Datasheet

LM4881
www.ti.com SNAS001D –SEPTEMBER 1997–REVISED MAY 2013
APPLICATION INFORMATION
SHUTDOWN FUNCTION
In order to reduce power consumption while not in use, the LM4881 contains a shutdown pin to externally turn off
the amplifier's bias circuitry. This shutdown feature turns the amplifier 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 supply to provide maximum device performance. By switching the shutdown pin to the V
DD
,
the LM4881 supply current 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 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 provides 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 enables the amplifier. If the switch is open, then the external pull-up resistor will
disable the LM4881. This scheme ensures 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 LM4881 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
LM4881 does not require heat sinking over a large range of ambient temperature. From Equation 1, assuming a
5V power supply and an 8Ω load, the maximum power dissipation point is 158 mW per amplifier. Thus the
maximum package dissipation point is 317 mW. The maximum power dissipation point obtained must not be
greater than the power dissipation that results from Equation 2:
P
DMAX
= (T
JMAX
− T
A
) / θ
JA
(2)
For package DGK0008A, θ
JA
= 230°C/W, and for package D0008A, θ
JA
= 170°C/W, and for package P0008E, θ
JA
= 107°C/W. T
JMAX
= 150°C for the LM4881. 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 T
A
reduced. For the typical application of a 5V power supply, with
an 8Ω load, the maximum ambient temperature possible without violating the maximum junction temperature is
approximately 96°C provided that device operation 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
dissipation point, the ambient temperature may be increased accordingly. Refer to the Typical Performance
Characteristics curves for power dissipation information for lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifer, 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. As displayed in the Typical Performance Characteristics section, 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 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 supply nodes of the LM4881. The selection of bypass capacitors, especially
C
B
, is thus dependent upon desired low frequency PSRR, click and pop performance as explained in Proper
Selection of External Components system cost, and size constraints.
PROPER SELECTION OF EXTERNAL COMPONENTS
Selection of external components when using integrated power amplifiers is critical to optimize device and
system performance. While the LM4881 is tolerant of external component combinations, consideration to
component values must be used to maximize overall system quality.
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