LM4859SP National Semiconductor, LM4859SP Datasheet - Page 24

• Manufacturer Part Number: LM4859SP
• Description: Audio Amp Headphone/Speaker 2-CH Stereo 1.6W Class-AB 28-Pin LLP EP T/R
• Manufacturer: National Semiconductor

Specifications of LM4859SP

• Package: 28LLP EP
• Function: Headphone/Speaker
• Amplifier Type: Class-AB
• Total Harmonic Distortion Noise: 0.04@32Ohm@15mW|0.05@8Ohm@400mW %
• Typical Psrr: 86 dB
• Output Signal Type: Differential|Single
• Output Type: 2-Channel Stereo
• Maximum Input Offset Voltage: 40@5V mV
• Maximum Load Resistance: 32 Ohm

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Application Information
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.
Equation (10) gives the maximum junction temperature T
MAX
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
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
θ
the case-to-sink thermal impedance, and θ
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
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
SA
. (θ
. If the result violates the LM4859’s 150˚C, reduce the
JC
is the junction-to-case thermal impedance, θ
T
JMAX
= P
DMAX-TOTAL
JA
. The heat sink can be created
JA
is the sum of θ
i
in Figure 1). In many
θ
JA
+ T
(Continued)
SA
A
is the sink-to-
JC
, θ
CS
, and
CS
(10)
is
J -
24
limited frequency response reap little improvement; by using
a large input capacitor.
The internal input resistor (R
produce a high pass filter cutoff frequency that is found using
Equation (13).
As an example when using a speaker with a low frequency
limit of 50Hz and R
0.19µF. The 0.22µF C
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
1). A high value output capacitor can be expensive and may
compromise space efficiency in portable design.
The speaker load (R
high pass filter with a low cutoff frequency determined using
Equation (14).
When using a typical headphone load of R
frequency limit of 50Hz, C
The 100µF C
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
nected to the BYPASS pin. Since C
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
2), the smaller the turn-on pop. Choosing C
along with a small value of C
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing C
sary for the desired bandwidth helps minimize clicks and
pops. C
times the value of C
eliminated when the LM4859 transitions in and out of shut-
down mode. Connecting a 2.2µF capacitor, C
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
wake-up time. The selection of bypass capacitor value, C
depends on desired PSRR requirements, click and pop per-
formance, wake-up time, system cost, and size constraints.
B
’s value should be in the range of 5 times to 10
O
shown in Figure 4 allows the LM4859 to drive
i
. This ensures that output transients are
L
i
f
) and the output capacitor (C
i
c
f
c
shown in Figure 4 allows the LM4859
= 20kΩ, C
= 1 / (2πR
= 1 / (2πR
O
is 99µF.
i
) and the input capacitor (C
i
(in the range of 0.1µF to
L
i
i
, using Equation (13) is
C
C
O
i
)
B
i
)
B
no larger than neces-
determines how fast
, the capacitor con-
L
= 32Ω with a low
B
B
equal to 2.2µF
B
, between the
will increase
O
O
in Figure
) form a
(12)
(11)
DD
B
i
)
/
,