first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Application Information (Continued)
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the LM4871 as possible. The capacitor
connected between the bypass pin and ground improves the
internal bias voltage’s stability, producing improved PSRR.
The improvements to PSRR increase as the bypass pin
capacitor increases. Typical applications employ a 5V regu-
lator with 10µF and a 0.1µF bypass capacitors which aid in
supply stability. This does not eliminate the need for bypass-
ing the supply nodes of the LM4871 with a 1µF tantalum
capacitor. The selection of bypass capacitors, especially CB,
is dependent upon PSRR requirements, click and pop per-
formance as explained in the section, Proper Selection of
External Components, system cost, and size constraints.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
reproduce signals below 100Hz to 150Hz. Thus, using a
large input capacitor may not increase actual system perfor-
mance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
Ci. A larger input coupling capacitor requires more charge to
reach its quiescent DC voltage (nominally 1/2 VDD). This
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.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4871 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half- supply. It is best to switch between ground and
supply to provide maximum device performance. By switch-
ing the shutdown pin to VDD, the LM4871 supply current
draw will be minimized in idle mode. While the device will be
disabled with shutdown pin voltages less then VDD, the idle
current may be greater than the typical value of 0.6µA. In
either case, the shutdown pin should be tied to a definite
voltage to avoid unwanted state changes.
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the bypass capacitor value. Bypass
capacitor, CB, is the most critical component to minimize
turn-on pops since it determines how fast the LM4871 turns
on. The slower the LM4871’s outputs ramp to their quiescent
DC voltage (nominally 1/2 VDD), the smaller the turn-on pop.
Choosing CB equal to 1.0µF along with a small value of Ci (in
the range of 0.1µF to 0.39µF), should produce a virtually
clickless and popless shutdown function. While the device
will function properly, (no oscillations or motorboating), with
CB equal to 0.1µF, the device will be much more susceptible
to turn-on clicks and pops. Thus, a value of CB equal to
1.0µF is recommended in all but the most cost sensitive
designs.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown. Another so-
lution is to use a single-pole, single-throw switch in conjunc-
tion with an external pull-up resistor. When the switch is
closed, the shutdown pin is connected to ground and en-
ables the amplifier. If the switch is open, then the external
pull-up resistor will disable the LM4871. This scheme guar-
antees that the shutdown pin will not float thus preventing
unwanted state changes.
AUDIO POWER AMPLIFIER DESIGN
Design a 1W/8Ω Audio Amplifier
Given:
Power Output
Load Impedance
Input Level
1 Wrms
8Ω
1 Vrms
PROPER SELECTION OF EXTERNAL COMPONENTS
Input Impedance
Bandwidth
20 kΩ
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4871 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
100 Hz–20 kHz 0.25 dB
A designer must first determine the minimum supply rail to
obtain the specified output power. By extrapolating from the
Output Power vs Supply Voltage graphs in the Typical Per-
formance Characteristics section, the supply rail can be
easily found. A second way to determine the minimum sup-
ply rail is to calculate the required Vopeak using Equation 3
and add the output voltage. Using this method, the minimum
The LM4871 is unity-gain stable which gives a designer
maximum system flexibility. The LM4871 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.
supply voltage would be (Vopeak + (VOD
+ VODBOT)), where
VOD
and VOD
are extrapolated frToOmP the Dropout Volt-
TOP
age BvOsT Supply Voltage curve in the Typical Performance
Characteristics section.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1. The input coupling capacitor, Ci, forms a
(3)
Using the Output Power vs Supply Voltage graph for an 8Ω
load, the minimum supply rail is 4.6V. But since 5V is a
standard voltage in most applications, it is chosen for the
supply rail. Extra supply voltage creates headroom that al-
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