LM4842 National Semiconductor, LM4842 Datasheet - Page 19

• Manufacturer Part Number: LM4842
• Description: Stereo 2W Amplifiers with DC Volume Control Transient Free Outputs/ and Cap-less Headphone Drive
• Manufacturer: National Semiconductor

Application Information
maximum power dissipation point given by Equation (3)
must not exceed the power dissipation given by Equation
(4):
The LM4842’s T
to a DAP pad that expands to a copper area of 5in
PCB, the LM4842’s θ
ages soldered to a DAP pad that expands to a copper area
of 2in
For the LM4842MT package, θ
ambient temperature T
mum internal power dissipation supported by the IC packag-
ing. Rearranging Equation (4) and substituting P
P
mum ambient temperature that still allows maximum stereo
power dissipation without violating the LM4842’s maximum
junction temperature.
For a typical application with a 5V power supply and an 4Ω
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 45˚C for the MH
package.
Equation (6) gives the maximum junction temperature
T
reduce the maximum junction temperature by reducing the
power supply voltage or increasing the load resistance. Fur-
ther allowance should be made for increased ambient tem-
peratures.
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 (2) is greater than that of Equation
(3), 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
copper area around the package, with connections 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
junction-to-case thermal impedance, θ
thermal impedance, and θ
impedance.) Refer to the Typical Performance Character-
istics curves for power dissipation information at lower out-
put 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 capacitor to stabilize
JMAX
DMAX
2
. If the result violates the LM4842’s 150˚C T
' results in Equation (5). This equation gives the maxi-
on a PCB, the LM4842MH’s and LQ’s θ
JA
. The heat sink can be created using additional
JA
P
JMAX
T
T
A
is the sum of θ
DMAX
JMAX
= T
JA
= 150˚C. In the LQ package soldered
' = (T
A
JMAX
, use Equation (4) to find the maxi-
= 20˚C/W. In the MH and LQ pack-
= P
SA
DMAX
– 2*P
JMAX
is the sink-to-ambient thermal
JA
JC
θ
− T
DMAX
, θ
JA
= 80˚C/W. At any given
CS
A
+ T
CS
)/θ
, and θ
θ
A
JA
JA
is the case-to-sink
(Continued)
SA
JA
. (θ
is 41˚C/W.
DMAX
JC
2
JMAX
is the
on a
(4)
(5)
(6)
for
,
19
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
LM4842’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
capacitors between the LM4842’s power supply pin and
ground as short as possible. Connecting a 1µF capacitor,
C
the internal bias voltage’s stability and improves the amplifi-
er’s PSRR. The PSRR improvements increase as the by-
pass pin capacitor value increases. Too large a capacitor,
however, increases turn-on time and can compromise the
amplifier’s click and pop performance. The selection of by-
pass capacitor values, especially C
sired PSRR requirements, click and pop performance (as
explained in the section, Proper Selection of External
Components), system cost, and size constraints.
PROPER SELECTION OF EXTERNAL COMPONENTS
Optimizing the LM4842’s performance requires properly se-
lecting external components. Though the LM4842 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4842 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 ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain circuits demand input
signals with greater voltage swings to achieve maximum
output power. Fortunately, many signal sources such as
audio CODECs have outputs of 1V
refer to the Audio Power Amplifier Design section for more
information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (0.33µF in Figure 2). A high
value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals below
150Hz. Applications using speakers with this limited fre-
quency response reap little improvement by using a large
input capacitor.
Besides effecting system cost and size, the input coupling
capacitor has an affect on the LM4842’s click and pop per-
formance. When the supply voltage is first applied, a tran-
sient (pop) is created as the charge on the input capacitor
changes from zero to a quiescent state. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Higher value capacitors need more time to reach a quiescent
DC voltage (usually V
rent. The amplifier’s output charges the input capacitor
through the feedback resistor, R
mized by selecting an input capacitor value that is no higher
than necessary to meet the desired −3dB frequency.
As shown in Figure 1, the input resistors (RIN = 20K) and the
input capacitosr (CIN = 0.33µF) produce a −6dB high pass
filter cutoff frequency that is found using Equation (7).
BYPASS
, between the BYPASS pin and ground improves
DD
/2) when charged with a fixed cur-
f
. Thus, pops can be mini-
BYPASS
RMS
(2.83V
, depends on de-
P-P
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). Please