LM4830 National Semiconductor, LM4830 Datasheet - Page 9

• Manufacturer Part Number: LM4830
• Description: Two-Way Audio Amplification System with Volume Control
• Manufacturer: National Semiconductor

Application Information
POWER AMPLIFIER HANDSFREE MODE
As shown in Figure 1 , amplifier A1 can be used in one of two
modes, bridged output or single-ended output. This IC was
intended to be used in systems requiring both internal
speaker drive and external mono-headphone drive capabil-
ity. Headphones generally have a much higher impedance
than that of speakers since headphones don’t require as
much output power. This also allows headphones to be
driven single-endedly. Shown in Figure 1 , the output can be
automatically switched from bridged speaker drive to
single-ended headphone drive using a control pin in the
headphone jack that is tied to the Headset (HS) pin, pin 3.
When the voltage at the HS pin input changes from 0V to 5V,
V
ance. This allows the permanently connected internal
speaker of the system to be disabled when a headphone is
plugged into the headphone jack. Output V
headphone single-endedly through the output coupling cap,
C
be amplified. Since C
must be big enough to allow frequencies down to 20 Hz to be
amplified. The following equation should be used for proper
component selection.
As usual, the output drive limitations are the maximum sup-
ply voltage swing, current drive capability, and power dissi-
pation. In bridged-output drive mode, the power amplifier will
drive 4
ever, trying to put a sinewave through the amplifier at the
worst case power dissipation point could cause the amplifier
to go into thermal shutdown.
In single-ended drive mode, the amplifier is intended to drive
32
limitations of voltage swing and current drive capability. The
result of driving lower impedance loads single-endedly is
lower achievable output power.
Headset and Shutdown Pin Table
X — “Don’t Care” NA — Not Applicable
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 2 states the maximum power
dissipation point for a bridged amplifier operating at a given
supply voltage and driving a specified output load.
Although the LM4830 has three amplifiers in the package,
the bridged amplifier produces the majority of the power dis-
sipation because it supplies the largest amount of output
power. If each of the amplifiers in the LM4830 were of the
same power level, each of their power dissipations would
need to be taken into account. However, this is not the case
and the bridged power amplifier is the only major power dis-
sipation contributor.
O2
C
HS Pin
. C
High
Low
X
of the bridged amplifier output is put into high imped-
C
headphones. It will drive lower impedances with the
C
C
should be chosen to allow the full audio bandwidth to
= 1/(2 (20 Hz)(R
or 8
SD Pin
High
Low
Low
with normal music signals over time. How-
P
DMAX
C
and R
= 4(V
All Outputs On
1/2 A1 On
(V
Whole IC Off
L
IC Operation
O1
)) where 16
DD
On Only)
L
create a high-pass filter, C
)
2
/(2
2
R
L
O1
)
R
then drives the
L
Microphone
MIC1 On
MIC2 On
600
NA
(1)
(2)
C
9
Even with the large internal power dissipation created by the
bridged amplifier, the LM4830 does not require heatsinking
over a large range of ambient temperatures. Using Equation
2, assuming a 5V power supply and a 8 load, the maximum
power dissipation point is 633 mW.
For the LM4830 surface mount package,
T
of the system surroundings, Equation 3 can be used to find
the maximum internal power dissipation supported by the IC
packaging. If the result of Equation 2 is greater than that of
Equation 3, then either the supply voltage must be de-
creased, the load impedance increased, or the ambient tem-
perature reduced. For the typical application of a 5V power
supply, with a bridged 8
perature possible without violating the maximum junction
temperature is approximately 100˚C provided that device op-
eration is around the maximum power dissipation point. The
average power dissipation caused by typical music material
played at a reasonable level is generally lower than the
maximum power dissipation point. Refer to the Typical Per-
formance Characteristics curves for power dissipation in-
formation for lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. The capacitor location on both the half-supply bypass
and power supply pins should be as close to the device as
possible. The effect of a larger half-supply bypass capacitor
is improved low frequency PSRR due to increased
half-supply stability. Typical applications employ a 5V regula-
tor with 10 µF and a 0.1 µF bypass capacitors which aid in
supply stability, but do not eliminate the need for bypassing
the supply nodes of the LM4830. The selection of bypass ca-
pacitors, especially C
frequency PSRR, system cost, and size constraints.
GROUNDING
In order to achieve the best possible performance, there are
certain grounding techniques that should be followed. All in-
put reference grounds should be tied with their respective
source grounds and brought back to the power supply
ground separately from the output load ground returns.
Those input grounds should also be tied in with the
half-supply bypass ground, pin 16. As an example, the AC in-
put ground reference for the power amplifier, A1, is V
7. This ground should be tied as close as possible to the By-
pass ground (pin 16), as shown in Figure 1 . In order to tie in
the signal source ground, the audio jack ground on V
should also be tied to the Bypass ground.
As stated above, the ground returns for the output loads
should be brought back to the supply ground individually.
This will keep large signal currents on those ground lines
from interfering with the stable AC input ground references.
In addition, the signal ground reference for the preamp, A2,
(the ground end of capacitor C
the mic inputs’ signal ground reference from the microphone.
LAYOUT ISSUES
As stated in the Grounding section, placement of ground re-
turn lines is imperative in maintaining the highest level of
system performance. It is not only important to route the cor-
rect ground return lines together, but also equally important
to be aware of where those ground return lines are routed in
conjunction with each other. As an example, the output load
JMAX
= 150˚C. Depending on the ambient temperature, T
P
DMAX
b
, is thus dependent upon desired low
= (T
load, the maximum ambient tem-
JMAX
I
) should be tied together with
− T
A
)/
JA
JA
= 79˚C/W and
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IN+
, pin
(3)
IN−
A
,