LM4880N National Semiconductor, LM4880N Datasheet - Page 11

• Manufacturer Part Number: LM4880N
• Description: IC AMP AUDIO PWR .325W STER 8DIP
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4880N

• Output Type: 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 325mW x 2 @ 8 Ohm
• Voltage - Supply: 2.7 V ~ 5.5 V
• Features: Shutdown, Thermal Protection
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM4880N

Application Information
switch with one of the output pins. The output of this circuit
(the voltage on pin 5 of the LM4880) has two states based
on the state of the switch. When the switch is open, signify-
ing that headphones are inserted, the LM4880 should be
enabled. When the switch is closed, the LM4880 should be
off to minimize power consumption.
The operation of this circuit is rather simple. With the switch
closed, R
gate voltage of less than 5 mV. This gate voltage keeps the
NMOS inverter off and R
LM4880 to the supply voltage. This places the LM4880 in
shutdown mode which reduces the supply current to 0.7 µA
typically. When the switch is open, the opposite condition is
produced. Resistor R
which turns on the inverter and produces a logic low signal
on the shutdown pin of the LM4880. This state enables the
LM4880 and places the amplifier in its normal mode of
operation.
This type of circuit is clearly valuable in portable products
where battery life is critical, but is also benefical for power
conscious designs such as “Green PC’s”.
AUTOMATIC SWITCHING CIRCUIT
A circuit closely related to the Automatic Shutdown Circuit
is the Automatic Switching Circuit of Figure 3. The Auto-
matic Switching Circuit utilizes both the input and output of
the NMOS inverter to toggle the states of two different audio
power amplifiers. The LM4880 is used to drive stereo single
ended loads, while the LM4861 drives bridged internal
speakers.
In this application, the LM4880 and LM4861 are never on at
the same time. When the switch inside the headphone jack
is open, the LM4880 is enabled and the LM4861 is disabled
since the NMOS inverter is on. If a headphone jack is not
present, it is assumed that the internal speakers should be
on and thus the voltage on the LM4861 shutdown pin is low
and the voltage at the LM4880 pin is high. This results in the
LM4880 being shutdown and the LM4861 being enabled.
Only one channel of this circuit is shown in Figure 3 to keep
the drawing simple but the typical application would a
LM4880 driving a stereo external headphone jack and two
LM4861’s driving the internal stereo speakers. If only one
internal speaker is required, a single LM4861 can be used as
a summer to mix the left and right inputs into a single mono
channel.
PROPER SELECTION OF EXTERNAL COMPONENTS
Selection of external components when using integrated
power amplifiers is critical to optimize device and system
performance. While the LM4880 is tolerant of external com-
ponent combinations, care must be exercised when choos-
ing component values.
The LM4880 is unity-gain stable which gives a designer
maximum system flexibility. The LM4880 should be used in
low gain configurations to minimize THD + N values, and
maximize the signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1 Vrms are available
from sources such as audio codecs. Please refer to the
section, Audio Power Amplifier Design, for a more com-
plete explanation of proper gain selection.
Besides gain, one of the major design considerations is the
closed-loop bandwidth of the amplifier. To a large extent, the
p
and R
o
form a resistor divider which produces a
p
pulls the gate of the NMOS high
sd
pulls the shutdown pin of the
(Continued)
11
bandwidth is dictated by the choice of external components
shown in Figure 1. Both the input coupling capacitor, C
the output coupling capacitor, C
filters which limit low frequency response. These values
should be chosen based on needed frequency response for
a few distinct reasons.
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly a certain sized
capacitor is needed to couple in low frequencies without
severe attenuation. But in many cases the transducers used
in portable systems, whether internal or external, have little
ability to reproduce signals below 100 Hz–150 Hz. Thus
using large input and output capacitors may not increase
system performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
C
reach its quiescent DC voltage (normally 1/2 V
charge comes from the output via the feedback and is apt to
create pops upon device enable. Thus, by minimizing the
capacitor size based on necessary low frequency response,
turn-on pops can be minimized.
Besides minimizing the input and output capacitor sizes,
careful consideration should be paid to the bypass capacitor
size. The bypass capacitor, C
nent to minimize turn-on pops since it determines how fast
the LM4880 turns on. The slower the LM4880’s outputs ramp
to their quiescent DC voltage (nominally 1/2 V
smaller the turn-on pop. Choosing C
with a small value of C
should produce a virtually clickless and popless shutdown
function. While the device will function properly, (no oscilla-
tions or motorboating), with C
will be much more susceptible to turn-on clicks and pops.
Thus, a value of C
mended in all but the most cost sensitive designs.
AUDIO POWER AMPLIFIER DESIGN
Design a Dual 200 mW/8Ω Audio Amplifier
Given:
A designer must first determine the needed supply rail to
obtain the specified output power. Calculating the required
supply rail involves knowing two parameters, V
the dropout voltage. As shown in the Typical Performance
Curves, the dropout voltage is typically 0.5V. V
determined from Equation (3).
For 200 mW of output power into an 8Ω load, the required
V
minimum supply voltage is twice the sum of V
Since 5V is a standard supply voltage in most applications, it
is chosen for the supply rail. Extra supply voltage creates
headroom that allows the LM4880 to reproduce peaks in
excess of 200 mW without clipping the signal. At this time,
the designer must make sure that the power supply choice
along with the output impedance does not violate the condi-
tions explained in the Power Dissipation section. Remem-
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. A larger input coupling capacitor requires more charge to
Power Output: 200 mWrms
Input Level: 1 Vrms (max)
Bandwidth: 100 Hz–20 kHz
is 1.79V. Since this is a single supply application, the
B
equal to 1.0 µF or larger is recom-
i
(in the range of 0.1 µF to 0.39 µF),
B
B
, is the most critical compo-
o
±
equal to 0.1 µF, the device
, form first order high pass
Input Impedance: 20 kΩ
0.50 dB
Load Impedance: 8Ω
B
equal to 1.0 µF along
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and V
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