LM4889MAX/NOPB National Semiconductor, LM4889MAX/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM4889MAX/NOPB
• Description: IC AMP AUDIO PWR 1W MONO 8SOIC
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4889MAX/NOPB

• Output Type: 1-Channel (Mono)
• Max Output Power X Channels @ Load: 1W x 1 @ 8 Ohm
• Voltage - Supply: 2.2 V ~ 5.5 V
• Features: Depop, Shutdown, Thermal Protection
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4889MAX

Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4889 has two operational am-
plifiers internally, allowing for a few different amplifier config-
urations. The first amplifier's gain is externally configurable,
while the second amplifier is internally fixed in a unity-gain,
inverting configuration. The closed-loop gain of the first am-
plifier is set by selecting the ratio of R
amplifier's gain is fixed by the two internal 20kΩ resistors.
Figure 1 shows that the output of amplifier one serves as the
input to amplifier two which results in both amplifiers produc-
ing signals identical in magnitude, but out of phase by 180°.
Consequently, the differential gain for the IC is
By driving the load differentially through outputs Vo1 and Vo2,
an amplifier configuration commonly referred to as “bridged
mode” is established. Bridged mode operation is different
from the classical single-ended amplifier configuration where
one side of the load is connected to ground.
A bridge amplifier design has an advantage over the single-
ended configuration, as it provides differential drive to the
load, thus doubling output swing for a specified supply volt-
age. Four times the output power is possible as compared to
a single-ended amplifier under the same conditions. This in-
crease in attainable output power assumes that the amplifier
is not current limited or clipped. In order to choose an
amplifier's closed-loop gain without causing excessive clip-
ping, please refer to the Audio Power Amplifier Design
section.
A bridge configuration, such as the one used in LM4889, also
creates a second advantage over single-ended amplifiers.
Since the differential outputs, Vo1 and Vo2, are biased at half-
supply, no net DC voltage exists across the load. This elimi-
nates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal IC
power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a suc-
cessful amplifier, whether the amplifier is bridged or single-
ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4889 has two opera-
tional amplifiers in one package, the maximum internal power
dissipation is 4 times that of a single-ended amplifier. The
maximum power dissipation for a given application can be
derived from the power dissipation graphs or from Equation
1.
It is critical that the maximum junction temperature T
150°C is not exceeded. T
power derating curves by using P
area. By adding additional copper foil, the thermal resistance
of the application can be reduced from a free air value of 150°
C/W, resulting in higher P
added to any of the leads connected to the LM4889. It is es-
pecially effective when connected to V
pins. Refer to the application information on the LM4889 ref-
erence design board for an example of good heat sinking. If
T
made. These changes can include reduced supply voltage,
JMAX
still exceeds 150°C, then additional changes must be
P
DMAX
= 4*(V
A
VD
DMAX
JMAX
= 2 *(R
DD
)
2
. Additional copper foil can be
/(2π
can be determined from the
f
DMAX
/R
2
R
i
)
f
DD
L
to R
)
and the PC board foil
, G
i
ND
while the second
(1)
, and the output
JMAX
of
9
higher load impedance, or reduced ambient temperature. In-
ternal power dissipation is a function of output power. Refer
to the Typical Performance Characteristics curves for
power dissipation information for different output powers and
output loading.
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 device as possible. Typical appli-
cations employ a 5V regulator with 10 µF tantalum or elec-
trolytic capacitor and a ceramic bypass capacitor which aid in
supply stability. This does not eliminate the need for bypass-
ing the supply nodes of the LM4889. The selection of a bypass
capacitor, especially C
ments, click and pop performance (as explained in the sec-
tion, Proper Selection of External Components), system
cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4889 contains a shutdown pin to externally turn off the
amplifier's bias circuitry. This shutdown feature turns the am-
plifier off when a logic low is placed on the shutdown pin. By
switching the shutdown pin to ground, the LM4889 supply
current draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages less than
0.5V
of 0.1µA. (Idle current is measured with the shutdown pin
grounded).
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch in conjunction with an
external pull-up resistor. When the switch is closed, the shut-
down pin is connected to ground and disables the amplifier.
If the switch is open, then the external pull-up resistor will en-
able the LM4889. This scheme guarantees that the shutdown
pin will not float thus preventing unwanted state changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications using
integrated power amplifiers is critical to optimize device and
system performance. While the LM4889 is tolerant of external
component combinations, consideration to component values
must be used to maximize overall system quality.
The LM4889 is unity-gain stable which gives the designer
maximum system flexibility. The LM4889 should be used in
low gain configurations to minimize THD+N values, and max-
imize the signal to noise ratio. Low gain configurations require
large input signals to obtain a given output power. Input sig-
nals 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 complete expla-
nation of proper gain selection.
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, C
high pass filter which limits low frequency response. This val-
ue should be chosen based on needed frequency response
for a few reasons.
SELECTION OF INPUT CAPACITOR SIZE
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
DC
, the idle current may be greater than the typical value
B
, is dependent upon PSRR require-
i
, forms a first order
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