LM4859_05 Datasheet, PDF (24/29 Pages) National Semiconductor (TI) – Stereo 1.2W Audio Sub-system with 3D Enhancement

Application Information (Continued)
For a typical application with a 5V power supply, stereo 8Ω
loudspeaker load, and the stereo 32Ω headphone load, the
maximum ambient temperature that allows maximum stereo
power dissipation without exceeding the maximum junction
temperature is approximately 93.4˚C for the SP package.
T_JMAX = P_DMAX-TOTAL θ_JA + T_A
(10)
Equation (10) gives the maximum junction temperature T_J-
MAX. If the result violates the LM4859’s 150˚C, reduce the
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases. If the result of Equation (7) is
greater than that of Equation (8), then decrease the supply
voltage, increase the load impedance, or reduce the ambient
temperature. If these measures are insufficient, a heat sink
can be added to reduce θ_JA. The heat sink can be created
using additional copper area around the package, with con-
nections to the ground pin(s), supply pin and amplifier output
pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the θ_JA is the sum of θ_JC, θ_CS, and
θ_SA. (θ_JC is the junction-to-case thermal impedance, θ_CS is
the case-to-sink thermal impedance, and θ_SA is the sink-to-
ambient thermal impedance.) Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion at lower output power levels.
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 stabi-
lize 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
LM4859’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4859’s
power supply pin and ground as short as possible.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (C_i in Figure 1). In many
cases, however, the speakers used in portable systems,
whether internal or external, have little ability to reproduce
signals below 50Hz. Applications using speakers with this
limited frequency response reap little improvement; by using
a large input capacitor.
The internal input resistor (R_i) and the input capacitor (C_i)
produce a high pass filter cutoff frequency that is found using
Equation (13).
f_c = 1 / (2πR_iC_i)
(11)
As an example when using a speaker with a low frequency
limit of 50Hz and R_i = 20kΩ, C_i, using Equation (13) is
0.19µF. The 0.22µF C_i shown in Figure 4 allows the LM4859
to drive high efficiency, full range speaker whose response
extends below 40Hz.
Output Capacitor Value Selection
Amplifying the lowest audio frequencies also requires the
use of a high value output coupling capacitor (C_O in Figure
1). A high value output capacitor can be expensive and may
compromise space efficiency in portable design.
The speaker load (R_L) and the output capacitor (C_O) form a
high pass filter with a low cutoff frequency determined using
Equation (14).
f_c = 1 / (2πR_LC_O)
(12)
When using a typical headphone load of R_L = 32Ω with a low
frequency limit of 50Hz, C_O is 99µF.
The 100µF C_O shown in Figure 4 allows the LM4859 to drive
a headphone whose frequency response extends below
50Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C_B, the capacitor con-
nected to the BYPASS pin. Since C_B determines how fast
the LM4859 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4859’s
outputs ramp to their quiescent DC voltage (nominally V_DD/
2), the smaller the turn-on pop. Choosing C_B equal to 2.2µF
along with a small value of C_i (in the range of 0.1µF to
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C_i no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops. C_B’s value should be in the range of 5 times to 10
times the value of C_i. This ensures that output transients are
eliminated when the LM4859 transitions in and out of shut-
down mode. Connecting a 2.2µF capacitor, C_B, between the
BYPASS pin and ground improves the internal bias voltage’s
stability and improves the amplifier’s PSRR. The PSRR im-
provements increase as the bypass pin capacitor value in-
creases. However, increasing the value of C_B will increase
wake-up time. The selection of bypass capacitor value, C_B,
depends on desired PSRR requirements, click and pop per-
formance, wake-up time, system cost, and size constraints.
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