lm4874mh National Semiconductor Corporation, lm4874mh Datasheet - Page 16

• Manufacturer Part Number: lm4874mh
• Description: 2.1w Differential Input, Btl Output Stereo Audio Amplifier With Selectable Gain And Shutdown
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4874 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 value increases.
Typical applications employ a 5V regulator with 10µF and a
0.1µF filter capacitors that aid in supply stability. Their pres-
ence, however, does not eliminate the need for bypassing
the LM4874’s supply pins. The selection of bypass capacitor
values, especially C
ments, click and pop performance (as explained in theSe-
lecting External Components section), system cost, and
size constraints.
MICRO-POWER SHUTDOWN
The LM4874 features an active-low micro-power shutdown
mode. The voltage applied to the SHUTDOWN pin controls
the LM4874’s shutdown function. Activate micro-power shut-
down by applying 0V to the SHUTDOWN pin. The logic
threshold is typically 0.4V for a logic low and 1.5V for a logic
high. When active, the LM4874’s micro-power shutdown
feature turns off the amplifier’s bias circuitry, disables the
internal V
into a high impedance state. The result is greatly reduced
power supply current. The low 0.1µA typical shutdown cur-
rent is achieved by applying a voltage to the SHUTDOWN
pin that is as near to GND as possible. A voltage that is
greater than GND may increase the shutdown current.
There are a few methods to control the micro-power shut-
down. These include using a single-pole, single-throw switch
(SPST), a microprocessor, or a microcontroller. When using
a switch, connect a 100kΩ pull-down resistor between the
SHUTDOWN pin and GND and the SPST switch between
the SHUTDOWN pin and V
eration by closing the switch. Opening the switch applies
GND to the SHUTDOWN pin, activating micro-power shut-
down. The switch and resistor guarantee that the SHUT-
DOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon-
troller, use a digital output to apply the active-state voltage to
the SHUTDOWN pin. Driving the SHUTDOWN pin with ac-
tive circuitry eliminates the pull-down resistor.
GAIN 0 GAIN 1
0
0
1
1
Table 1. Gain Settings and Input Resistance
DD
/2 generator, and forces the amplifier outputs
0
1
0
1
B
Selected Gain
, depends on desired PSRR require-
(dB)
15.6
21.6
10
6
DD
. Select normal amplifier op-
Input Resistance
(Continued)
90kΩ
70kΩ
45kΩ
25kΩ
(R
i
)
16
Table 2. Logic Level Truth Table for Shutdown
Operation
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4874’s performance requires properly se-
lecting external components. Though the LM4874 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues. The LM4874 is unity-gain stable, giving a designer
maximum design flexibility. The gain should be set to no
more than a given application requires. This allows the am-
plifier to achieve minimum THD+N and maximum signal-to-
noise ratio. These parameters are compromised as the
closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve maxi-
mum output power. Fortunately, many signal sources such
as audio CODECs have outputs of 1V
Please refer to the Audio Power Amplifier Design section for
more information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
high value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals with
frequencies below 150Hz. Applications using speakers with
this limited frequency response reap little improvement by
using large input capacitor.
Besides effecting system cost and size, CI - C4 can also
affect on the LM4874’s turn-on and turn-off transient (’click
and pop’) performance. When the supply voltage is first
applied, a transient may be created as the charge on the
input capacitor changes from zero to a quiescent state. The
magnitude of the transient is proportional to the value of, and
more importantly, the mismatch between, the capacitors
connected to a given pair of inverting and non-inverting
inputs. The better the match, the less the transient magni-
tude.
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
rent. This fixed current is supplied through amplifiers input
pins. Thus, selecting an input capacitor value that is no
higher than necessary to meet the desired -3dB frequency
will reduce turn-on time and help ensure that transients are
minimized.
The LM4874’s nominal input resistance (R
minimum) and the input capacitor, C
with a -3dB low frequency limit defined by equation (5).
As an example when using a speaker with a low frequency
limit of 150Hz, C
1 allows the LM4874 to drive high efficiency, full range
speaker whose response extends below 30Hz.
SHUTDOWN
High
Low
I
, is 0.047µF. The 0.47µF C
f
-3dB
DD
/2) when charged with a fixed cur-
= 1/2π(25kΩ)C
I
, C
2
Full Power, stereo
OPERATIONAL
and C
BTL amplifiers
Micro-power
Shutdown
i
, form high pass filter
MODE
3
i
, C
i
) is 25kΩ (20kΩ,
I
RMS
4
shown in Figure
) in Figure 1. A
(2.83V
P-P
(5)
).