NSC LM2896P

LM1896/LM2896 Dual Audio Power Amplifier
General Description
Features
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
(VS l 9V) the LM2896 is available in an 11-lead single-inline package. The LM2896 package has been redesigned,
resulting in the slightly degraded thermal characteristics
shown in the figure Device Dissipation vs Ambient Temperature.
Y
Y
Y
Y
Y
Y
Y
Y
Y
Low AM radiation
Low noise
3V, 4X, stereo Po e 250 mW
Wide supply operation 3V – 15V (LM2896)
Low distortion
No turn on ‘‘pop’’
Adjustable voltage gain and bandwidth
Smooth waveform clipping
Po e 9W bridged, LM2896
Applications
Y
Y
Y
Compact AM-FM radios
Stereo tape recorders and players
High power portable stereos
Typical Applications
TL/H/7920 – 1
FIGURE 1. LM2896 in Bridge Configuration (AV e 400, BW e 20 kHz)
Order Number LM1896N
Order Number LM2896P
See NS Package Number N14A
See NS Package Number P11A
C1995 National Semiconductor Corporation
TL/H/7920
RRD-B30M115/Printed in U. S. A.
LM1896/LM2896 Dual Audio Power Amplifier
February 1995
Absolute Maximum Ratings
Supply Voltage
LM1896
LM2896
Operating Temperature (Note 1)
Storage Temperature
150§ C
260§ C
Junction Temperature
Lead Temperature (Soldering, 10 sec.)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Thermal Resistance
iJC (DIP)
iJA (DIP)
iJC (SIP)
iJA (SIP)
VS e 12V
VS e 18V
0§ C to a 70§ C
b 65§ C to a 150§ C
30§ C/W
137§ C/W
10§ C/W
55§ C/W
Electrical Characteristics
Unless otherwise specified, TA e 25§ C, AV e 200 (46 dB). For the LM1896; VS e 6V and RL e 4X. For LM2896,
TTAB e 25§ C, VS e 12V and RL e 8X. Test circuit shown in Figure 2 .
Parameter
LM1896
Conditions
Min
Supply Current
Po e 0W, Dual Mode
Operating Supply Voltage
Output Power
LM1896N-1
LM1896N-2
LM2896P-1
LM2896P-2
Distortion
Typ
Max
15
25
3
THD e 10%, f e 1 kHz
VS e 6V, RL e 4X Dual Mode
VS e 6V, RL e 8X Bridge Mode
VS e 9V, RL e 8X Dual Mode
VS e 12V, RL e 8X Dual Mode
VS e 12V, RL e 8X Bridge Mode
VS e 9V, RL e 4X Bridge Mode
VS e 9V, RL e 4X Dual Mode
0.9
(
TA e 25§ C
LM2896
10
1.1
1.8
1.3
Max
25
40
mA
15
V
3
2.0
7.2
f e 1 kHz
Po e 50 mW
Po e 0.5W
Po e 1W
Units
Typ
2.5
9.0
7.8
2.5
W/ch
W
W/ch
W/ch
W
W
W/ch
0.09
0.11
0.14
%
%
%
2.1
TTAB e 25§ C
*
Min
0.09
0.11
Power Supply Rejection
Ratio (PSRR)
CBY e 100 mF, f e 1 kHz, CIN e 0.1 mF
Output Referred, VRIPPLE e 250 mV
b 40
b 54
b 40
b 54
dB
Channel Separation
CBY e 100 mF, f e 1 kHz, CIN e 0.1 mF
Output Referred
b 50
b 64
b 50
b 64
dB
Noise
Equivalent Input Noise RS e 0,
CIN e 0.1 mF, BW e 20 b 20 kHz
CCIR/ARM
Wideband
1.4
1.4
2.0
mV
mV
mV
1.4
1.4
2.0
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
Voltage Difference
between Outputs
5
LM1896N-2, LM2896P-2
10
Input Bias Current
120
5
20
10
120
mV
20
mV
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
THD and Gain vs Frequency
AV e 54 dB, BW e 30 kHz
THD and Gain vs Frequency
AV e 54 dB, BW e 5 kHz
THD and Gain vs Frequency
AV e 46 dB, BW e 50 kHz
THD and Gain vs Frequency
AV e 40 dB, BW e 20 kHz
THD and Gain vs Frequency
AV e 34 dB, BW e 50 kHz
AM Recovered Audio and Noise
vs Field Strength for Different
Speaker Lead Placement
Power Supply Rejection Ratio
(Referred to the Output)
vs Frequency
Channel Separation (Referred
to the Output) vs Frequency
Power Output vs
Supply Voltage
b 3 dB Bandwidth vs Voltage
Gain for Stable Operation
TL/H/7920 – 2
3
Typical Performance Curves (Continued)
Total Harmonic Distortion
vs Power Output
Power Dissipation vs
Power Output RL e 4X
Power Dissipation vs
Power Output RL e 8X
TL/H/7920 – 3
