Datasheet

LM4809, LM4809LQBD
SNAS126F –FEBRUARY 2001–REVISED APRIL 2013 www.ti.com
For package MUA08A, θ
JA
= 210°C/W. T
JMAX
= 150°C for the LM4809. Depending on the ambient temperature,
T
A
, of the system surroundings, Equation 2 can be used to find the maximum internal power dissipation
supported by the IC packaging. If the result of Equation 1 is greater than that of Equation 2, then either the
supply voltage must be decreased, the load impedance increased or T
A
reduced. For the typical application of a
5V power supply, with a 32Ω load, the maximum ambient temperature possible without violating the maximum
junction temperature is approximately 133.2°C provided that device operation is around the maximum power
dissipation point. Power dissipation is a function of output power and thus, if typical operation is not around the
maximum power dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical
Performance Characteristics curves for power dissipation information for lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitors to
stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient response.
However, their presence does not eliminate the need for a local 1.0µF tantalum bypass capacitance connected
between the LM4809's supply pins and ground. Keep the length of leads and traces that connect capacitors
between the LM4809's power supply pin and ground as short as possible. Connecting a 4.7µF capacitor, C
B
,
between the BYPASS pin and ground improves the internal bias voltage's stability and improves the amplifier's
PSRR. The PSRR improvements increase as the bypass pin capacitor value increases. Too large, however,
increases the amplifier's turn-on time. The selection of bypass capacitor values, especially C
B
, depends on
desired PSRR requirements, click and pop performance (as explained in the section, Selecting Proper External
Components), system cost, and size constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4809's performance requires properly selecting external components. Though the LM4809
operates well when using external components with wide tolerances, best performance is achieved by optimizing
component values.
The LM4809 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 amplifier 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 maximum output power. Fortunately, many signal sources
such as audio CODECs have outputs of 1V
RMS
(2.83V
P-P
). Please refer to the Audio Power Amplifier Design
section for more information on selecting the proper gain.
Input and Output Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value input and output coupling capacitors (C
I
and C
O
in
Figure 1). A high value capacitor can be expensive and may compromise space efficiency in portable designs. In
many cases, however, the speakers used in portable systems, whether internal or external, have little ability to
reproduce signals below 150Hz. Applications using speakers with this limited frequency response reap little
improvement by using high value input and output capacitors.
Besides affecting system cost and size, C
i
has an effect on the LM4809's click and pop performance. The
magnitude of the pop is directly proportional to the input capacitor's size. Thus, pops can be minimized by
selecting an input capacitor value that is no higher than necessary to meet the desired −3dB frequency. Please
refer to the Optimizing Click and Pop Reduction Performance section for a more detailed discussion on click and
pop performance.
As shown in Figure 1, the input resistor, R
I
and the input capacitor, C
I
, produce a −3dB high pass filter cutoff
frequency that is found using Equation 3. In addition, the output load R
L
, and the output capacitor C
O
, produce a
-3db high pass filter cutoff frequency defined by Equation 4.
f
I-3db
=1/2πR
I
C
I
(3)
f
O-3db
=1/2πR
L
C
O
(4)
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