Typical Performance Characteristics
Typical Application
Frequency Response
(Continued)
Typical Application
Frequency Response
Power Derating Curve
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Application Information
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the
LM4880 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 to provide maximum device performance. By
switching the shutdown pin to V
DD
, the LM4880 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 defi-
nite voltage because leaving the pin floating may result in an
unwanted shutdown condition.
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 LM4880. This scheme guarantees that
the shutdown pin will not float which will prevent unwanted
state changes.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design.
Equation (1)
states the maximum power
dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.
P
DMAX
= (V
DD
)
2
/(2π
2
R
L
)
(1)
Since the LM4880 has two operational amplifiers in one
package, the maximum internal power dissipation point is
twice that of the number which results from
Equation (1).
Even with the large internal power dissipation, the LM4880
does not require heat sinking over a large range of ambient
temperatures. From
Equation (1),
assuming a 5V power sup-
ply and an 8Ω load, the maximum power dissipation point is
158 mW per amplifier. Thus the maximum package dissipa-
tion point is 317 mW. The maximum power dissipation point
obtained must not be greater than the power dissipation that
results from
Equation (2):
(2)
P
DMAX
= (T
JMAX
-T
A
)/θ
JA
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8
For the LM4880 surface mount package,
θ
JA
= 170˚ C/W
and T
JMAX
= 150˚C. Depending on the ambient temperature,
T
A
, of the system surroundings,
Equation (2)
can be used to
find the maximum internal power dissipation supported by
the IC packaging. If the result of
Equation (1)
is greater than
that of
Equation (2),
then either the supply voltage must be
decreased, the load impedance increased, or the ambient
temperature reduced. For the typical application of a 5V
power supply, with an 8Ω load, the maximum ambient tem-
perature possible without violating the maximum junction
temperature is approximately 96˚C provided that device op-
eration is around the maximum power dissipation point.
Power dissipation is a function of output power and thus, if
typical operation is not around the maximum power dissipa-
tion point, the ambient temperature may be increased ac-
cordingly. Refer to the
Typical Performance Characteris-
tics
curves for power dissipation information 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 bypass and power
supply pins should be as close to the device as possible. As
displayed in the
Typical Performance Characteristics
sec-
tion, the effect of a larger half supply bypass capacitor is im-
proved low frequency PSRR due to increased half-supply
stability. Typical applications employ a 5V regulator with
10 µF and a 0.1 µF bypass capacitors which aid in supply
stability, but do not eliminate the need for bypassing the sup-
ply nodes of the LM4880. The selection of bypass capaci-
tors, especially C
B
, is thus dependant upon desired low fre-
quency PSRR, click and pop performance as explained in
the section,
Proper Selection of External Components
section, system cost, and size constraints.
AUTOMATIC SHUTDOWN CIRCUIT
As shown in
Figure 2,
the LM4880 can be set up to automati-
cally shutdown when a load is not connected. This circuit is
based upon a single control pin common in many head-
phone jacks. This control pin forms a normally closed switch
with one of the output pins. The output of this circuit (the volt-
age on pin 5 of the LM4880) has two states based on the
state of the switch. When the switch is open, signifying that
headphones are inserted, the LM4880 should be enabled.
When the switch is closed, the LM4880 should be off to mini-
mize power consumption.
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