Equivalent Schematic
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
TL/H/7920–5
Top View
TL/H/7920 – 6
Top View
4
Typical Applications (Continued)
TL/H/7920 – 8
TL/H/7920 – 7
6, 9 No connection on LM1896
( ) Indicates pin number for LM2896
FIGURE 2. Stereo Amplifier with AV e 200, BW e 30 kHz
External Components (Figure 2)
Components
1. R2, R5, R10, R13
2. R3, R12
3. Ro
4. C1, C14
5. C2, C13
6. C3, C12
7. C5, C10
8. C7
9. Cc
10. Co
11. CS
Comments
Sets voltage gain, AV e 1 a R5/R2 for one channel and AV e 1 a R10/R13
for the other channel.
Bootstrap resistor sets drive current for output stage and allows pins 3 and 12 to
go above VS.
Works with Co to stabilize output stage.
Input coupling capacitor. Pins 1 and 14 are at a DC potential of VS/2. Low
frequency pole set by:
1
fL e
2q RIN C1
Feedback capacitors. Ensure unity gain at DC. Also a low frequency pole at:
1
fL e
2qR2C2
Bootstrap capacitors, used to increase drive to output stage. A low frequency
pole is set by:
1
fL e
2qR3C3
Compensation capacitor. These stabilize the amplifiers and adjust their
bandwidth. See curve of bandwidth vs allowable gain.
Improves power supply rejection (See Typical Performance Curves). Increasing
C7 increases turn-on delay.
Output coupling capacitor. Isolates pins 5 and 10 from the load. Low frequency
pole set by:
1
fL e
2q CcRL
Works with Ro to stabilize output stage.
Provides power supply filtering.
5
Application Hints
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 is a 6 dB increase in voltage gain referenced to
Vi. The voltage gain in bridge is:
R5
Vo
e2 1a
Vi
R2
CB is used to prevent DC voltage on the output of amp 1
from causing offset in amp 2. Low frequency response is
influenced by:
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 Figure 2 is capable of a
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 AV 0o
AV(S) e
S a 0o
R2 a R5
1
, 0o e
where AV e
R2
R5C5
A curve of b3 dB BW (0o) vs AV is shown in the Typical
Performance Curves.
#
fL e
J
1
2q RBCB
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 VS e 6V and RL e 8X bridged, then the
maximum dissipation is:
2
VS
62
c2e
c2
20 RL
20 c 4
PD e 0.9 Watts
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
BW e
2qRC
where R e feedback resistor
C e feedback 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.
PD e
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 Figure 4 .
TL/H/7920 – 9
FIGURE 3. Improved AM Sensitivity over Conventional Design
6
Application Hints (Continued)
TL/H/7920 – 10
Figure 4. Bridge Amplifier Connection
Printed Circuit Layout
less than 50 kX to prevent an input-output oscillation. This
oscillation is dependent on the gain and the proximity of the
bridge elements RB and CB to the ( a ) input. If the bridge
mode is not used, do not insert RB, CB into the PCB.
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).
Printed Circuit Board Layout
Figure 5 and Figure 6 show printed circuit board layouts for
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
TL/H/7920 – 11
FIGURE 5. Printed Circuit Board Layout for the LM1896
7
Printed Circuit Layout (Continued)
TL/H/7920 – 12
FIGURE 6. Printed Circuit Board Layout for the LM2896
8
Physical Dimensions inches (millimeters)
Molded Dual-In-Line Package (N)
Order Number LM1896N
See NS Package Number N14A
9
LM1896/LM2896 Dual Audio Power Amplifier
Physical Dimensions inches (millimeters) (Continued)
Single-In-Line Package (P)
Order Number LM2896P
NS Package Number P11A
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