R5, R10, R13 Sets voltage gain, A
2. R3, R12 Bootstrap resistor sets drive current for output stage and allows pins 3 and 12 to
3. R
o
4. C1, C14 Input coupling capacitor. Pins 1 and 14 are at a DC potential of V
5. C2, C13 Feedback capacitors. Ensure unity gain at DC. Also a low frequency pole at:
6. C3, C12 Bootstrap capacitors, used to increase drive to output stage. A low frequency
7. C5, C10 Compensation capacitor. These stabilize the amplifiers and adjust their
8. C7 Improves power supply rejection (See Typical Performance Curves). Increasing
9. C
c
10. C
o
11. C
S
(Figure 2)
e1a
R5/R2 for one channel and A
for the other channel.
go above V
Works with Coto stabilize output stage.
V
.
S
frequency pole set by:
1
e
f
L
2q RINC1
1
e
f
L
2qR2C2
V
e1a
S
R10/R13
/2. Low
pole is set by:
1
e
f
L
2qR3C3
bandwidth. See curve of bandwidth vs allowable gain.
C7 increases turn-on delay.
Output coupling capacitor. Isolates pins 5 and 10 from the load. Low frequency
pole set by:
1
e
f
L
2q CcR
Works with Roto stabilize output stage.
L
Provides power supply filtering.
5
Application Hints
AM Radios
The LM1896/LM2896 has been designed fo fill a wide
range of audio power applications. A common problem with
IC audio power amplifiers has been poor signal-to-noise performance when used in AM radio applications. In a typical
radio application, the loopstick antenna is in close proximity
to the audio amplifer. Current flowing in the speaker and
power supply leads can cause electromagnetic coupling to
the loopstick, resulting in system oscillation. In addition,
most audio power amplifiers are not optimized for lowest
noise because of compensation requirements. If noise from
the audio amplifier radiates into the AM section, the sensitivity and signal-to-noise ratio will be degraded.
The LM1896 exhibits extremely low wideband noise due in
part to an external capacitor C5 which is used to tailor the
bandwidth. The circuit shown in
signal-to-noise ratio in excess of 60 dB referred to 50 mW.
Capacitor C5 not only limits the closed loop bandwidth, it
also provides overall loop compensation. Neglecting C2 in
Figure 2
, the gain is:
(S)
A
V
R2aR5
e
V
3dBBW(0o)vsAVis shown in the Typical
A curve of
where A
b
Performance Curves.
Figure 3
shows a plot of recovered audio as a function of
field strength in mV/M. The receiver section in this example
is an LM3820. The power amplifier is located about two
inches from the loopstick antenna. Speaker leads run parallel to the loopstick and are 1/8 inch from it. Referenced to a
20 dB S/N ratio, the improvement in noise performance
over conventional designs is about 10 dB. This corresponds
to an increase in usable sensitivity of about 8.5 dB.
Bridge Amplifiers
The LM1896/LM2896 can be used in the bridge mode as a
monaural power amplifier. In addition to much higher power
output, the bridge configuration does not require output coupling capacitors. The load is connected directly between the
amplifier outputs as shown in
e
R2
SaA
a
S
Figure 4
Figure 2
V0o
0
o
,
0
o
.
is capable of a
1
e
R5C5
Amp 1 has a voltage gain set by 1
a
R5/R2. The output of
amp 1 drives amp 2 which is configured as an inverting
amplifier with unity gain. Because of this phase inversion in
amp 2, there isa6dBincrease in voltage gain referenced to
V
. The voltage gain in bridge is:
i
V
o
e
V
i
2#1
R5
a
R2
J
CBis used to prevent DC voltage on the output of amp 1
from causing offset in amp 2. Low frequency response is
influenced by:
1
e
f
L
2q RBC
B
Several precautions should be observed when using the
LM1896/LM2896 in bridge configuration. Because the amplifiers are driving the load out of phase, an 8X speaker will
appear as a 4X load, and a 4X speaker will appear as a 2X
load. Power dissipation is twice as severe in this situation.
For example, if V
maximum dissipation is:
e
6V and R
S
2
V
S
e
P
D
20 R
L
P
D
c
e
0.9 Watts
e
8X bridged, then the
L
2
6
e
2
20c4
c
2
This amount of dissipation is equivalent to driving two 4X
loads in the stereo configuration.
When adjusting the frequency response in the bridge configuration, R5C5 and R10C10 form a 2 pole cascade and the
b
3 dB bandwidth is actually shifted to a lower frequency:
0.707
e
BW
2qRC
where Refeedback resistor
Cefeedback capacitor
To measure the output voltage, a floating or differential meter should be used because a prolonged output short will
over dissipate the package.
Figure 1
shows the complete
bridge amplifier.
FIGURE 3. Improved AM Sensitivity over Conventional Design
TL/H/7920– 9
6
Application Hints (Continued)
Figure 4. Bridge Amplifier Connection
Printed Circuit Layout
Printed Circuit Board Layout
Figure 5
and
Figure 6
the LM1896 and LM2896. The circuits are wired as stereo
amplifiers. The signal source ground should return to the
input ground shown on the boards. Returning the loads to
power supply ground through a separate wire will keep the
THD at its lowest value. The inputs should be terminated in
show printed circuit board layouts for
TL/H/7920– 10
less than 50 kX to prevent an input-output oscillation. This
oscillation is dependent on the gain and the proximity of the
bridge elements R
mode is not used, do not insert R
To wire the amplifer into the bridge configuration, short the
capacitor on pin 7 (pin 1 of the LM1896) to ground. Connect
together the nodes labeled BRIDGE and drive the capacitor
connected to pin 5 (pin 14 of the LM1896).
and CBto the (a) input. If the bridge
B
B,CB
into the PCB.
FIGURE 5. Printed Circuit Board Layout for the LM1896
7
TL/H/7920– 11
Printed Circuit Layout (Continued)
FIGURE 6. Printed Circuit Board Layout for the LM2896
8
TL/H/7920– 12
Physical Dimensions inches (millimeters)
Molded Dual-In-Line Package (N)
Order Number LM1896N
See NS Package Number N14A
9
Physical Dimensions inches (millimeters) (Continued)
LM1896/LM2896 Dual Audio Power Amplifier
Single-In-Line Package (P)
Order Number LM2896P
NS Package Number P11A
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systems which, (a) are intended for surgical implant support device or system whose failure to perform can
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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Corporation Europe Hong Kong Ltd. Japan Ltd.
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