Application Information (Continued)
POWER DISSIPATION
Whether the power amplifier is bridged or single-ended,
power dissipation is a major concern when designing the
amplifier. Equation 1 states the maximum power dissipation
point for a single-ended amplifier operating at a given supply
voltage and driving a specified load.
P
DMAX
=
(V
DD
)
2
/(2π
2
R
L
): Single-Ended (1)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in in-
ternal power dissipation. Equation 2 states the maximum
power dissipation point for a bridge amplifier operating at the
same given conditions.
P
DMAX
=
4
*
(V
DD
)
2
/(2π
2
R
L
): Bridge Mode (2)
Since the LM4863 is a dual channel power amplifier, the
maximum internal power dissipation is 2 times that of Equa-
tion 1 or Equation 2 depending on the mode of operation.
Even with this substantial increase in power dissipation, the
LM4863 does not require heatsinking. The power dissipation
from Equation 2, assuming a 5V power supply and an 8Ω
load, must not be greater than the power dissipation that re-
sults from Equation 3:
P
DMAX
=
(T
JMAX
−T
A
)/θ
JA
(3)
For packages M16A and MTA20, θ
JA
=
80˚C/W, and for
package N16A, θ
JA
=
63˚C/W. T
JMAX
=
150˚C for the
LM4863. Depending on the ambient temperature, T
A
,ofthe
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 an 8Ω bridged load, the maximum ambient tem-
perature possible without violating the maximum junction
temperature is approximately 48˚C provided that device op-
eration is around the maximum power dissipation point and
assuming surface mount packaging. Internal power dissipa-
tion is a function of output power. If typical operation is not
around the maximum power dissipation point, the ambient
temperature can be increased. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation for different 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 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
PSRR due to increased half-supply stability. Typical applica-
tions employ a 5V regulator with 10 µF and a 0.1 µF bypass
capacitors which aid in supply filtering. This does not elimi-
nate the need for bypassing the supply nodes of the
LM4863. The selection of bypass capacitors, especially C
B
,
is thus dependent upon desired PSRR requirements, click
and pop performance as explained in the section, Proper
Selection of External Components, system cost, and size
constraints.
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4863 contains a shutdown pin to externally turn off the
amplifier’s bias circuitry. This shutdown feature turns the am-
plifier off when a logic high is placed on the shutdown pin.
The trigger point between a logic low and logic high level is
typically half supply. It is best to switch between ground and
the supply V
DD
to provide maximum device performance. By
switching the shutdown pin to V
DD
, the LM4863 supply cur-
rent draw will be minimized in idle mode. While the device
will be disabled with shutdown pin voltages less than V
DD
,
the idle current may be greater than the typical value of
0.7 µA. In either case, the shutdown pin should be tied to a
definite voltage to avoid unwanted state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry which pro-
vides a quick, smooth transition into shutdown.Another solu-
tion 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 enables the
amplifier. If the switch is open, then the external pull-up re-
sistor will disable the LM4863. This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
HP-IN FUNCTION
The LM4863 possesses a headphone control pin that turns
off the amplifiers which drive +OutA and +OutB so that
single-ended operation can occur and a bridged connected
load is muted. Quiescent current consumption is reduced
when the IC is in this single-ended mode.
Figure 2
shows the implementation of the LM4863’s head-
phone control function using a single-supply headphone am-
plifier. The voltage divider of R1 and R2 sets the voltage at
the HP-IN pin (pin 16) to be approximately 50 mV when there
are no headphones plugged into the system. This logic-low
voltage at the HP-IN pin enables the LM4863 and places it in
bridged mode operation. Resistor R4 limits the amount of
current flowing out of the HP-IN pin when the voltage at that
pin goes below ground resulting from the music coming from
the headphone amplifier. The output coupling capacitors pro-
tect the headphones by blocking the amplifier’s half supply
DC voltage.
When there are no headphones plugged into the system and
the IC is in bridged mode configuration, both loads are es-
sentially at a 0V DC potential. Since the HP-IN threshold is
set at 4V, even in an ideal situation, the output swing cannot
cause a false single-ended trigger.
When a set of headphones are plugged into the system, the
contact pin of the headphone jack is disconnected from the
signal pin, interrupting the voltage divider set up by resistors
R1 and R2. Resistor R1 then pulls up the HP-IN pin, en-
abling the headphone function. This disables the second
side of the amplifier thus muting the bridged speakers. The
amplifier then drives the headphones, whose impedance is
in parallel with resistors R2 and R3. Resistors R2 and R3
have negligible effect on output drive capability since the
typical impedance of headphones are 32Ω. Also shown in
Figure 2
are the electrical connections for the headphone
jack and plug. A 3-wire plug consists of a Tip, Ring and
Sleave, where the Tip and Ring are signal carrying conduc-
tors and the Sleave is the common ground return. One con-
trol pin contact for each headphone jack is sufficient to indi-
cate to control inputs that the user has inserted a plug into a
jack and that another mode of operation is desired.
The LM4863 can be used to drive both a pair of bridged 8Ω
speakers and a pair of 32Ω headphones without using the
HP-IN pin. In this case the HP-IN would not be connected to
the headphone jack but to a microprocessor or a switch. By
enabling the HP-IN pin, the 8Ω speakers can be muted.
LM4863
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