lm4820mm-6 National Semiconductor Corporation, lm4820mm-6 Datasheet - Page 11

• Manufacturer Part Number: lm4820mm-6
• Description: Fixed Gain 1 Watt Audio Power Amplifier
• Manufacturer: National Semiconductor Corporation

Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4820-6 has two operational
amplifiers internally. Figure 1 shows that the output of am-
plifier one serves as the input to amplifier two which results
in both amplifiers producing signals identical in magnitude,
but out of phase by 180˚. Consequently, the differential gain
for the IC is
By driving the load differentially through outputs Vo1 and
Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classical single-ended amplifier configura-
tion where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration, as it provides differential
drive to the load, thus doubling output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
A bridge configuration, such as the one used in LM4820-6,
also creates a second advantage over single-ended amplifi-
ers. Since the differential outputs, Vo1 and Vo2, are biased
at half-supply, no net DC voltage exists across the load. This
eliminates the need for an output coupling capacitor which is
required in a single supply, single-ended amplifier configura-
tion. Without an output coupling capacitor, the half-supply
bias across the load would result in both increased internal
IC power dissipation and also possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power
delivered to the load by a bridge amplifier is an increase in
internal power dissipation. Since the LM4820-6 has two
operational amplifiers in one package, the maximum internal
power dissipation is 4 times that of a single-ended amplifier.
The maximum power dissipation for a given application can
be derived from the power dissipation graphs or from Equa-
tion 2.
It is critical that the maximum junction temperature T
150˚C is not exceeded. T
power derating curves by using P
area. By adding additional copper foil, the thermal resistance
of the application can be reduced from a free air value of
150˚C/W, resulting in higher P
can be added to any of the leads, bumps or vias connected
to the LM4820-6. It is especially effective when connected to
V
information on the LM4820-6 reference design board for an
example of good heat sinking. If T
then additional changes must be made. These changes can
include reduced supply voltage, higher load impedance, or
reduced ambient temperature. Internal power dissipation is a
function of output power. Refer to the Typical Performance
Characteristics curves for power dissipation information for
different output powers and output loading.
DD
, G
ND
, and the output pins. Refer to the application
P
DMAX
= 4*(V
A
VD
JMAX
= 2 *(R
DD
)
can be determined from the
DMAX
2
/(2π
DMAX
f
/R
JMAX
i
2
)
. Additional copper foil
R
and the PC board foil
L
still exceeds 150˚C,
)
JMAX
(1)
(2)
of
11
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is critical for
low noise performance and high power supply rejection. The
capacitor location on both the bypass and power supply pins
should be as close to the device as possible. Typical appli-
cations employ a 5V regulator with 10 µF tantalum or elec-
trolytic capacitor and a ceramic bypass capacitor which aid
in supply stability. This does not eliminate the need for
bypassing the supply nodes of the LM4820-6. The selection
of a bypass capacitor, especially C
PSRR requirements, click and pop performance (as ex-
plained in the section, Proper Selection of External Com-
ponents), system cost, and size constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4820-6 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the
amplifier off when a logic low is placed on the shutdown pin.
By switching the shutdown pin to ground, the LM4820-6
supply current draw will be minimized in idle mode. While the
device will be disabled with shutdown pin voltages less than
0.5V
value of 0.1µA. (Idle current is measured with the shutdown
pin grounded).
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry to provide a
quick, smooth transition into shutdown. Another solution is to
use a single-pole, single-throw switch in conjunction with an
external pull-up resistor. When the switch is closed, the
shutdown pin is connected to ground and disables the am-
plifier. If the switch is open, then the external pull-up resistor
will enable the LM4820-6. This scheme guarantees that the
shutdown pin will not float thus preventing unwanted state
changes.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4820-6 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4820-6 is unity-gain stable which gives the designer
maximum system flexibility. The LM4820-6 at 6dB of fixed
gain is a low gain configuration which minimizes THD+N
values, and maximizes the signal to noise ratio. Low gain
configurations require large input signals to obtain a given
output power. Input signals equal to or greater than 1 Vrms
are available from sources such as audio codecs. Please
refer to the section, Audio Power Amplifier Design, for a
more complete explanation of proper gain selection.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1. The input coupling capacitor, C
first order high pass filter which limits low frequency re-
sponse. This value should be chosen based on needed
frequency response for a few distinct reasons.
Selection Of Input Capacitor Size
Large input capacitors are both expensive and space hungry
for portable designs. Clearly, a certain sized capacitor is
needed to couple in low frequencies without severe attenu-
ation. But in many cases the speakers used in portable
systems, whether internal or external, have little ability to
DC
, the idle current may be greater than the typical
B
, is dependent upon
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