lm4898mmx National Semiconductor Corporation, lm4898mmx Datasheet - Page 13

• Manufacturer Part Number: lm4898mmx
• Description: 1 Watt Fully Differential Audio Power Amplifier With Shutdown Select
• Manufacturer: National Semiconductor Corporation

Application Information
function of output power. If typical operation is not around the
maximum power dissipation point, the LM4898 can operate
at higher ambient temperatures. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection ratio (PSRR). The capacitor location on both the
bypass and power supply pins should be as close to the
device as possible. A larger half-supply bypass capacitor
improves PSRR because it increases half-supply stability.
Typical applications employ a 5V regulator with 10µF
and0.1µF bypass capacitors that increase supply stability.
This, however, does not eliminate the need for bypassing the
supply nodes of the LM4898. Although the LM4898 will
operate without the bypass capacitor C
decrease. A 1µF capacitor is recommended for C
value maximizes PSRR performance. Lesser values may be
used, but PSRR decreases at frequencies below 1kHz. The
issue of C
and click and pop performance as explained in the section
Proper Selection of External Components.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4898 contains shutdown circuitry that is used to turn off
the amplifier’s bias circuitry. In addition, the LM4898 con-
tains a Shutdown Mode pin, allowing the designer to desig-
nate whether the part will be driven into shutdown with a high
level logic signal or a low level logic signal. This allows the
designer maximum flexibility in device use, as the Shutdown
Mode pin may simply be tied permanently to either V
GND to set the LM4898 as either a "shutdown-high" device
or a "shutdown-low" device, respectively. The device may
then be placed into shutdown mode by toggling the Shut-
down Select pin to the same state as the Shutdown Mode
pin. For simplicity’s sake, this is called "shutdown same", as
the LM4898 enters shutdown mode whenever the two pins
are in the same logic state. The trigger point for either
shutdown high or shutdown low is shown as a typical value
in the Supply Current vs. Shutdown Voltage graphs in the
Typical Performance Characteristics section. It is best to
switch between ground and supply for maximum perfor-
mance. While the device may be disabled with shutdown
voltages in between ground and supply, the idle current
maybe greater than the typical value of 0.1µA. In either case,
the shutdown pin should be tied to a definite voltage to avoid
unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut-
down high or low application). This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
B
selection is thus dependant upon desired PSRR
B
(Continued)
, the PSRR may
B
. This
DD
or
13
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical when optimizing
device and system performance. Although the LM4898 is
tolerant to a variety of external component combinations,
consideration of component values must be made when
maximizing overall system quality.
The LM4898 is unity-gain stable, giving the designer maxi-
mum system flexibility. The LM4898 should be used in low
closed-loop gain configurations to minimize THD+N values
and maximize signal to noise ratio. Low gain configurations
require large input signals to obtain a given output power.
Input signals equal to or greater than 1Vrms are available
from sources such as audio codecs. Please refer to the
Audio Power Amplifier Design section for a more complete
explanation of proper gain selection. When used in its typical
application as a fully differential power amplifier the LM4898
does not require input coupling capacitors for input sources
with DC common-mode voltages of less than V
allowable input common-mode voltage levels are actually a
function of V
Equation 5:
Special care must be taken to match the values of the
feedback resistors (R
matching the input resistors (R
Figure 1). Because of the balanced nature of differential
amplifiers, resistor matching differences can result in net DC
currents across the load. This DC current can increase
power consumption, internal IC power dissipation, reduce
PSRR, and possibly damaging the loudspeaker. The chart
below demonstrates this problem by showing the effects of
differing values between the feedback resistors while as-
suming that the input resistors are perfectly matched. The
results below apply to the application circuit shown in Figure
1, and assumes that V
DC coupled inputs tied to ground.
Similar results would occur if the input resistors were not
carefully matched. Adding input coupling capacitors in be-
tween the signal source and the input resistors will eliminate
this problem, however, to achieve best performance with
minimum component count it is highly recommended that
both the feedback and input resistors matched to 1% toler-
ance or better.
Tolerance
20%
10%
5%
1%
0
V
CMi
<
DD
(V
, R
R
0.8R
0.9R
0.95R
0.99R
R
DD
f
1
i
-1.2)*((R
, and R
f
DD
1 and R
R
= 5V, R
R
1.2R
1.1R
1.05R
1.01R
R
f
/R
f
f
2
f
+(R
i
=A
i
and may be determined by
1 and R
f
2) to each other as well as
VD
i
L
)/(R
= 8Ω, and the system has
Vo2-Vo1
-0.5V
-0.250V
-0.125V
-0.025V
R
f
)-V
i
2) to each other (see
DD
*(R
i
/2R
www.national.com
I
62.5mA
31.25mA
15.63mA
3.125mA
0
LOAD
DD
f
)
. Exact
(5)
(6)